Drivetrain

TH400 rebuild

The recommendation holds, and the verified numbers are in the file (/private/tmp/claude-501/-Users-topper-sandbox/e4b3498d-606b-46e4-bce7-6dfd5da5a94b/scratchpad/bonanza/sections/06-trans-th400.md). For Path B (400-450 hp, 93 octane), do a DIY TH400 rebuild with the Alto 031901HPPWRSK kit ($545), a 34-element…

Research note: many big vendor pages (Summit, Jegs, CJ Pony, ck5.com) returned 403 to the fetch tool, so a number of prices below come from search-result snippets or secondary retailers. Each is labelled "snippet" or "page", and anything unconfirmed says "price not confirmed". All checked 2026-10-12.

The pick

Good
About $1,500-$2,500 DIY. Alto kit $545, TransGo $83-$152, Derale 13502 about $105, stainless lines $95, custom converter $550-$900, 34-element sprag upgrade (price not confirmed, budget a few hundred dollars). A shop rebuild is about $700-$1,400 (forum snippet).
Better
About $2,700-$3,400 DIY, or a built unit. DIY is the Alto kit, Coan COA-22817B drum $1,019, TransGo, a custom converter and cooler. Built options are ATI Street/Strip $2,395 (no converter), TCI 211000 $2,696 sale (no converter), or the ATI Street Rod combo $3,495 (includes converter and cooler).
Best
About $5,000-$6,300. Gearstar Level 3 is $5,995 + $650 core + $200 ATF and includes a street-spec converter (800 hp, 4x4 listing, confirm the 2WD fit). A built TH400 plus Gear Vendors is $3,195 for the unit plus a driveshaft and fitting, and runs about $6,300 all-in from an older Buick owner's $5,880 (ballpark only).

Pick for this truck:

  1. If the owner wants the simplest, cheapest path and accepts a loud 70 mph: DIY TH400 with the Alto 031901HPPWRSK kit ($545, page-verified) or an Alto/Raybestos Level 2 kit ($315-$475, snippets), a 34-element sprag drum upgrade (price not confirmed; budget a few hundred dollars), a TransGo 400-1&2 street kit ($83-$152), a Derale 13502 cooler (about $105), stainless lines ($95), a custom 2,200-2,500 converter ($550-$900; about $625 for a Coan-class unit, snippet), and 3.42 gears. Total roughly $1,500-$2,500 DIY (the "good" tier in the tier summary is $1,000-$1,800 for the lean end of this). Machine/part extras are not included. For Path B this is enough.

  2. For Path A (500+ lb-ft), build it as: Coan COA-22817B Super Sprag 4-clutch drum ($1,019, page-verified), Alto kit ($545), TransGo ($152), custom converter ($625-$900), cooler, new lines. Total roughly $2,700-$3,400 DIY, or a ready-built ATI Street/Strip ($2,395, no converter), TCI StreetFighter 211000 ($2,696 sale, no converter), ATI Street Rod combo ($3,495, includes converter and cooler) or Gearstar Level 3 ($5,995 + $650 core, includes converter) as the lowest-effort routes.

  3. The bottom line: a TH400 is the right answer for strength and cost. It does not fully satisfy the "comfortable 65-70 mph commute" requirement unless you add overdrive. If the commute matters more than cost, do not buy a Gear Vendors ($3,195 plus a 2-piece shaft, plus crossmember and tunnel work, with an older Buick-based owner total of $5,880 per source 22, roughly $6,300 with today's GV price). Spend that on a 4L80E or T56 instead (see sections for those options). Gear Vendors is only sensible if the owner loves the TH400 for originality and wants overdrive and a built-in 6-ratio feel.

My pick: TH400 for the Path B build (bolt-ons, 400-450 hp) with 3.42 gears and 30" tires, in a rebuilt, shift-kitted, custom-converter configuration. For Path A (500+ hp), the TH400 is still adequate but should be built with the drum upgrade, and the owner should weigh the 4L80E/L85E as the better highway answer. The decision depends on the engine path and the rear gear choice (3.42 gears and 30" tires are the only combination where the plain TH400 is acceptable on the interstate).

Why it matters

The TH400 is the transmission the truck already has, so it is the lowest-risk, lowest-cost path. Its gears are 2.48 / 1.48 / 1.00 (reverse 2.07). It is a 3-speed with no overdrive and no lockup, and a stock one is rated roughly 450 lb-ft input (source 14). Aftermarket "street" builds are advertised at 550-600 hp (sources 11, 19). Your 500-540 lb-ft target for Path A sits at the edge of a stock-hard-parts TH400, so Path A needs the hard-parts upgrades below. Path B (about 400-450 hp, roughly 450-500 lb-ft) is comfortable with a good rebuild plus a few known-weak-point upgrades.

