TH350 Filter and Pan Service Guide: Interval, Torque, Gaskets and Refill

A TH350 pan drop is about as simple as transmission service gets: 13 bolts, two filter screws, one gasket. That simplicity is exactly why it gets done badly. Pans get cranked down until the rails dish, sealer gets smeared on a gasket that was never meant to have it, the old filter gasket stays stuck to the valve body under the new one, and a deep pan goes on without the filter extension so the pickup ends up sucking air in a hard turn. None of those mistakes show up in the driveway. They show up a month later as a drip, a whine, or a delayed engagement.

This guide covers the whole job on a Turbo Hydra-Matic 350 (including the 1980-up TH350C): how often GM wanted it done, which filter and gasket to buy, how to read what is in the pan, the torque numbers, deep pan installs, and how much fluid goes back in. It is built from GM's own service literature and the pan, filter and gasket makers' installation sheets. The full source list is at the bottom.

Recommended TH350 parts

For a pan service done once and done right: a finned aluminum TH350 pan with a drain plug so the next service is a bolt, not a mess; GM-licensed DEXRON VI to refill; and a TH350 cooler if the pan showed burnt fluid. Ships from the USA. Secure checkout.

TH350 Finned Aluminum Transmission Pan Kit with Drain Plug

$131.99View part

ACDelco GM Original Equipment 10-9395 Dexron VI Automatic Transmission Fluid, 1 Gallon

$59.99View part

TH350 Transmission Cooler

$65.31View part

The Quick Answer

How Often GM Wanted It Done

The 1976 Chevrolet Light Truck Service Overhaul Supplement (ST-330-76) is a good snapshot of what GM told its dealers. In the lubrication section, the book interval is fluid and filter every 60,000 miles under normal driving, and every 15,000 miles if the truck regularly sees any severe condition: heavy city traffic, outside temperatures that regularly reach 90 F, very hilly or mountainous country, frequent trailer pulling, or commercial use like taxi, police or delivery work.

Flip to the TH350 pages in Section 7A of the same book and you find older wording: drain the pan and clean the strainer every 30,000 miles, or every 15,000 in heavy service. The two numbers disagree because the 7A text was carried over from earlier literature, but the severe-service number is the same in both places, and that is the one that matters. Nearly every TH350 still on the road today is a tow rig, a hot rod, or a truck that sits for weeks and then works hard. Treat yours as severe service and drop the pan every 15,000 miles or every couple of years, whichever comes first.

One more instruction from that same page that most people skip: GM says to drain the fluid immediately after operation, before it has had a chance to cool. Hot fluid carries its suspended debris out with it; cold fluid lets it settle in the corners of the pan and on the filter. The tradeoff is burns. B&M's deep pan instructions point out that automatics run between roughly 150 F and 250 F and suggest letting the vehicle cool for a few hours. A reasonable middle ground is a short drive, then a 20 to 30 minute wait with gloves and eye protection ready.

Parts to Have on the Bench

• Filter. The GM part is 8640848, used across 1969-1986 TH350s. Raybestos Powertrain (Allomatic) lists its 515615 as the direct replacement and notes an economy 016615 version also exists. Their 515-series numbers are filter only, no gasket, so buy the gasket separately or buy a kit (Raybestos kit number 7615).
• Pan gasket. Raybestos lists cork gaskets (03G615, 63G615) and a nitrile rubber gasket (22G615) for the same 13-bolt pan. Cork is cheap and seals well on a straight rail, but it crushes easily if overtightened. A rubber or steel-core gasket tolerates a slightly imperfect rail better and is less likely to squeeze out.
• Filter gasket. Usually included in a kit. It sits between the filter and the valve body.
• Fluid. GM specified DEXRON II. Valvoline's DEXRON VI product page states the fluid can be used wherever DEXRON II or III is recommended, so any GM-licensed DEXRON VI is a correct fill. Have at least 4 quarts for a stock pan and 7 for a deep pan conversion.
• Tools. B&M's list covers it: a 1/2-inch socket and ratchet, a 1/2-inch wrench, a flat blade screwdriver, a scraper, and an inch-pound or 0-50 ft-lb torque wrench. Add a drain pan wide enough for a pan that will dump from one corner.

