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Correcting Bearing Clearances

Practical Engine Building

Discussion and questions related to the course Practical Engine Building

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If you come to find your bearing clearances are out of spec, how do you go about correcting this when it's either too large or too small?

If it's too large a clearance, not too much you can do except check bearing suppliers if they have slightly undersize bearing shells available - I know KING have them for the more popular engines, and can make a custom set up if required. Sometimes you may find the manufacturing tolerances as sufficient to make the difference up by swapping shels around.

If there isn't enough clearance, it isn't uncommon to have the journals micro-ground or polished to remove a fe tenths of a thou' for clearance, however, again, some bearing companies may have shells available that are slightly oversize, for clearance.

With the shells, you may be able to buy them as pairs, rather than a full set, if it's just one or two journals that are the problem.

What engine is it, and the clearance problem(s)?

I didn't personally run into any problems... yet. This was just a question I came up with while watching the Bearing Clearances video in the practical engine building course. That being said, I will be building an Audi 2.5 I5 out of my 2019 TT RS and want to be prepared.

When I was rebuilding my engine, my main bearings left a clearance outside of the specification, both new and the OEMs, with no signs of wear on the OEM ones I took out.

That being said the service manual has a tolerance range and a service limit. The service limit for my engine was about 3 times larger than the tolerance range, which sounds silly to me, but I had to be out of the tolerance range because I could not get the bearings I needed to close the gap and I wasn't going to get my journals ground. I put the engine together with clearances slightly larger than tolerance, but well within the service limit and I run a slightly heavier oil.