The real problem is highway behavior, not strength. With no overdrive, direct 3rd gear means engine rpm is just axle ratio and tire diameter. With 3.73 gears and 28-30" tires you turn about 2,700-3,100 rpm at 65-70 mph. With 3.08-3.42 gears it is about 2,250-2,900 (table below). For a 454 that is not "spun to the moon", since 3,000 rpm is mid-range for a motor you are building to 5,500-6,000. It is loud and thirsty rather than harmful, but it does fall short of "comfortable" on a 35-minute interstate commute. That gap is what the overdrive options (Gear Vendors here; 4L80E and T56 in other sections) solve.

Converter choice interacts with the cam. A lopey cam bleeds vacuum and idle quality. A stock converter in a truck is large (about 13") and tight, so it will feel dull and lug against a big cam. A loose 2,400-2,800 converter lets the cam work, but with no lockup it also adds slip and heat at cruise (engine rpm higher than the gear math says for the road speed). In a non-lockup TH400, converter stall is the main thing that makes the truck feel "spun up" on the highway, so do not oversize it.

Caution on advertised stall numbers (added in verification): catalog stall ratings are quoted for a typical engine, not yours. B&M's Holeshot 2400 page says "2,300-2,500 RPM stall" for an engine making about 230 lb-ft at 2,500 rpm (page, checked 2026-10-12), and B&M's Tork Master 2400 page says "Not recommended for big block, supercharged, plate-type nitrous oxide or trans-brake applications." A 454 making 450-540 lb-ft will flash a "2,400" off-the-shelf converter noticeably higher than its label. That is the main reason this section recommends a custom-built converter spec'd from the cam card over a catalog "2,400".

Options

A. Rebuild kits (parts only, DIY)

Tier Brand & model Part # Price (vendor, checked 2026-10-12) Link Notes
Good Raybestos Stage 1 "Super Rebuild" (TH400, 1965-90) RMCSKSG1K-012 $363.00 (transmissioncenter.net via search result; listing OUT OF STOCK; page 404 on direct fetch 2026-10-12, so treat price as stale) transmissioncenter.net (page 404 on direct fetch) Part number is real (appears in search results) but the vendor page is dead/out of stock. Good for Path B. Confirm kit includes bands, seals, frictions, and steels.
Good (verified, in stock) Alto TH400 Master Rebuild Kit with Red Eagle Power Packs + Kolene steels, high-performance intermediate band, new modulator, filter, thrust washers, bushings 031901HPPWRSK $545.00 (Suncoast Diesel, page fetched 2026-10-12) suncoastdiesel.com Added in verification. This is the one kit in this table whose page and price were confirmed live. Its contents (new modulator, HP band) make it the best DIY pick for Path B.
Good Generic Raybestos Stage 1 Red master kit not confirmed $245 (eBay seller, snippet); Level 2 version $289-$315 (snippet) ebay.com (snippet only) Prices only from search snippets. Check what is in the box.
Better Alto Red Eagle / Kolene deluxe kit, Level 2 not confirmed $475 (eBay, snippet); Raybestos Stage 1 Red deluxe $425 (snippet) ebay.com "Ultra Performance Powerpack" with Alto Red and Kolene listed at $555 (snippet). Good value for Path A when paired with the hard parts below.
Best Transtar "Super Kit" Stage 2, TransTec overhaul + BorgWarner HP frictions + Kolene steels TSP121009A-ST2 (that is a 4L80E-class kit, not a TH400 kit; I could not find a TH400-specific Transtar Stage 2 number) $3,273.81 snippet (transend.us) -- not applicable to TH400; Stage 1 for the same family $796.28 transend.us Treat as price not confirmed for a TH400. Use Raybestos/Alto above instead, or ask a builder to assemble a custom kit.

B. Hard-parts upgrades (needed for Path A, nice for Path B)

Tier Item Part # Price (vendor, 2026-10-12) Link Notes
Good 34-element sprag upgrade in stock direct drum not confirmed price not confirmed bteracing.com BTE: good to about 750 lb-ft; the original sprag/roller "isn't reliable above 500 ft-lbs". Fits your torque target.
Better Coan direct drum assembly, Super Sprag, 4-clutch intermediate COA-22817B $1,019.26 (Suncoast Diesel, page) suncoastdiesel.com USA-made steel; drop-in, no case work. Right call for Path A.
Best BTE aluminum direct drum with Pro Mod sprag 443920 price not confirmed bteracing.com 7075 hybrid, 36-element sprag, 5-clutch. Overkill for pump-gas street; needed only with "clean neutral" valve bodies.
Also Replace plastic thrust washers with bronze/steel; builder may add a 4th intermediate clutch -- washers usually in the better kits -- Common practice; builder's call. (The earlier Speedmaster link was removed: that page is a $1,725.50 race-style 9-plate forward+direct drum assembly with 36-element sprag, sold without frictions, not a thrust-washer or 4th-clutch item.)

