Main bearing clearance

[thexkeprocess]

Hi,
Ive been slowly refreshing my engine. I was checking my main crankshaft bearing clearances and found that the rear main bearing was 2 thou while the front bearing was 3 thou. I used new standard size bearings bought years ago and used them to test. Are they passable or is 3 thou too wide a clearance? It seems odd that the rear bearing has less clearance than the front bearing, why would that be? I was hoping I would get away with not sending the crank to machine shop.
This is a 69 4.2.
Mark

[abowie]

Quoting Christopher Story in another thread:

"The main bearing clearance given in the factory manual for both 3.8 and 4.2 for all the main bearings is .0025" to .0042" "

A quick look through the Jaguar service manuals confirms this for the 3.8 but I haven't been able to pin it down conclusively for the 4.2.

[christopher storey]

Andrew : where the 4.2 section of the manuals gives no information, it means it is unchanged from 3.8. Neither the BX nor the BY sections have any information, so one can take it that the bearing clearances are as I originally posted i.e. .0025 to .0042" . Thus in this case 2 thou is fractionally tight, but in my opinion not worth worrying about, and 3 thou is right in the middle of the permissible clearances

[Series1 Stu]

I agree with Christopher, although I thought I did see the clearance figures quoted in the series 2 manual.

I would counsel caution in your measurements if you don't have truly accurate measuring equipment that can measure to around a 10th of a thousandth of an inch. An Aldi special digital caliper is definitely not the right tool for the job.

You also need to be able to check for roundness otherwise your bearing clearances will vary as the crank rotates.

Regards

[abowie]

Andrew : where the 4.2 section of the manuals gives no information, it means it is unchanged from 3.8

I hadn't considered that but it makes sense.