Step by Step: Stock Pan

1. Get it in the air, safely. B&M suggests one to two feet of clearance on stands, ramps or a lift. Some chassis need the crossmember dropped to clear the pan; if so, support the tail of the transmission so the engine does not tip back into the firewall and crack the distributor.
2. Drain without a bath. Remove the pan bolts one at a time working toward the front, then loosen the last two slowly so the pan tilts down and empties over one corner. If the pan is stuck to the old gasket, break it loose with a gentle pry before those last two bolts come out. Do not lever on the rail in one spot; that is how rails get bent.
3. Pull the filter. Two screws hold it to the valve body. Remove them, drop the filter, and find the filter gasket. If it stayed on the valve body, peel it off. Stacking a new gasket on an old one is a classic cause of a vacuum leak at the pickup, which shows up as noise and delayed engagement.
4. Read the pan before you clean it. See the next section.
5. Clean both sealing surfaces. Scrape every trace of old gasket from the case rail; B&M flags leftover gasket material as a leak cause. Wash the pan, wipe it dry, and lay a straightedge across the rail. Check each bolt hole for a dimple pushed in by years of overtightening. Tap those flat on a block of wood before reinstalling. A pan with a dished rail will leak no matter what gasket you use.
6. Install the new filter and gasket. Line up the holes, run the two screws in snug. They thread into aluminum; tight is good, stripped is a teardown.
7. Hang the pan. New gasket on a clean, dry rail. Moroso's instructions say plainly not to use sealer on the gasket, because small pieces can break off and clog the transmission internals. Start all 13 bolts by hand, then snug them in a crisscross pattern.
8. Torque in steps. B&M specifies 12-13 ft-lb on the TH350 pan bolts and warns that overtightening causes leaks. Moroso specifies 10 ft-lb for its pan with its own gasket. Pick the number from your gasket or pan maker's sheet, bring every bolt up to it in two passes, and stop. Do not go back later and give them another quarter turn to chase a weep.
9. Refill and set the level. GM lists about 2-1/2 U.S. quarts to bring a TH350 back to the mark after a sump drain. Add that, start the engine, then set the level by the dipstick procedure below.

What the Pan Is Telling You

The pan is the cheapest diagnostic tool you will ever use, and GEARS magazine's advice on high-mileage service starts with it: before deciding how aggressive a fluid service should be, drive the vehicle so you know how it behaves, then look at what is in the pan. Here is how builders read it:

• A thin gray film or paste in the bottom. Normal wear on an older unit. Clean it out and move on.
• Dark, gritty material that looks like coffee grounds. Friction material from clutches or bands. A little on a high-mileage unit is expected; a layer of it, especially with a slip complaint, means the unit is wearing out internally and a fluid service will not fix it.
• Brass or bronze-colored flakes. Bushings and thrust washers. Usually means wear in the pump, the stator support, or the gear train.
• Steel chips or slivers you can feel. Something hard is breaking. Stop and plan a teardown.
• Strong burnt smell with dark fluid. The fluid has been overheated. Fix the cooling problem before you put fresh fluid through it. Our TH350 overheating prevention guide covers cooler sizing and plumbing.

If the pan is clean and the unit drove well, this is just maintenance. If the pan is ugly and the unit already slips, fresh fluid sometimes makes it worse in the short run because the old, debris-loaded fluid was helping the worn clutches hold. On an unknown high-mileage unit, follow GEARS' approach and do partial changes over several services rather than a single complete exchange. Our TH350 common problems guide walks through what usually fails.

Deep Pan Conversions

A deep pan is the most popular upgrade done during a TH350 pan service, and it is a good one: B&M points out that the extra capacity keeps a larger volume of fluid at the pump pickup to prevent starvation, adds cooling, and their pans add a drain plug so future services do not mean dumping fluid across your arms. Their cast aluminum version also adds cooling fins and a 1/8-inch NPT port for a temperature sender.

