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I am looking at swapping out my cast iron powerglide to a TH350 in a 58 chevy w/348 engine. Can anyone tell me exactly what I have to change out in the swap. I know the drive shaft needs to be shortened and I believe the front yoke spline count is different. Is the existing flexplate useable with the 350 and what about the starter? I know new shift linkage is required also.Spedo gear?
Any help is greatly appreciated.
 

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The spline count for a cast iron power glide was really coarse (I'm thinking around 17 splines). The TH350 uses a 27 spline output yoke available in any bone yard for the price of scrap metal.

You will not have to shorten the drive shaft as the PowerGlide, and the TurboHydramatic 350 are the same length and width. Luckily the TH350 uses the same starter motor as the PowerGlide, the flex plate is the same size and uses the same starter ring (153 tooth) but the holes that mount the torque converter are slightly different.

You can drill them out, but I would point out that a flex plate is called that because it flexes all the time as the torque converter moves in and out on the input shaft. That constant bending of the plate can and will cause work hardening stress cracks that can cause it to fail. It is safer to replace the flex plate. Most transmission shops also replace the torque converter at the time of a rebuild to keep old contaminates out of the tranny and to take advantage of any new upgrades (such as a locking torque converter).

You can buy the transmission shift linkage kit (includes longer shift rods and a gear shift selector comb) and these kits also often include a new indicator decal or plate to reflect the added gear for the steering column.

Big Dave
 
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