Interference Fitting of Bearings

Mike Vitetta

When I purchased my car the engine made a real racket especially after the Breakfast on the beach run last year. I replaced the timing gear, (it was a real mess) and found this procedure on the web. I have done it on the rods but not yet on the crank.

Mike Flanagan has posted his description of interference fitting of the main bearings in the Model A engine. Because of the weight of the flywheel and the flexibility of the Model A crank, plastigauge may not be a reliable method of adjusting the bearing clearances.


Here's Mike:

How do you check the clearance on the bearings? It is not QED but can be done if you can lay on your back and work with oil and grit and grime falling in your eyes.

First you gotta drop the pan, let it sit overnight to get all the oil that will fall out to do so. You are still going to get oily. 

Pull the damn oil pump out and lay it aside. You can shine a flashlight up in there to see which way the tank is oriented to reinstall and with it out there won't be so damn much blood on it to clean off it. 

Then I start with the center main. This one requires that the valve cover be taken off as the nut is inside there. The other nut is between 2 and three outside on the other side. Remove the cap being careful to keep the shims on the same side as they came from. 

Visually inspect the bearing for cracks and places at both the front and back that look as if it has been compressing the babbitt and pushing it forward or to the rear. Bad signs if so. 

To check the clearance, use a piece of aluminum foil .002 thick. The heavy duty she buys to do heavy-duty things is .002 but the regular like you cook a baked potato in is .001 so you fold it. You want a piece as long as the cap and about 1/2 wide. Lay this in the cap and with the SHIMS BACK IN THERE put the cap back on and tighten it up. 

Now you want to see if the motor will turn with the obstruction in place. Stick a 6" punch in one of the balance holes in the crank throw so the pull will be consistent each time you check. You are looking for lockup with the obstruction in there and free without. The shims are increments of .002 or .003 depending on which the rebuild used. The steel shims are generally in .002 and the brass in .003. Peel 1 increment from one side at a time, one from one side and if that doesn't lock it up then do one from the other side. 

Yes it is tedious; just be glad you didn't by a space shuttle. 

Once you get this bearing to lockup then remove the obstruction and replace it and tighten it up again and see if it will rotate. If it won't turn with the 6" punch then put that dame shim back in and run it another 3k.

Do the rear next then the front. The rods adjust exactly the same way. When you finally get a bearing adjusted like you want it put some pre-lube (engine assembly lube) on there before putting it on there for the last time. Torque to whatever you has been using (75 minimum) and go. The rod nuts torque to 40.

To put the pan on there install the oil pump using a 3/8 fine thread bolt in the plug hole in the side of the block to retain the pump while you install the pan. Finger tight as the threads are actually national pipe threads but the bolt will work in there. Get some 5/16 all thread and make yourself some cheaters by cutting 4 pieces 4 or 5 inches long. Screw them into the pan rail and let 'em hang down. Slip the pan over them and start a nut. Then it is a matter of tightening the nuts incrementally in turn and thereby winching the pan up against the spring on the pump.

The gaskets and sealer is a matter of choice but my opinion is that the silicone belongs in the bathroom. I use permatex red personally. When you put it all back together put some of the oil down the dist hole so the valve galley is oiled. It should not have lost any oil but I like to make sure.

Lotta work laying on your back.