1. GM TH350

GM TH350 Transmissions

The TH350 three speed automatic transmission is largely recognized as one of the greatest automatic transmissions ever built. It’s strength, durability and simplicity has earned this transmission the respect it well deserves.

The TH350 transmission’s humble beginnings started as a joint project by Chevy and Buick in hopes of finding a potential successor for Chevy’s Powerglide and Buick’s Super Turbine 300 transmission. GM’s Hydromatic Division had little participation in the initial development so early tech manuals referred the transmission as a CBC350 (Chevrolet-Buick Combined). As General Motors support for the transmission grew, The Transmission took on the Turbo Hydramatic name and officially became known as the TH350. There were still many doubters as to the durability of the transmission when compared with the TH400. Its reputation however quickly grew largely based upon its strength and versatility. Even today the TH350 is a favorite choice for adapting into 4wd vehicles. Its short case and respected strength are ideal for even some of the shortest wheel base vehicles. 

  

    The TH350 transmission was first mass produced in 1969 for both 2WD and 4WD vehicles and remained prevalent through 1982. The transmission was standard for most V6 and V8 vehicles. The Th350 transmission can be identified by its’ 21-1/2" long one piece aluminum case. The transmission has an integral or non-removable bellhousing. The engine bolt pattern for the front of the transmission was either unique for the block (i.e. Chevy or Buick), but there were many dual drilled with both Chevy and B.O.P (Buick, Olds, Pontiac) patterns as this picture clearly shows. The dust cover and not the case has the starter pocket cast into it, so the dual drilled transmissions can be used for odd-ball engine conversions like Olds that may have starter pockets located on the driver side of the block. The dual drilled th350 transmissions are also often desirable for someone wanting to do a Buick V6 conversion, but with possibilities of a Chevy V8 later down the road.

 

 The TH350 weighs approximately 120 lbs. As with most transmissions, the oil pan can be used as a further way to i.d. the transmission. The transmission oil pan has a unique chamfer at the rear passenger corner, as well as a modulator also located at the right rear of the case. The 4WD version of the transmission was mostly found mated to NP203 and NP205 transfer cases. These 4wd version transmissions have a stubby 27 spline output shaft that protrudes past the 4 bolt flanged case by only ¾ of an inch. It is this short output shaft that makes the transmission an even more desirable choice for adapting. Advance Adapters has purposely designed most of our adapters utilizing this output shaft. This is especially beneficial as no tear down of the transmission is required. Caution should be used when buying a 4WD TH350 that was mated to the NP208, as this transmission used a significantly longer output shaft. 2WD transmissions also use longer output shafts and are obviously easily identified by the conical tailhousing tapering down to a yoke. Both 2WD and 4WD versions of the TH350 can be used for conversions and internally there are no benefits of one over the other. There were however some differences with clutch pack and band specs. As a general rule, transmissions found behind larger engines will have stronger components. The aftermarket has had several decades to offer aftermarket components for the TH350. If your plans are to include a rebuild, your local transmission shop can assist you in selecting upgrades. These upgrades can greatly improve in the performance of shifting, heat reduction, clutch wear and overall life expectancy of your transmission. There are also many styles of torque converters and stalls which further aid in the shifting of the transmission.

 

In 1979 GM introduced the TH350-C. This transmission had a lockup torque converter version. The TH350-C lockup version can be identified by an electrical plug on the left side of the transmission. There are several aftermarket transmission suppliers that offer this version coupled with a stand alone TCC control unit. The TH350-C transmissions are often difficult to locate in salvage yards do to the limited years of production. This version of the TH350 is also heavily sought after do to the overall benefits of the lock up feature. The lock up feature of the TH350-C increases fuel mileage at cruising speeds by at least 8 percent. It also aids in reducing generated heat.

The standard TH350 used part codes: M33, M38 & M39. The TH350C used codes: MV4, MX2, and MX3 & MX5.

There were downsized versions of the TH350 especially seen on some Buick applications. These used codes: TH200, TH200C, TH250, and TH250C. A bigger stronger version of the transmission known as the TH375 was also offered, as well as a stronger case known as the “K” case.  

The TH350 has a case length of 21.50" long and the gear ratio's are 1st 2.52:1, 2nd 1.52:1, 3rd 1:1.