MTFCA Forum

After reading several discussions on 4th main bearings on this forum, I wanted to pose a question.
My 24 roadster came with a ball bearing 4th main as well as a needle bearing drive shaft bushing. I have had no trouble with either, and have put approximately 10000 miles on the car with those in it. Both of the bearings do not seem to be sloppy or worn out. Right now I have the engine out and disassembled to pour new mains and put in some new pistons and rings and valves.
The question I have is should I replace the ball bearing 4th main with an original babbitt one, also replace the needle bearing on the drive shaft and if I should, can I use the drive shaft tube with the ball cut back about 3/8" for the ball bearing 4th main, or do I need to replace the drive shaft tube also? Or should I just forget about it and continue using the ball bearing 4th main and needle bearing on the drive shaft.

If it ain't broken.......

The tail shaft is often machined to accept a smaller inside diameter ball bearing. If so, put the ball bearing back in. If not, you could swap back to babbit, but both have proven to perform reliably. The trimmed drive shaft tube is necessary with the ball bearing, but makes no difference paired with a stock bearing.

I’m not a big fan of the drive shaft needle bearings, primarily because the shaft is not surface hardened. The drive shaft is not radial loaded, so may not be much of a concern. I would inspect the drive shaft for wear under the needle bearing, and swap back to the bushing if you see anything unusual

I'd make sure the universal joint and the square sockets and shaft ends are in top condition. A worn joint will give the bearings a hard time.

If it mine, I would replace all the "modified" parts. But, that's me, I have all those parts on hand.
These modified parts too often fail. I am all about reliability & doing a motor once.

D.S. : The ball end is a ball joint. Cutting the end usually adds a lot of unwanted slop.
Depending on the type of 4th main,(cast brass as opposed to a modified original ball cap) a strong bump can break it.
If you go back to an original 4th main replace the DS tube.
Either way, you should remove the DS & look @ the bearing surface for wear from the needle bearing.

Otherwise a very careful look that all the parts are in order. Are the bearing seals still keeping the u-joint dry?
Was the output plate butchered? Are the rivets tight?
Very important that your crank case is straight, 4th main goes into position perfectly centered & the Transmission center shaft & output shaft are running true within .001"
The Babbitt 4th main should have .0015-.002" clearance. Buying a standard .010 under wont necessarily fit correctly. Custom machining to keep a tight clearance is usually in order.

mcnallyf wrote: ↑

Tue Oct 28, 2025 12:42 pm

After reading several discussions on 4th main bearings on this forum, I wanted to pose a question.
.......
The question I have is should I replace the ball bearing 4th main with an original babbitt one, also replace the needle bearing on the drive shaft and if I should, can I use the drive shaft tube with the ball cut back about 3/8" for the ball bearing 4th main, or do I need to replace the drive shaft tube also? Or should I just forget about it and continue using the ball bearing 4th main and needle bearing on the drive shaft.

As was previously stated "if it ain't broke, don't fix it'. You have 10,000 miles on it, proof of a working update.

If you have a roller bearing at the front of the
driveshaft how is the thrust taken into account? Is there an adjustable bearing assembly at the diff end? Just another thing to consider if making changes.
Allan from down under.

No expert but from what I've learned from AN EXPERT...DUMP IT. Good way to bust a crank I've been told.