C. Shift kits / valve body

Tier Brand & model Part # Price Link Notes
Good TransGo TH400 Stage 1 & 2 shift calibration kit SC-TG400-1 (Suncoast SKU); TransGo part number "400-1&2" (confirmed in search results) $152.00 (Suncoast, page fetched 2026-10-12); eBay listings seen at about $83-$138, so shop around tamelessperformance.com Street or full race; holds 1st/2nd to any rpm. Pick the street end of the range.
Better Same, from another retailer -- about $136-$140 (snippet) search result only Cheaper, price not verified on a page.
Best Valve-body-matched kit from a builder, or TransGo Hi-Lo variant price not confirmed -- -- Not worth it for a street truck. Skip full-race or "clean neutral" valve bodies.

Note on firm shifts: a TransGo kit raises line pressure and firms shifts, which is good for clutch life with a big-torque engine, but it will feel harsh in a nice truck. Pick the mildest street setting.

D. Kickdown (the TH400 uses an electric switch, not a cable)

Tier Item Part # Price Link Notes
Good B&M TH400 kickdown switch kit 20297 $84.95 (Ecklers) to $97.69 (Walmart) (snippet) 888murrays.com (price not on page) Mounts to a TH350/200/700R4 throttle cable bracket; also lets you choose to prevent part-throttle kickdown.
Better Lokar Hi-Tech electric TH400 kickdown kit with 48" stainless-housed cable (switch box mounts with the trans pan bolts) KD-2400HT $135.83 (Summit, via search result; Summit page fetch returned empty 2026-10-12) summitracing.com (search-result URL; page not readable). Alternate listing: cjponyparts.com (search result, not fetched) Cable-style, cleaner install. Product description confirmed via search.
Best Same as above plus wiring through the stock 1977 truck detent switch circuit -- included -- Your factory C10 already has the carb-linkage kickdown switch and wiring. Re-using it costs nothing; with a new Edelbrock or Holley carb the linkage bracket may not match.

If you go to EFI or a different carb, the factory carb-mounted switch bracket may not fit. Plan on the B&M or Lokar kit. The kickdown connects to a trans-case terminal (12V to the solenoid at wide-open throttle).

E. Cooler and lines

Tier Item Part # Price Link Notes
Good Derale Series 8000 13-row plate and fin cooler kit (13,500 BTU, 18,000 GVW rating, about 11" x 7.25" x 7/8", 5/16" line kit) 13502 $104.39 (Mancini Racing, search result); other US listings $106.99-$111.56 (Walmart) carid.com (search result; page 403 to fetch) Corrected in verification: the earlier draft said "11,000 GVW" and linked the 24-row 13504 page on an Australian site (AUD prices). 13502 is the 13-row, 18,000 GVW unit. Cheap and adequate. A stand-alone cooler matters because a loose converter makes heat.
Better Stainless TH400 lines, pre-bent for 1976-87 C10, 2WD (Classic Tube) CTT1021S $94.99 sale, $117.99 MSRP (CJ Pony, search result; page 403 to fetch) cjponyparts.com Pre-bent lines for the factory radiator cooler. The CJ Pony description says "3/4 ton 2WD Chevrolet pickups"; confirm it also fits a 1/2-ton C10 before ordering (call CJ Pony).
Also Original-steel version of the same lines CTT1021O price not confirmed cjponyparts.com Restoration-style.
Best Use both: the radiator cooler plus an auxiliary plate-fin cooler in series -- combined about $105 plus lines -- With a Cold Case radiator, make sure it has a trans cooler fitting or you must use an external cooler only.

