TH350 Input Drum 1969-1986

If your Turbo 350 backs up fine but will not pull forward, slips on every launch once it warms up, or keeps creeping with the shifter in Neutral, the forward clutch is the first place to look. This remanufactured TH350 input drum is the forward clutch housing with the input shaft pressed in, listed for 1969-1986 transmissions in Chevrolet, GMC, Pontiac, Oldsmobile and Buick cars and light trucks. The listing photo shows the housing and shaft together with a clutch plate set, a piston and the seals and snap rings that go with it. If you need to know exactly which pieces ship before you order, ask us and we will confirm against the item.

What the forward clutch drum does in a TH350

In the TH350 the input shaft is the link between the converter turbine and the geartrain, and GM presses it into the forward clutch housing so the two turn as one whenever the engine runs. Fluid for the forward piston comes from the pump cover through two sealing rings on the pump hub. When the driver picks D, 2 or 1, that pressure pushes the piston against the plates, the pack grabs the input ring gear, and power reaches the front gearset. Nothing else in the unit can move the car forward, so this one clutch is applied in every forward gear. Reverse does not use it at all, which is why a forward clutch failure leaves Reverse untouched.

Symptoms of a failing forward clutch drum

• No movement in D, 2 or 1 while Reverse still engages normally
• Engine flares on take-off in every forward range, often only after the fluid is hot
• Car pulls in Neutral because a stuck pack will not release; GM notes this also causes no Reverse
• Black, burned forward frictions in the pan or on the filter
• Worn ring lands on the input shaft that let forward clutch pressure bleed off

How to confirm the forward clutch is the problem

Rule out the cheap causes first: correct fluid level, a manual linkage that actually reaches the D detent, and a filter that is not plugged. Then put a gauge on the line pressure port. GM's diagnosis chart for no drive in Drive lists a cracked forward piston, missing or damaged piston seals, burned plates, broken or missing forward rings on the pump cover, a leaking feed circuit or misplaced pump gasket, and a stuck or missing ball check in the drum. Good pressure with no drive means the leak or the burned pack is inside the drum. Our TH350 common problems and fixes guide lays out how to separate a forward clutch failure from a modulator or valve body fault, and the delayed engagement guide covers slow engagement into Drive.

Fitment

This drum is listed for the TH350 from 1969 through 1986. That range spans the original non-lockup TH350 (GM option codes M33, M38 and M39) and the TH350C lockup unit built from late 1979 (codes MV4, MX2, MX3 and MX5). A TH350C has an electrical plug on the left side of the case and a C cast into the front face near the pump bore. Because the forward housing and input shaft are serviced as separate pieces, lay your old shaft beside this one and compare length, the front spline and the sealing ring lands before you build. If you are not sure which version you have, send us the year and VIN or a photo of your case and we will check it. The rest of our Turbo 350 parts are on the TH350 parts page.

Install notes

• Stack the pack in GM order: cushion spring first, then alternate steel and lined plates, then the pressure plate and its retaining ring
• Check pressure plate to top friction clearance with a feeler gauge; GM allows .011 to .082 inch and offers three selective pressure plate thicknesses to correct it
• Feed the piston in past the inner and outer seals with a lip tool made from .020 inch music wire, as the factory procedure shows
• Blow through the ball check exhaust in the drum before assembly and air-check the piston on the bench
• Look over the two forward clutch rings on the pump hub; GM's service replacements are cast iron hook-type rings
• Refill with DEXRON ATF and set the final level hot

The forward housing rides directly behind the direct clutch, separated by a needle bearing, so check that drum while the unit is apart. A loaded TH350 direct clutch drum replaces a worn 2-3 pack, and the TH350 master repair kit supplies the gaskets, seals and plates for the rest of the rebuild.

Is this the forward clutch drum or the direct drum?

It is the forward clutch housing with the input shaft installed. The direct clutch drum is the separate drum in front of it that carries the intermediate overrun roller clutch.

My TH350 has Reverse but no Drive. Is this the right part?

That symptom points at the forward clutch because Reverse never applies it. Check fluid level, linkage and line pressure first, and inspect the two forward rings on the pump hub, since broken rings cause the same no-drive condition.

Will it fit my lockup TH350C?

The listing covers 1969-1986, which includes the TH350C. Compare the input shaft length, front spline and ring lands with your old shaft before installing, or send us your year and VIN and we will confirm.

What clearance should the forward clutch have?

GM specifies .011 to .082 inch between the pressure plate and the top lined plate, corrected with one of three selective pressure plates if you are outside that range.

What should I replace with the forward drum?

Replace the piston seals, gaskets and any worn plates, inspect the pump hub sealing rings, and check the direct clutch drum and the needle bearing between the two drums while the unit is apart.

What fluid goes back in after the install?

DEXRON automatic transmission fluid. Fill, start the engine, cycle through the gears and check the level on the dipstick at operating temperature.

• TH350 Common Problems and Fixes: Modulator, No 2nd, No Reverse, Worn Bores
• TH350 Delayed Engagement: Causes and Fix
• TH350 Pump Failure Symptoms and Replacement
• TH350 Fluid Type and Capacity: DEXRON Spec, Refill, Dry Fill and Converter Quarts