Stop by your local auto parts store and get some "plastic-gage". Remove the
bearing caps, remove all oil from bearing and crank, lay a small piece of
"P-gage" across the bearing, reinstall the cap and torque to specs, remove
cap again and measure width of 'P-gage". This will tell you how much
clearance you have. 'P-gage" comes in different thickness' for different
clearances, you might have to follow this process several times. This
procedure should be done every time and on every bearing that you
If you find that you are close to 10 thousandths you can then "scrape" some
material off the new 30 thousandths undersized bearings, or have a m-shop
ream to size. Not a job for the UN or semi-skilled. Any-ways the "P-gage"
will let you know where you stand.
Don't forget to check for visual signs of wear.
> From: Giles Nelson <Giles.Nelson@cl.cam.ac.uk>
> To: firstname.lastname@example.org
> Subject: Undersize big-end bearings
> Date: Sunday, September 21, 1997 9:40 AM
> The big-end bearings in my TR6 sound worn. There's a rumble at low-revs
> the characteristic higher frequency `knocking' at low revs when I put
> my foot down (not the high-frequency pre-ignition knock).
> However I replaced the bearings approximately 8,000 miles ago.
> The crank must have been reground in the past as the bearings I replaced
> 20 thousandths undersize. I used similarly sized bearings.
> For a while everything seemed tight, but now this is obviously not the
> Has anyone on the list any suggestions to what I should do?
> I don't want to take the engine out of the car. Everything else
> is functioning satisfactorily. I'd like the engine to do another
> 30,000 miles before I give it a full rebuild. I'm happy to replace the
> bearings with 20 thou undersize again. However big-end bearings last
> longer than 8000 miles and so my conclusion is that the crank is more
> than 20 thou undersize. Can I use 30 thou undersize bearings without a
> crankshaft regrind.
> Has anyone on the list any advice?
> Many thanks.
> Giles Nelson.