F. Torque converters (non-lockup, for TH400; bolt pattern 10.75" on the small-diameter units)

Tier Brand & model Part # Price Link Notes
Good Stock-style rebuilt converter (a factory 454/TH400 truck converter is about 13", roughly 1,800-2,000 stall per owner reports) n/a price not confirmed gmsquarebody.com The "near-stock" pick. Quiet, cool, will feel soft with the lopey cam.
Good TCI Fast Lap, 10", 1,800-2,000 stall 242125 $449.95 (snippet, Jegs); speedwaymotors page did not load jegs.com Circle-track design. Not ideal for a street truck.
Better TCI 10", 2,600-2,800 flash stall, anti-ballooning plates TCI-242120 $739.95 (Butler Performance, page) butlerperformance.com TCI's page says "recommended for 350+ ci, 2600+ lbs" (the 2,600 is minimum vehicle weight, so a roughly 4,000-lb truck is within range; the earlier draft misread this as "lighter cars"). Small (10.75") bolt pattern. Still, a 2,600-2,800 flash with 450+ lb-ft will run looser than labelled and has no lockup, so it is on the loose side for 70 mph cruise.
Better Hughes Street Master GM25, 2,500 stall, 6 pad, non-lockup HUGGM25 (HD version HUGGM25HD) $480.99 (Jegs listing on eBay, snippet); $643.99 (Summit, snippet); $706.43 (Walmart, snippet) jegs.com Smooth idle in gear. Street Master series is 2,200-2,500 (source 10). Walmart lists the HD as a 12" unit for 1969-1979 TH350/400 (out of stock when checked). The HD version is for stronger drivelines. The price spread is wide ($481-$706); the Summit $643.99 price is the best reference. Prices are via search results; Jegs/Summit pages 403.
Better B&M Holeshot 2400 130-20412 / 20412 $525.95 on sale from $584.95 (Jegs, snippet); $691.53 (JB's, AUD, snippet) jegs.com (403) B&M says its Tork Master 2400 (20404) is NOT for big blocks (source 8, confirmed on page). The Holeshot page (888murrays, fetched) says 2,300-2,500 stall, 10.75" bolt circle, "requires small 10.75-inch bolt circle flexplate", sized for ~230 lb-ft engines, and has no big-block note either way. Other US prices seen: $505.95 (Walmart), $522.44 (Tint World).
Best Custom-built unit from Coan, ATI, Hughes, or D&P to your cam card custom Coan "Pro-Street" about $625 (snippet, not re-verified); forum posts on that thread quote $130 (D&P, Sacramento) to $310 (Hughes) but those are old (undated) and should not be budgeted. Budget $550-$900 for 2026. gmsquarebody.com Best fit: give the builder the full cam specs, gear, tire, vehicle weight. Ask for an actual flash of 2,200-2,500 rpm on this engine (not a catalog label; section 90 reconciles this with the 4L80E's 2,300-2,600) with a 11-12" diameter and a good stator.
Not recommended Jegs 3,500-3,800 and B&M Tork Master 3000 555-60404, 20402 -- jegs.com Race-style, too loose for 70 mph cruise.

Converter rule from the forum search: with a mild cam of 224-230 deg duration at .050 and 3.42-3.73 gears, a 2,400-2,800 converter is a typical match (source 10). That matches a mildly lopey 454. A larger duration pushes toward 3,000+, which is too loose for this truck. Choose the cam first, then the converter.

Lockup note: Coan makes lockup kits for the TH400 with a 1-1/4" input shaft, billet pump and adjustable regulator (source 12). Current prices from Suncoast listings (search result, 2026-10-12): Ultimate XST Lock-Up Kit COA-22880-AL-LU $2,823.75; Billet Instant Reaction Lock-Up / Trans Brake Kit COA-22027-AL-LU $1,110.88. Both are drag/bracket-oriented, not street cruise lockup, and still need a lockup converter. It would cut cruise slip but the cost is high, and at that point a 4L80E (factory lockup, overdrive) is the better buy. There is no good bolt-on street lockup for a TH400.

G. Built, ready-to-run TH400s

Tier Brand & model Part # Price Link Notes
Good ATI Street/Strip T400, 600 hp 401000 $2,395.00 (ATI, page) atiracing.com Chevy case. No converter.
Better ATI Street Rod Package Combo T400 + converter + cooler (up to 600 hp) 401800 $3,495.00 (ATI, page fetched 2026-10-12) atiracing.com A complete package and a good match for Path A.
Better ATI Competition T400 (700 hp) 401200 $2,495.00 (ATI, page) atiracing.com Overbuilt for the street, but only $100 more.
Better TCI StreetFighter TH400, 4" tailshaft, non-lockup (listed for Chevy C10 60-87), no converter 211000 $2,695.95 sale / $3,124.95 MSRP (CJ Pony, search result); compcams also lists $3,124.95 cjponyparts.com (search result; 403 to fetch) Rated 600 lb-ft, non-supercharged, pump gas. Valve body is manual/auto selectable. Added in verification (earlier draft said price not confirmed). Confirm the tailshaft length matches your driveshaft/crossmember (4" short tail vs stock truck tail).
Best Gearstar Performance TH400 Level 3 (rebuilt, with custom 12"/13" Yank converter 1,600-2,200 stall, TransGo reprogram, Raybestos Stage 1 frictions) Level 3 $5,995.00 + $650 core + $200 ATF (Gearstar, page fetched 2026-10-12) gearstar.com Added in verification as a missing turn-key option. Rated 800 hp / 700 lb-ft. The page is the 4x4 listing: confirm the 2WD configuration with Gearstar. Overbuilt for this truck but the most complete "built, warrantied, pump-gas friendly" option, and the converter is street-spec'd.
Reference B&M Street/Strip TH400 112002 price not confirmed bullfrogperformance.aftermarketwebsites.com Rated 600 hp / 550 lb-ft (snippet).
Reference Monster, Bowler, Performance Automatic -- not confirmed in this pass -- I could not retrieve current price pages. Ask for quotes.
Real-owner cost FTI Stage 4 TH400 + 3400 converter + driveshaft + Gear Vendors -- owner totaled $5,880 (TH400 $1,930, converter $900, driveshaft $300, Gear Vendors $2,750) turbobuick.com Page fetched and figures confirmed, but note it is a turbocharged Buick (Grand National/T-Type) and the prices are old (GV was $2,750, now $3,195). Updating GV alone gives roughly $6,300 before any C10 driveshaft/crossmember work. A 3,400 stall converter is also not what this truck wants. Use as a ballpark only.

