If your Turbo 350 slips into second once it is hot, flares on the 2-3 shift, takes its time picking up Drive or Reverse, or has already been pulled and split on the bench, this ATP SMS-9 Master Repair Kit Plus is the box that covers the wear side of the job. It is listed for the GM TH350 and the lockup TH350C from 1969 through 1986, the three-speed found behind small blocks and inline sixes in Chevrolet, GMC, Pontiac, Oldsmobile and Buick cars and light trucks. The SMS-9 is the "Plus" version of the ATP TH350 master kit: on top of the friction plates, steel plates, gaskets, seals, sealing rings and O-rings, the listing includes a pan filter, so the unit goes back together with a fresh pickup instead of a reused screen.
What a master repair kit replaces in a TH350
A TH350 moves the car with four multi-plate clutches and one band. The forward clutch is applied in every forward gear, the intermediate clutch works with the intermediate roller clutch for second, the direct clutch comes on for third and Reverse, and the low and reverse clutch holds the reaction carrier in manual low and Reverse. The intermediate overrun band only holds for engine braking in manual second. Every one of those clutch packs is a stack of lined frictions sandwiched between steel plates, every apply piston is sealed with rubber lip seals, and the rotating drums are fed through sealing rings. Those are the parts that glaze, harden or crack with heat and mileage, and they are what the SMS-9 replaces in one order. The photo shows separate friction and steel sets for each pack, the valve body and separator plate gaskets, the pump-to-case gasket, the pan gasket, the front pump seal and rear seal, the sealing rings and the filter.
Symptoms that call for a full TH350 overhaul
- Slips on take-off in every range once the fluid is hot, with dark, burned-smelling fluid
- Slips or flares on the 1-2 shift, or starts out in second (intermediate clutch or piston seals)
- First and second only with no 2-3 shift, or no Reverse (direct clutch seals or pump hub sealing rings)
- Delayed engagement into Drive or Reverse after the car sits overnight
- Friction material, metal flakes or debris in the pan and filter
- Leaks at the front pump, rear seal or pan rail that a pan job did not cure
How to confirm you need a rebuild kit and not just a repair
Do the cheap checks first. Set the fluid level hot, check the vacuum line and the modulator, and confirm the detent cable and manual linkage are adjusted. GM's diagnosis sequence then calls for a road test with a pressure gauge on the line port so you can isolate which clutch or circuit is failing. A stuck modulator valve, a worn pressure regulator or a valve body fault can mimic a burned clutch, and our TH350 common problems and fixes guide walks through separating those from internal clutch damage. If the complaint is slow engagement, read the delayed engagement guide before you commit to pulling the transmission. Once the pan shows friction debris or the pressure test points inside the case, a full kit is the right buy.
Fitment
The SMS-9 is listed for the TH350 and TH350C, 1969-1986. The lockup TH350C arrived late in the 1979 model year and has an electrical connector on the left side of the case. Within that run GM varied intermediate and low-reverse plate counts and pistons between applications, so lay your old packs out in order as you tear down and match counts and tooth patterns to the plates in the box. If you are unsure whether your unit is a TH350, a TH350C or a TH250, send us the year, the vehicle and the VIN or the transmission ID tag and we will confirm before it ships. The full lineup of parts for this transmission is on our TH350 parts page.
Install notes
- Soak every new friction in clean ATF for at least 30 minutes before stacking the packs.
- Inspect each drum, piston and ball check; GM lists a stuck piston ball check as a cause of burned clutch plates.
- Check clutch pack clearance with the new plates and pick the correct selective pressure plate before final assembly.
- Bushings are not part of this kit; on a high-mileage core replace the pump, stator, drum, sun gear and case bushings as needed.
- Air-check each clutch circuit before the valve body goes on, then refill with Dexron-type ATF. Capacities and the dry-fill amount are in our TH350 fluid type and capacity guide.
An overhaul is the right time to add a TransGo SK350 shift kit for firmer, shorter shifts. If the intermediate roller clutch is worn or its outer race has cracked the drum hub, replace the TH350 direct clutch drum with roller clutch rather than reusing it. Changed the rear gear ratio or tire size since the last rebuild? A governor shift point kit moves the shift points back where they belong.
