VCCA Home Forums TECHNICAL - 1945 AND EARLIER 1912-1928 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 Chevy engines?

Thanks, Dean

Make sure the bore is OK and not scored. I have seen instances where the bushing does not wear as much as the harder mating surface. Also with a pressed fit both items need to be measured when they are at the same temperature. I am not qualified to answer your tolerance question but I would check with a machine shop. I am guessing the above is good. Once you get that answer and are ready to install I think putting the bushing in the freezer overnight will make the job easier. I am curious if you have determined why the bushing spun inside the crank bore so you don't have a repeat performance.

Steve,

You ask very pertinent questions. ;-)

Yes, the bore in the crankshaft is smooth, as far as I can see and feel. My pinky finger JUST fit in there. I intend to measure the bore more carefully (at different places) and make sure my figures above are accurate. I was using a borrowed telescopic gauge when I came up with the .818" bore diameter. I have since bought myself a set of these gauges so I can play with them.

Also, the measurements in my first post were taken with all the parts and engine at the same temperature. Outside in my driveway in the low 70s in degrees F.

One thing I did not mention is that I slid the old bushing into the crank hole and it slipped in without using any force and could spin around as there is nothing to keep it from spinning.

I did the same test with the 'new' bushing and got the same result.

The difference between the old and new bushing was extra wear in the ID of the old bushing, meaning the old bushing was more sloppy on the transmission input shaft than the new one.

BTW, the tranny input shaft end is also smooth and not scored in any way, thankfully!

Keep the comments coming! Dean

I have replaced a handful of those bushings over the years and none of them went in very tight. I use an old worn out transmission input shaft as a driver. Light taps with a regular claw hammer gets it done. I've also observed the wear pattern on the 28 motors is only about 50% deep on the bushing so the tip of the input shaft doesn't have much engagement. Perhaps that's why the bushings tend to fail. Wild guess: the addition of the transmission to frame cross-member in 28 set the transmission back about 1/8" and resulted in less engagement. I think your comment about 0.001" for every 1 inch diameter is reasonable. Perhaps a little less. An Oilite bushing is porous and brittle. It will probably squish a little so double check the ID after driving it home. Good luck.

Thanks for your input coachman28.

I performed some more detailed measurements and found that the hole in the crankcase is slightly tapered and egg shaped.

I had a detailed discussion about this topic over on the Stovebolt site and the end result is that:

1. I'm going to use the new bushing that I bought from the vendor as is. It is .0025" - .009" smaller than the misshaped hole in the crank because of the taper/egg characteristics of that hole. Yes, the bushing can wiggle around a fair amount.

2. I'm going to slide the bushing into the crank hole with a coating of green Loctite stud and bearing compound around the outside of the bushing. According to the specs, the Loctite can 'fill up' gaps up to .015".

3. Right after I slide the bushing in, I'm going to bolt the tranny back in place (without the clutch, pressure plate assembly, or throw out bearing) so that the input shaft of the tranny will automagically 'center' the bushing in crank hole while the Loctite cures (24 hours).

After that, I should be able to back off the tranny and install the new clutch with a rehabbed pressure plate and a new throw out bearing. Then, this part of the project will be done.

BTW, here is a link to that discussion on Stovebolt: https://www.stovebolt.com/ubbthread...it-for-pilot-bearing-in-a-28-engine.html

Cheers, Dean

Dean, I checked the Stovebolt post and it was informative reading. The mechanics at the truck shop I worked at used the stud and bearing mount with good success. The only other thing i can think of is to check the bearing on the input shaft in the transmission.

Im getting ready to do his project.Thanks for posting the bushing info. Just what I needed to know

Dean,
I do not have an alternate suggestion, but I wonder if your transmission will provide a truly accurate center for the bushing. If your front bearing is absolutely tight, and without wiggle, it's the best shot you have.

Mike

This can be measured if you want to separate the bellhousing from the transmission, bolt it to the engine, and dial-indicate the opening for concentricity, same as if you were installing a manual trans in a modern car. I once had a 28 motor that kept breaking the center out of clutch discs. I found the stamped steel bellhousing was not concentric with the crank centerline. It took a few tries to find a combination of parts that was close and that solved the problem.

Cheers