H. Having a shop rebuild it (instead of DIY)

Item Cost Source
Basic rebuild, local shop $700-$1,400 ck5.com thread (snippet), source 24
Full rebuild with upgrades on a 454 closer to $2,000 ck5.com thread (snippet), source 24
Estimated range $800-$1,500 basic; $2,000-$3,000 comprehensive marketmuscles (snippet), source 25
Typical labor rate in Kansas City $75-$115/hr transmissionrepaircostguide.com (page)
Shops listed in KC area (not verified for TH400 performance work) Cottman (8401 Wornall Rd, 816-363-5550), Anderson's Transmission Service (13128 4th St, Grandview, 816-966-1504), BRB Transmission Center (5730 Troost, 816-363-3888), DTE Transmission Service (4506 Independence Ave, 816-241-6921), Certified Transmission (820 W 23rd St S, Independence, 816-252-7868). Keith's Transmission Service (KCK) was removed: it is not on the cited page and could not be verified. transmissionrepaircostguide.com

Honest limitation: I could not find a KC-area shop confirmed to build high-performance TH400s. Call each and ask whether they use the hard parts above and what they warranty. DIY rebuild is realistic for this owner (a TH400 is regarded as an easy rebuild; one source describes tooling as fabricable from scrap, source 26).

I. Gear Vendors overdrive add-on (TH400 + OD)

Item Detail
Product Gear Vendors underdrive/overdrive auxiliary gearbox, bolts on in place of the TH400 tail housing
Price $3,195.00 for 3-speed automatic units (TH350/TH400), per Gear Vendors' own site (page fetched 2026-10-12). Older pages say about $2,600. An owner paid $2,750 (source 22).
Ratio 0.78 overdrive, "gear splitting" yields 6 forward ratios
Modes Automatic: OD engages at 47 mph; or manual dash/floor switch
Capacity Gear Vendors claims 1,200+ hp (page) / "upwards of 4,000" in trade copy
Link gearvendors.com

Driveshaft and fit in a C10 (real-owner report): on an 85 C20 regular cab, long bed, 2WD, auto, the OD does NOT shorten the driveshaft enough to go one-piece. The builder needed a 2-piece shaft with a relocated carrier bearing, and eliminated the supplied slip yoke because of runout (source 3). His measurements: pinion to carrier 39", carrier to GV yoke 17.875". He estimated $3,500 for a DIY install including driveshaft and fluid. Another owner (TH400 in a car) had to "persuade" the tunnel with a hammer and notch the crossmember for the GV oil pan, and saw about 2,300 rpm at 71 mph with OD versus 3,000 without (source 22). Use Lucas 75-90 synthetic gear oil rather than ATF in truck-model units per source 3, because ATF made shifts harsh. Correction from verification: the earlier draft said a C10 is "shorter-wheelbase than the C20 long bed". That is only true for the short bed. A single-cab C10 comes as a 117.5" short bed or a 131.5" long bed (same 131.5" as the C20 long bed in that report). The source truck was the long wheelbase, which is the worst case for a one-piece shaft (about 57" overall from pinion to GV yoke: 39" + 17.875"). A 117.5" truck has roughly 14" less shaft, so a one-piece shaft may be possible there; that is an inference, not confirmed by any source, so treat the 2-piece shaft as "likely on a long bed, possible-to-avoid on a short bed, measure before buying". Also note the report is a C20 (3/4-ton) with its own driveline angles; a C10 differs.

Speedo: GV uses a speed-sensor module and requires knowing tire size and axle ratio when ordering. A 3.73-gear truck drops from about 2,900-3,100 rpm to about 2,300-2,450 at 65-70 mph (table). That is the same effect as swapping 3.73 to about 2.9.

