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Technical Reference #7

TH350 replacing Powerglide

Authored by Wes Vann, last modified on November 4, 1998

PLEASE NOTE; Always use jack stands while working under a car. I would also recommend that you have another person present when lifting a transmission into place.

I'm sorry about a lack of photos. If anybody has a photo that shows the difference between a long and short version of the TH350, please contact me (just be sure that it isn't copyrighted).

November 4,1998;

I've gotten a fair amount of questions sent to me in regards to hooking up the original floor shifter. Now, I REALLY don't like it looking like I'm endorsing a manufacturer, but here goes. (part of the reason that I don't feel bad about it is that there is only one source that I know of)

I ended up getting feed-back from a guy named Brian who used one of the "Shiftworks" conversion kits. He was happy with the kit, however, the cost for everything seems to really add up. Be sure that you add up ALL the costs before making the jump (then think about the fact that there may not be any other option).

Shiftworks has a web site and to get there, click here. You will have to use your "back" button to return here. If you end up buying one of their kits, let them know that you heard of them from here.

Normally I would include additional notes at the end of the page, however, there are several items that need noting at the outset.

1. A Pontiac TH350 will not bolt up to a Chevy engine! The engine bolt pattern for Buick, Oldsmobile, and Pontiac (BOP) are different from Chevy.

2. TH350's come in three different lengths (measured from the engine to the end of the trans). The difference is in the tailshaft housing and output shaft. The only one to use is the short version due to the fact that it is the same length at the Powerglide. The short version has a tailshaft housing that measures 6" from where it bolts to the trans body to the end (where the rear seal is). The other lengths are 9", and also 12". Per B&M, 90% of the TH350's were the short version.

3. When I have done this conversion, I was able to use the existing driveshaft yoke. Check the spline count on both trans, and compare before assuming that you need a different one. Another idea is to have a spare yoke that can be placed in the trans any time you pull the driveshaft in order to prevent the fluid from spilling all over the floor.

4. Both transmissions require a vacuum line from the engine that goes to the "vacuum modulator". This line comes from the rear of the intake manifold and not from the carburetor.

5. The Powerglide has kick-down linkage, where as the TH350 has a kick-down cable (a TH400 has an electrical switch mounted at the carb that controls the kick-down function).

6. This is dumb, but I have to say it, you can't use the old converter (at least I'd find it real hard to believe it's possible)

This page is written with a 65 Chevelle as an example. This should be typical of all GM models. The steps are going to be written in real general terms.

Drain the trans fluid by unbolting the pan. In order to keep the mess to a minimum, remove all of the pan bolts except for the four corners. Loosen the bolts at the corners and crack the gasket loose. Then remove two of the bolts on one of the edge so that the pan can tilt and drain into the drain pan. (if you can drain an automatic trans without getting at least some fluid on the floor, you are a hero in my book)

Remove the parking brake cable, driveshaft, kick-down linkage (all of it, from the carb, down to the lever on the side of the trans), speedometer cable, dipstick tube, and vacuum line.

Remove the torque converter cover plate and unbolt the torque converter from the flywheel. You should be able to slide the torque converter towards the trans body approximately 1/2 inch.

Block up the rear of the engine (if you don't do this, when the trans is removed, the engine will tilt back and rest with the distributor or oil idiot light sender fitting the firewall). Put a "stable" roll-around jack under the trans and remove the trans mount. Remove the trans to engine bolts. Remove the trans, being careful that the converter doesn't fall out (and spill fluid every where). Without a special fixture for your roll-around jack, the trans WILL want to slide off. HAVE A FRIEND HELP!

If the new torque converter is dry (new), add a quart of trans fluid to it prior to sliding it on the input shaft. Slide it in as far as it will go. You will have to rotate it in order to get it to fully seat. By adding the fluid, the inner stator will resist rotating freely and will engage easier.

With that friend helping, set the new trans on the roll-around jack and put it into position. Bolt the trans to the engine. It may help to have the engine tilted back as far as possible, without any thing hitting the firewall. You may have to disconnect the exhaust lines in order to get the engine to tilt enough.

If the torque converter is correctly in the trans, it will still spin freely. DON'T USE THE TRANS BOLTS TO FORCE THE TORQUE CONVERTER INTO THE TRANS!

Raise the trans into position so that the trans mounts and crossmember can be installed. Should the trans hit the floor, you will have to remove it and "dimple" the floor pan for clearance. (I had to on my car, but it was real minor)

If the trans clears ok, add a new TH350 rubber trans mount and slid the trans mount (cross member) into position. You may have to drill new holes through the frame rails.

Slide the torque converter forward (toward the engine) and bolt it up using grade 8 bolts (don't use standard bolts! If you don't know how to tell the grade, ask!). Put on the converter cover plate (the Powerglide one will not fit).

Replace the driveshaft, parking brake cable, and hook up the speedometer cable. Hook up the vacuum line to the modulator If the original line can't be bent to fit, go to a auto parts store and pick up a length of "bundy flex" brake line. Cut off the fittings at the end and then bend the line to fit. Be sure to flush out the line of any debris.

Hook up the fluid lines that go to the cooler.

Install the dipstick tube and rod. (the Chevy dealers sell one for trucks that is short and unobtrusive) Always make sure that the dipstick rod and tube are a matched set (matched lengths)!

Hook the kick-down cable to the trans and run the line up toward the drivers side of the engine. The brackets for mounting the end of the cable housing on the intake manifold can be purchased from Chevy (or a speed shop).

For hook-up of the cable on the carburetor linkage I recommend that you get a factory manual.

Add trans fluid. After having the engine running and putting the trans in all gears, recheck the level of the fluid (with the engine running).

Good luck and be careful (a transmission falling off the jack onto your hand isn't a pretty site!)