The part people get wrong is the pickup. A deeper pan moves the floor down, so the filter has to move down with it. Both B&M and Moroso include a filter extension (spacer) that bolts between the valve body and the filter with a gasket on each side. Skip it and the filter sits high in the sump, which defeats the point of the deeper pan and can uncover the pickup under braking or cornering. A few specifics from the instructions:

• B&M's kit uses its own filter (No. 10288) on the extension, held with supplied screws and nuts, and notes that one hole is blocked by the filter with no effect on function.
• B&M's cast pan needs longer bolts (supplied); its chrome steel pan reuses the stock bolts. Both get 12-13 ft-lb, and the drain plug gets 20-25 ft-lb.
• Moroso's 42010 pan uses two 1/4-20 x 1.5-inch bolts through the spacer, tightened to the factory filter spec, and 13 serrated flange bolts at 10 ft-lb with no sealer.
• B&M says to add about 6 quarts after installing its deep pan, then set the level by the dipstick. Moroso simply notes the pan holds more than a stock shallow pan and to fill to the dipstick mark. Either way, do not pour in a stock 2-1/2 quarts and call it done.

B&M's sheet recommends its own Trick Shift fluid or Type F over DEXRON. That reflects a performance-shift preference, not GM's spec. For a street TH350 with stock clutches and valve body, stay with a GM-licensed DEXRON fluid.

Setting the Level After the Job

Overfilling after a pan service is more common than running it low, and GM's ST-330-76 explains why: the level rises more than 3/4 inch as fluid temperature goes from 60 F to 180 F, so a cold dipstick reads low and people keep adding. GM warns that overfilling causes foaming and fluid loss through the vent, and that slippage and transmission failure can result. B&M's deep pan sheet repeats the warning: overfill causes foaming and overheating.

• Check with the engine running, the shifter in Park, and the vehicle level.
• After the initial fill, shift slowly through every range with your foot on the brake, return to Park, and recheck.
• Cool fluid (65 to 85 F): the level should be 1/8 to 3/8 inch below ADD. Hot fluid: between ADD and FULL.
• GM notes it takes only one pint to go from ADD to FULL on a hot transmission, so add in small amounts.
• Recheck for leaks around the rail after the first heat cycle. A weep at one corner usually means a dimpled hole you missed, not a bolt that needs more torque.

For the full fluid spec and every capacity number, see our TH350 fluid type and capacity guide, and for what a shop charges versus doing it yourself, the TH350 fluid change cost breakdown.

FAQ

How often should a TH350 filter and pan be serviced?

GM's 1976 service literature calls for fluid and filter every 60,000 miles in normal use and every 15,000 miles in severe service, which includes towing, heavy city traffic, hot climates and hilly country. Most TH350s still in use fit the severe category, so a pan drop every 15,000 miles or every couple of years is the safe plan.

What is the torque spec for TH350 pan bolts?

B&M specifies 12-13 ft-lb for the TH350 pan bolts and Moroso specifies 10 ft-lb for its pan and gasket. Bring all 13 bolts up evenly in a crisscross pattern in two passes and stop. Overtightening crushes the gasket and dimples the pan rail, which causes the leaks people are trying to prevent.

Should I use sealer on a TH350 pan gasket?

No. Moroso's instructions specifically say not to use sealer on the gasket because small pieces can break off and clog the transmission internals. Use a new gasket on a clean, dry, flat rail and torque it correctly instead.

How much fluid does a TH350 take after a filter change?

GM lists about 2-1/2 U.S. quarts to refill a TH350 after draining the stock pan. A deep pan holds more; B&M says to add about 6 quarts after installing its deep pan. In both cases, finish by setting the level on the dipstick with the engine running in Park.

Do I need a filter extension with a TH350 deep pan?

Yes. A deeper pan lowers the sump floor, so the filter has to drop with it. B&M and Moroso both supply a filter extension or spacer that bolts between the valve body and filter with a gasket on each side. Without it, the pickup sits too high and can starve under braking or cornering.