Fitment & gotchas

  • Flexplate (corrected in verification): the Mark IV 454 (1970-90, 2-piece rear main) takes a 168-tooth, EXTERNALLY balanced flexplate (with the weight). CJ Pony ATF43, which the earlier draft cited, is "168-Tooth Flexplate W/O Weight, 292" (the 292 inline-six, no weight) and is WRONG for a 454. The correct CJ Pony listing is ATF39, "168-Tooth Flexplate W/ Weight 454 Chevy C10 1973-1987" (cjponyparts.com, found in search results; page 403 to fetch, price not confirmed). Aftermarket option: B&M 20233 SFI steel flexplate, 168-tooth, external balance, dual 10.75"/11.5" converter bolt circle, fits 1970-90 454 with TH400 (page fetched, price not on page). For Path A, a forged crank/rotating assembly can be ordered internal or external balance, and the flexplate and converter must match whatever is ordered.
  • Converter bolt pattern: two patterns exist. Small-diameter aftermarket converters (B&M, Hughes, TCI small-bolt units) use a 10.75" bolt circle; the stock 13" TH400 truck converter uses 11.5". Check the flexplate bolt circle before ordering a converter (a dual-pattern flexplate such as B&M 20233 fits both). The bellhousing is integral to the TH400 case.
  • Dipstick: TH400 stick is 24 1/8" versus TH350 27 1/8" (ck5 thread, source 17). A flexible bellhousing-mounted dipstick exists (17 1/8", Speedway 231400) if the tube is missing.
  • Crossmember: moving from TH350 to TH400 needs the crossmember set back; C10s have the holes (source 18). If you already have a TH400 this is not an issue, but check it with a Gear Vendors or 4L80E.
  • Cooler lines: CJ Pony pre-bent lines for 1976-87 C10 TH400 2WD (CTT1021S/O).
  • Kickdown: electric, via 12V switch; wide-open-throttle only. Without it, the truck will not downshift at WOT.
  • Vacuum modulator: TH400 uses a vacuum modulator; lower manifold vacuum means HIGHER line pressure, so a lopey cam's low, unsteady idle vacuum gives harsh engagement, late/firm upshifts and inconsistent shift points (the earlier draft said "soft"; soft shifts and burned clutches come from the opposite case, a leaking modulator or too little line pressure). Use an adjustable modulator ($19.50 snippet, Automaxx eBay store) or hold with a shift kit, and replace the modulator at rebuild (the Alto kit above includes one). Check vacuum at idle and set the modulator accordingly (the same vacuum also runs the power-brake booster; see Interactions).
  • Output shaft/tail housing: truck TH400s may be long-tail with 1310 or 1350 yokes. Confirm the yoke and spline when buying a built unit.
  • Tunnel: Gear Vendors needs hammering/notching; a TH400 alone does not.
  • Rear gear / U-joints: a 12-bolt takes 1330 or 1350 joints; confirm.

How it touches other systems

  • Cam vs. converter: pick the cam first. A big cam (long duration, tight LSA) needs more stall; a milder cam tolerates the 2,200-2,500 range. If the stall is too high with no lockup, highway rpm and heat go up.
  • Cam vs. power brakes: a lopey cam makes low vacuum; the vacuum modulator, power brake booster and A/C vacuum all pull from the same supply. Consider a vacuum pump or hydroboost (see brakes section).
  • Rear gear and tire: the table above. 3.73 with a TH400 is about 3,000 rpm at 70.
  • Fuel system/ignition: unaffected, but a carb vs. EFI changes the kickdown switch bracket.
  • Cooling: a Cold Case radiator may or may not include a trans cooler; plan an external cooler anyway.
  • Exhaust: full-length headers and the TH400 crossmember do not conflict on a 2WD C10 in general, but verify clearance around the TH400 pan and the steering box.
  • Driveshaft: any change in tail housing (long tail, Gear Vendors, 4L80E) changes shaft length; with Gear Vendors on a long bed a 2-piece shaft and carrier-bearing relocation is likely (short bed: measure; see section I).
  • Tire diameter: the highway table stops at 30". The brief allows big rear meats; a 33" rear tire with 3.42 gears gives about 2,440 rpm at 70 (70 x 3.42 x 336.1 / 33, before slip), which makes 3.42 plus a taller tire the cheapest way to calm a TH400, at the cost of launch and speedo error.

Check on the truck

  1. Is the current TH400 really a TH400? The tag and pan shape should confirm; look for a "TH400" marking and the 24 3/8" length.
  2. What is the existing converter diameter (13" truck unit?) and what flexplate is on the engine?
  3. Does the truck have the original electric kickdown switch and wiring?
  4. Gear ratio: pull the cover and read the axle tag (3.08 / 3.42 / 3.73?).
  5. Wheelbase: 117.5" or 131.5", and tire height planned (28 / 29 / 30")?
  6. Which is more important: lowest cost (TH400) or quiet highway cruise (overdrive)?
  7. Which engine path (A or B)? That decides sprag/drum and converter spec.
  8. Is the owner willing to DIY the rebuild (a TH400 is reasonable), or does a shop need to do it?

