Rear Main Bearing - Tech - Service Bulletin

[James_Douglas]

In another recent thread the issue of the rear main bearings came up. Chrysler used two different main bearings. Once for use with the standard transmission and Fluid Drive and then a later one that was used with the Fluid-Torque Drive as well as the Powerflite Automatic Transmission.

 

This is of importance to anyone rebuilding and engine that has a torque converter or anyone swapping to a modern automatic.

 

Apparently, there must have been a thrust loading issue on the rear main due to torque multiplication. Chrysler did away with everything on the bearing but the single groove in both the top and bottom half's of the bearings.  Attached it the tech note from my collection in which they talk about it.

 

I happen to have a couple of Thomson Products later bearings in my parts bin.  I also want to thank the poster in the other post who put up the Federal Mogul parts books with the part numbers.

With that information I was able to go hunting and found some more bearings so I am stocked for the rebuild of the 265...not knowing yet what crankshaft bearing oversize it will need. One of the bearings I found was specifically a "ream to size" rear main.  I have not seen one of those in a long time!.

 

Anyone running a torque converter should print this out and put it in their own books for reference.

 

Best, James.

 

Edited December 3, 2019 by James_Douglas

[Bob Riding]

Great info James. I will be putting a GM TH200 4R automatic behind my 230, using a Wilcap adapter plate. Would you think I would need to upgrade the rear main to something beefier?

[Plymouthy Adams]

does this no refer to the engine oil fed torque converter.....not to be confused with the standard sealed fluid coupler....

[James_Douglas]

I think this refers to any Flathead six with a torque converter regardless of the transmission. The date of the service note and the engine number mentioned is when the Powerflite went into production. Although it is not written to be clear, the fact that in the first paragraph they refer to a "torque Converter" and in the second a "Fluid Torque Drive", tells me that they put the new bearing in all the powerflite applications and are warning to use it in the last couple of years of fluid torque drive cars and to be on the watch for engine swaps into either the fluid torque drive cars or the powerflite cars and to make sure you have the later bearing.

 

That makes sense as the powerflite cars did not share the engine oil like some of the fluid torque drive cars.

 

I think the Torque Multiplication was putting a strain on the rear main bearing. This is why they removed the extra outer grooves to increase the surface area to the max area they thought they could get away with oil groove wise.  I suspect, although I have no data to back it up, that rear main bearings may have been cracking at or near the thrust side at the front or rear oil groove. That is why they went to a bearing with only one groove in the middle. But, like I said, just a guess.

 

James.