MTFCA Forum

I am replacing the rear main cap. What is the best way to to remove babbit to set end play? My cap is slightly too long.


John

If you can borrow a facing tool they work the best. If you where not so far away I would tell you to bring it over and we could knock it out.

Whatever size the rear main diameter is, turn a mandrel in your lathe to .001" smaller diameter and leave it in the lathe. You can now clamp the rear main in the lathe using another bearing to clamp in place and then face off one end of the bearing. Swap ends to do the other end, but brother, you need to make sure you're setting the crank in the correct location to be able to properly shim the mag.

That's a whole 'nother story.

Before you fit your cap length, look very closely at your crankshaft. On either side of the rear main diameter is a thrust surface, (both front and rear). Just like the thrust surfaces on the main cap, the crankshaft thrust surfaces also wear. The wear pattern on the crank thrust surfaces forms grooves in the thrust faces. The problem is, when you install the new main cap, you have to pass it by the unworn portions of the crankshaft thrust faces. Therefore, the cap has to be trimmed so short, in order to pass by the unworn areas, that when it's finally seated, and now aligned with the worn spots, you may find that you have just as much end play is you did when you began. For most engines I've worked on, (besides those that were total rebuilds), I've had to remove the crankshaft and true up the thrust faces to reestablish a consistent dimension across the entire thrust surface.

What he said^ also that area needs to be as smooth as the crank journal. Any roughness/ridges/grooves are going to act like a file on the babbitt.

I'm fortunate enough to have a thrust flange cutter. It works great and makes the job super easy. If you know someone who has one, that's the route I would go.

Stephen

Steven makes a good point. Any wear at the thrust face on the crankshaft should be measured. If that wear exceeds that allowable, there is no point in fitting the main cap until that wear is ground out. Should the wear be within the allowable tolerance, the main cap can be worked down to a close sliding fit and then bolted down.
Two points to consider. If you have a machined surface on the ends of the cap, I would work those down using a surface plate and wet rubbing paper. Frequent checking can be made to see just how much more is needed to get a tight slide in fit.
Secondly, the diameter of the worn face in the crankshaft and the white metal in your cap may well not be the same. This does not really matter. With a greater diameter on the white metal, a close slide in fit you will have minimal end float. As the cap gets to know the flange, you will develop the end float allowed by the wear on the flange.

Allan from down under.

I turn down the rear main in the lathe as on the picture .
Toon

Joe,thanks for the offer. The engine that you did for me 5 or 6 years ago is still doing fine and the owners have driven the car on the Hillbilly tour every year.
Thanks to all for the information. I was thinking of clamping the bearing to an angle plate in the mill,but the mandrel sounds a lot better.
The crank surfaces are very good,no wear marks.
To center it I was going to take the same amount off each end.

John