Highway rpm table (direct 3rd, TH400, no overdrive; rpm = mph x axle x 336.1 / tire height in inches; assumes zero converter slip)

Axle Tire 65 mph 70 mph With Gear Vendors 0.78 OD: 65 / 70 mph
3.08 28" 2,403 2,588 1,874 / 2,019
3.08 30" 2,243 2,415 1,749 / 1,884
3.42 28" 2,668 2,874 2,081 / 2,241
3.42 30" 2,491 2,682 1,943 / 2,092
3.55 28" 2,770 2,983 2,160 / 2,327
3.55 30" 2,585 2,784 2,016 / 2,172
3.73 28" 2,910 3,134 2,270 / 2,445
3.73 30" 2,716 2,925 2,119 / 2,282

(29" tire values fall between the 28" and 30" rows. Computed by me; see formula above. Tire-diameter basis (added in the integration pass, 2026-10-12): the rpm figures here use spec-sheet (unloaded) tire diameters. Published revolutions per mile imply a loaded rolling diameter about 3% smaller (Nitto NT555 G2 275/50R17: 27.87 in spec, 746 rev/mi = 27.03 in; BFGoodrich Radial T/A 275/60R15: 28.0 in spec, 743 rev/mi = 27.14 in; section 91), so real cruise rpm is about 3% higher than tabulated. Example: the recommended 4L80E + Ford 9" 3.70 + 275/50R17 is 2,175 / 2,343 rpm at 65 / 70 mph at spec diameter and about 2,240 / 2,415 measured. The recommended rear tire is 27.9 in spec (27.0 in rolling), not the 29-30 in tire some tables here assume; choose the gear after the tire. Forum cross-check: "3.50 gears runs 3100 rpm at 70" on a 28" tire for TH400 (source 15), consistent with the 2,983 computed above plus slip.) Real converter slip on a non-lockup unit adds roughly 100-200 rpm at cruise, more with a loose converter.

Judgment: a TH400 meets "not spun to the moon" only with 3.08 or 3.42 gears and 30" tires (about 2,250-2,700 rpm). That is tolerable but loud. Run 3.73 and it is about 3,000 rpm at 70, which is what the owner said to avoid. With a 454 that makes torque low, 3.08-3.42 is workable for a 4,000+ lb truck, but then 1st-gear launch is lazier (see Interactions).

Fact-check notes

Adversarial fact-check run 2026-10-12.

URLs: 28 fetched or confirmed in search results. Page-verified live: ATI TH400 page, Gear Vendors ?p=1099, Suncoast Coan COA-22817B, Tameless/Suncoast TransGo, 888murrays B&M 20297, 20404, 20233 and 20412, Butler TCI-242120, turbobuick GV thread, gmsquarebody posts 543519 and 538326, BTE article, dieselhub, streetmusclemag, dragzine, transmissionrepaircostguide, Suncoast Alto kit, Gearstar Level 3. Dead or unreadable: transmissioncenter.net Raybestos page (404), Summit KD-2400HT (empty), Jegs, CJ Pony, ck5 and Speedway pages (403/empty), transmissioncenter ?p=30202 (no Derale content). Wrong target: the Derale link pointed to a 24-row 13504 page in AUD.

Prices spot-checked live (page): ATI Street/Strip $2,395 (401000), Competition $2,495 (401200), Street Rod combo $3,495 (401800); Gear Vendors $3,195; Coan COA-22817B $1,019.26; TransGo SC-TG400-1 $152; TCI-242120 $739.95; Alto 031901HPPWRSK $545; Gearstar Level 3 $5,995 + $650 core. All matched the draft except as noted. Via search result: Derale 13502 $104.39, Hughes GM25 $481-$706, B&M Holeshot $505-$692, CTT1021S $94.99, TCI 211000 $2,695.95, Lokar KD-2400HT $135.83.

