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How to set bearing clearance?

Engine Building Fundamentals

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Discussion and questions related to the course Engine Building Fundamentals

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The process of measuring clearance is a topic that is very thoroughly discussed and very well known.

But how to properly SET clearances, and what techniques and options are available to do that, seems to be less discussed.

Obviously there's different sized bearings available. I know I can get STD size, .001 over, and .001 under.

And I know I can mix'n'match the bearing shells for adjusting in half-thou increments.

But what if I need to be more precise than that? Or is that generally precise enough for most cases, and I don't need to worry any more about it?

For my application, I've been told by a pro that I want to be right at the loose end of the factory tolerance. That seems like a VERY fine mark to try to hit. A half-thou adjustment could easily overshoot it...

I haven't started my build yet, still gathering all the parts I need. And still waiting for the block from the machine shop, so I won't be able to measure anything for a few more weeks. I guess I'll wait until then, so I know what I'm dealing with. Until then I'm just trying to plan ahead and know what to do once I get to that point.

You must have a conversation with your engine machinist before he starts machining your crank and line honing your block!

Get it right now instead of trying to after he has machined your components!

You could have your crank micro-polished to your desired size -- perhaps your machine shop can give you options.

I normally adjust the bearings themselves to get required clearances but some say it is not right way of doing it. However I never had a single issue wity such approach...

So what I'm hearing from Benny and David, sounds like there's nothing I can do if I want that kind of precision, I'd have to have the machine shop do it. I do get that, but I'm not really satisfied with that.

Georg, you're suggesting to finely sand the back of the bearing shell, or something like that?

Yeah, I've heard some people say that's a thing. But other people say it's a really bad idea. The obvious problem with that, is it's easy to cause tapering or out-of-round of the bearing. However, it's easy to re-measure and verify that everything is all good after re-installing the bearing. It might involve going through a couple sets of bearings if it's not done right the first time.

What does everyone think of adjusting the bearing thickness by hand like that? Assuming that I'd verify the work with multiple measurements?

I don't work on bearings back but top of bearing half that is getting crushed when proper torque is applied resulting in some bearing clearance. The more material you remove from to of the bearing legs, the less clearance is going to be to some extend. After that if you remive too much the clearance will get bigger again as there is not enough crushing on bearing to press it enough.