The Stovebolt Site Forums The Parking Lot Early 'Bolts (1916-1936 1st Half) Press fit for pilot bearing in a '28 engine

HI all,

I need to replace the pilot bearing in Lurch's engine. When I separated the transmission from the flywheel housing, the bearing (bushing) just fell out. This could be the smoking gun that points to the squealing noise that he's been making.

I ordered a new bearing from one of the usual vendors and its outside diameter (OD) is also too small for a press fit.

Here's the stats:

1. As best that I can measure, the inside diameter (ID) of the hole at the end of the crankshaft is .818" in diameter.

2. The OD of the new brass bushing is .8155" in diameter, or .0025" smaller that the hole in the crank. So, it slips right in and would spin nicely, which it isn't supposed to do.

3. The ID of the new bushing fits the OD of the input shaft of the transmission well. They both seem to be .625-ish".

I first thought of knurling the OD of the new bushing to expand its OD a little, but I read online (in a machinist discussion) that this method is not a good solution. In the long run, the knurl would wear down and become loose.

So, I ordered a new Oilite Bronze bushing that has an ID of .625" and an OD of .875" so I can turn it down to what I need on my lathe.

The BIG question is, what constitutes a 'press fit'? Or put differently, what OD should I shoot for to get a good press fit in a .818" hole?

Keep in mind that I don't want too much of a press fit, which would compress the bushing and then I would have to ream out the ID of the bushing to get back to the good .625" for the tranny input shaft.

I've read online that adding .001" for every 1 inch in diameter will give a good press fit.

What say you folks? How do you figure out the optimal OD for the pilot bearing in your old Chevys?

Thanks, Dean

.001” per 1” OD is a good rule of thumb for a press fit. You can always try the bushing on your pilot shaft to see how much play you have (or measure it) just to see what you have to play with.

Phat is right about the .001 per 1 inch diameter rule of thumb.
But, make sure the bore you are shrinking it into is not egg shaped, tapered or just plain whollered out.
You seem to have a good machining background, so I think you can make it work!
Please let us know how you corrected the issue!

Don

Use a lathe file to taper the oversize bushing slightly where it will fit about halfway into the hole with a firm push with your thumb, and use green LocTite "stud and bearing" compound to keep it tight in the crankshaft bore. Even if the hole is a little egg shaped, the liquid will even things out as it cures. I've used it on big wrist pin bushings for Cummins Diesel rods and never had one come loose, to the best of my knowledge. That's a lot more stress than a pilot bushing will ever see.
Jerry

Have used green LocTite on some quite loose press fit parts with good success. It is also called "sleeve lock" and "bearing lock". Takes heat to release once set.

First, thanks for all your comments and suggestions. I especially like the idea of using the green Loctite to help secure the outside of the bushing to the hole in the crank. ;-)

I'm an amateur machinist, but I think I can tackle this job.

After cleaning the crankshaft hole some more and taking some careful measurements, it looks like the hole is a little tapered and egg shaped. ;-(

Using a new telescoping measuring gauge and a micrometer, I determined the figures you see in the diagram below. I measured multiple times in each location until I got at least three semi-consistent numbers.

Keep in mind that the engine is still sitting in Lurch, so I am contorting my upper body to get these measurements from leaning down into the hole where the floor boards would go. Luckily, I'm still flexible enough to do so.

The vertical dimension of the hole seems to taper from .8195-ish" at the inside to .822-ish" close to the outside.

The horizontal dimension of the hole tapers from .818-ish" to a whopping .8245-ish" close to the outside.

At some point in this crankshaft's life, there must have been a whole lotta shakin' goin' on in there!

I guess what I need to do is come up with a plan for shaping the OD of the pilot bushing with enough clearance so that when I put it in with the Loctite around it, the center hole is concentric with the the flywheel diameter and I need to hold the bushing in that particular place long enough for the Loctite to fully cure, which seems to be 24 hours.

Am I over-thinking this?

Thanks for your combined history and knowledge!!

Dean

Let the transmission input shaft snout be its own alignment fixture- - - -Give the pilot bushing a little clearance so it's a snug slip fit, put a few drops of Loctite on the OD, and install the transmission without the clutch so the bushing self centers while the goop cures. Instant perfect alignment! Loctite says the green stuff will take up .015" of slop. Just be sure the hole in the crank is CLEAN so the Loctite gets a good bond- - - - -and don't get any goop on the ID of the bushing! If you ever need to remove the bushing again, a Propane torch and 500 degrees F. breaks the bond.
Jerry

Great info and suggestions, Jerry!

Thanks, Dean

I wonder if the 1920's machining methods might be responsible for the variation in the size of the hole in the crankshaft?
Jerry

I have no idea if the crankshaft hole varies from one 1920s crankshaft to another, since I have not measured any other than the one that I am working on.

However, this whole discussion has come full circle.

I started this thread because the bushing that I bought from one of the Chevy 4 cylinder vendors was not a press fit. It is .8155" in diameter, whereas the hole in the crankshaft varies from .818" to .8245".

Based on Jerry's info (that Loctite green will handle up to .015" of slop), I can:

1. Use the bushing that I bought from the vendor as is.

2. Using Jerry's suggestion to let the transmission input shaft do the aligning and the Loctite green to 'fill in the gaps', the bushing will be perfectly aligned with the flywheel, even though the crankshaft hole is slightly egg shaped.

Thanks to all for a great conversation that arrived at a very doable solution!!

Happy happy! Dean