Changed:

  • Flexplate: ATF43 is for the 292 six with no weight; wrong for a 454. Replaced with ATF39 (454, with weight) and B&M 20233. Flagged dual 10.75"/11.5" converter bolt circle.
  • Derale 13502 specs and link corrected (13-row, 18,000 GVW, not 11,000).
  • TCI 242120 note corrected ("2600+ lbs" is minimum vehicle weight, not a lighter-car limit). Added catalog-stall-vs-big-block caveat (ratings quoted for about 230 lb-ft).
  • Vacuum modulator explanation fixed (low vacuum raises line pressure; not "soft shifts").
  • Speedmaster drum link removed (wrong product). Keith's Transmission removed (not on cited page). Hughes citation fixed (source 10, not 21). Garbled "rpm-less-than-expected-speed" sentence rewritten.
  • Gear Vendors wheelbase statement corrected; one-piece shaft may be possible on a 117.5" (short-bed) truck (inference, flagged, no source). The owner's bed length is not yet known: on a 131.5" long bed the cited real-owner report (85 C20 long bed) needed a two-piece shaft with a carrier bearing for a Gear Vendors unit, and a 4L80E (1.5-2 in. longer than the TH400) on a long bed needs the shaft builder's measurement; do not assume a one-piece shaft until bed length and ride-height measurements are in (section 90, 2.14). Turbobuick cost labelled as an older Buick example.
  • Filled "price not confirmed" gaps: TCI StreetFighter 211000, CTT1021S, ATI Street Rod combo part number (401800), Alto kit part and price, Coan lockup kit prices.
  • Added missing options: Gearstar Level 3 turn-key, TCI 211000, Alto 031901HPPWRSK, taller-tire option.
  • Reconciled the DIY totals in the recommendation with the tier summary.

Math: all 16 highway rpm table values recomputed and correct (rpm = mph x axle x 336.1 / tire height).

Recommendation check: still sound. TH400 Path B with 3.42 gears and a custom converter is the correct cheap and durable pick; the section honestly says it does not meet "comfortable 65-70" without overdrive. Unverified: 34-element sprag price, KC-area performance TH400 shops, ck5/Jegs/Summit pages blocked, and whether the CTT1021S lines fit a 1/2-ton C10 (listing text says 3/4 ton).

Sources 43

  1. atiracing.com (ATI TH400 part numbers and prices, page)
  2. gearvendors.com (Gear Vendors GM 2-3 speed, $3,195, page)
  3. gmsquarebody.com (C20 Gear Vendors lessons learned)
  4. gmsquarebody.com (Gear Vendors in K30 4L80E discussion)
  5. suncoastdiesel.com (Coan COA-22817B, $1,019.26)
  6. bteracing.com (drum and sprag upgrades)
  7. tamelessperformance.com (TransGo, $152)
  8. 888murrays.com (B&M Tork Master 2400, not for big blocks)
  9. butlerperformance.com (TCI-242120, $739.95)
  10. streetmusclemag.com (Hughes Street Master, 2,200-2,500)
  11. compcams.com (TCI StreetFighter, via search snippet)
  12. dragzine.com (Coan lockup kit)
  13. gmsquarebody.com (factory converter 13", about 1,800-2,000; owner chose B&M 2,700-3,000)
  14. dieselhub.com (TH400 specs: 24 3/8", about 150 lb, 450 lb-ft)
  15. ck5.com (3100 rpm at 70 with 3.50, via search snippet; page 403)
  16. gmsquarebody.com (cooler lines, via snippet)
  17. ck5.com (dipstick, via snippet)
  18. cjponyparts.com (via snippet)
  19. bullfrogperformance.aftermarketwebsites.com (via snippet)
  20. jegs.com (via snippet)
  21. walmart.com (Hughes HUGGM25HD, via snippet)
  22. turbobuick.com (owner cost $5,880, 2,300 rpm at 71)
  23. transmissionrepaircostguide.com (KC shops, labor rates)
  24. ck5.com (rebuild cost, via snippet)
  25. update.marketmuscles.com (rebuild cost ranges, via snippet)
  26. gmsquarebody.com (rebuild vs replace, TH400 easy)
  27. transend.us (Transtar kits, via snippet)
  28. transmissioncenter.net (Raybestos RMCSKSG1K-012, via snippet; page 404)
  29. ebay.com (via snippet)
  30. 888murrays.com (B&M 20297)
  31. summitracing.com (Lokar KD-2400HT, via snippet)
  32. carid.com (Derale 13502, via search result; replaces the wrong 13504 Australian page)
  33. jegs.com (B&M Holeshot, via snippet)
  34. jegs.com (TCI Fast Lap, via snippet)
  35. (removed: speedmaster79 TH400-Drum-Kit is a $1,725.50 race drum assembly, not what was claimed)
  36. cjponyparts.com (454 flexplate with weight, via search result; ATF43 is the 292 six, no weight, wrong for this truck)
  37. suncoastdiesel.com (Alto 031901HPPWRSK, $545, page fetched)
  38. gearstar.com (Gearstar Level 3, page fetched)
  39. cjponyparts.com (TCI 211000, via search result)
  40. 888murrays.com (B&M 20233 flexplate, page fetched)
  41. 888murrays.com (B&M Holeshot page, fetched)
  42. speedwaymotors.com (via snippet)
  43. jegs.com (via snippet)