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Hagerty Employee

Piston Slap: 426 Hemi vs. ballooning torque convertor?

Dave writes: I have a Dodge Dart with a 426 Gen-II Hemi and a 727 auto transmission. During the summer of 2018, I started having some issues and also started to diagnose them. Long story short, I found that the rear wall of the thrust bearing was compromised.
Advanced Driver

It has been my experience that on Chrysler 727 transmissions, torque convertor ballooning is never a problem.
To check, take the convertor loose from the flex plate, 4 bolts. The convertor should slide back into the transmission about a quarter of an inch. If it doesn't, there is a problem with the transmission, or perhaps the flex plate is wrong for the application. It is possible, with difficulty, for the flex plate to be installed backwards. That assumes Chrysler components. Aftermarket convertors and flex plates could present their own problems.

I have seen Chrysler engines with this problem, and it has always been due to a problem with the initial machining of either the block or the crank. With the engine assembled, but the torque convertor disconnected, check for end play. I don't remember the specs, but there should be some.

It has been quite a while since I worked on old Chryslers, but then again, these are old parts we are talking about.

BB Chrysler spec book end play numbers are pretty sloppy compared to other manufactureres. .002-.007 max .010. I've seen some as sloppy as .017 with miles piled without serious problems. Maybe a dripp from the rear main but then old Chrysler V8s of all displacements are notorious for bottom end oil leaks anyhow.

All great points, and the probable cause is likely in one or more of the options offered, but I would add one more thing to consider: faulty bearings.  A lot of aftermarket stuff is made to look like O.E.M. nowadays, but falls well short of original spec as far as materials are concerned.  If your thrust bearings were made of inferior material, they could have failed way early simply due to not being able to hold up to "spirited" use - or even little old lady driving.  If all of the clearances check out, I think inferior bearings might actually be the simplest solution.

Community Manager

Yes, that's a very good point. 


HEMI Dart…. Gotta Love It ! All comments on point…Good Luck getting her done…ENJOY

Be sure to grease the converter pilot hole in the crank flange center before tranny install. When the Bell Housing is aligned in place cross bolt the housing to the block with 2 bolts and tighten. Turn the converter over several times to make sure the pilot hub is free and aligned properly in the crank flange. To index the converter to the drive plate there is a 1/8 hole in the plate that aligns horozontally with a ">" mark on the converter.