Advice on Twin cam main bearing clearances

[Limey]

My +2 has a had a very long lay up, so as a precaution, I decided to strip the engine and check it over.
I recall the oil pressure being circa 10lbs at hot idle, and about 35/40 at cruising speeds the last time it was run.

The main bearing clearances are between 3 and 4 thou. I have measured them with a bore gauge and rechecked with plastigauge.
Checking in the Miles Wilkins twin cam book: the max recommended clearance is 3; and the max journal under size is 30 thou; which is the size my crank is at already.

The question is, can I get away with these clearances?
If not, then Burton have +40 bearings, so I would like to know if anyone has fitted +40 bearings with a crank regrind and had good results?

Steve

[AHM]

Steve,

You can read too many books! The workshop manual gives the running clearance as .0005/.0022 in (.013/.513 (??) mm) and David Vizard's book gives the max regrind as .020.

I would suggest that you satisfy yourself of the condition of your crank by measuring the journal diameter as this is an indication of the wear.

Are you using new bearings to measure - you haven't indicated.

Simon

[Limey]

The bearings have only done about 5000 miles.

My workshop manual states 1.5 - 3.0 thou for the mains, and 0.5 - 2.2 for the big ends

I have measured the crank journals

Main Big End
1 2.0945 1.9065
2 2.0940 1.9065
3 2.0945 1.9065
4 2.0940 1.9060
5 2.0945

Being 30 under the min sizes should be 2.0955 mains and 1.9070 big ends.
So it looks as though the crank is about 1 thou under what it should be.
It looks like the large clearances are due to the undersize crank, and oversize bearings.

I think I'll get some new +30 bearings and check to see if the clearances come into spec.

Steve

[Chancer]

I ran and competed with a pretty hot X/flow for several years that was reground to -40 on the big ends and - 30 mains without any problem, it was lightened and balanced and the rods had a lot of material removed followed by shot peened.

I was running a semi-chambered head and pre X/flow flat top pistons which I believe are lighter than the X/flow ones, I guess a similar mass to the twin cam pistons.