Monthly Archives: August 2010

How to measure converter clearance to engine / adapter plates.

First off, install the converter into the housing. on a Th350/Th400 you will install the torque to where you feel it click in three times and it spins freely. if it does not spin freely when it is fully seated in the tranny, you got a problem.

Next, lay a straightedge across the face of the bellhousing (engine side). I usually use a large carpenter’s square. With the converter installed in the transmission, take a ruler and measure the distance from the straight edge down to the mounting pad of the converter. The measurement should be as follows:
GM TH-350: 1 1/8″ from bellhousing to mounting pads
GM TH-400: 1 3/16″ from bellhousing to mounting pads
GM Powerglide: 1 1/8″ from bellhousing to mounting pads
Ford C-6: 1 1/8″ from bellhousing to mounting pads
Chrysler Torqueflite 727: 1 1/4″ from bellhousing to ring gear
Note: Distance may vary either way .050″.

when using a stock replacement converter, tranny, and engine there is usually no need to worry about this.

With the transmission installed to the engine ,you should have between 1/8″ to 3/16″ of clearance between the torque converter and flexplate before pulling the converter forward and bolting it to the flexplate. So, when using an adapter plate/distributor protector, you can use this as a guide to select the right thickness of shims to properly set your converter to not bind the engine, housing, or most of all the pump gears into the back face of the pump. for derby i like to lean more toward 3/16″ as the converter needs as much room room to expand as possible as it heats up quickly. It may seem like if isn’t in the pump that far but trust me, as it gets hot it will be in there just right.

commonly an adapter plate comes with shims of the correct thickness and these measurements become somewhat pointless when using a factory converter on a factory tranny. However, when you mix and match aftermarket performance converters it can be an issue. the only time i had a real problem with one though is when using a powerglide core custom built with a turbo spline installed in a th400.

also when using washers from the hardware store as converter shims you gotta be careful. most of the cheap washers like that don’t have identical thickness and can really screw things up. you are usually better off cutting shims from the same material as the adapter plate. you should also be aware of how the snout of the converter centers in the end of the crankshaft. some of these ungodly thick adapter plates i have seen can pull the converter out of the end of the crank and the converter never centers- causing a lot of problems and vibration.

Th350 Center Support Weakness

390707193Can you see the crack? this tranny placed 3rd after a 2 1/2 hr feature run in a demo derby, one of the most extreme things to ever put a Transmissions through. snow plow trucks are very similar as well as drag racing.  the TH350 was designed not to shift to low 1 at high rpm and/or road speed. This was done through the use of both a manual low control valve and an internal shift control valve that is part of the 1-2 shift valve in the valve body.

The shift control valve basically has line psi to the back side of it with the modulator valve unhooked. to engage reverse, oil flows through this valve to the rear holding clutch pack and does not need to move….giving you reverse. to get manual low 1(which engages part of the rear clutch pack to help hold low 1 in addition to the center support) it must shuttle this valve to expose the passage to flow oil to the rear holding clutch pack.

so what am i getting at here? in demo derby and drag racing, the rear low 1 clutchpack in a nutshell is not allowed to move because the tranny believes it is hauling ass down the road with the modulator valve unhooked. leaving the center support and rear sprag to bear the entire load of holding member for 1st gear. however, unlike the th400 line, the th350 and it’s later counterpart the 700R4 underwent many different variations of valve body calibration and control for different vehicles. therefore this is not a golden rule.

So, as a rule of thumb for the Th350, during a rebuild i always recommend retro-fitting in a center support assembly out of a later model 700R4/4L60 transmission, even on driver transmissions. they are a larger sprag and can take more abuse. also always run your modulator valve hooked up. if you are doing any kind of performance for street. strip an aftermarket valve body kit or shift calibration kit is recommended as well. for demo derby….just go get a th400 or run a 3 speed. yes th350’s can run and last for demo derby…..i have had guys swear by 700R4 trannys too for derby. don’t do it. the results are inconsistent.