Wrong size main bearing shells

[pjhicks2]

Hi - This is my first post and I think I know the answer, but I'm just checking that Jaguar V12's engines don't do something different.

Story is that I am in the process of rebuilding the engine. When I pulled the shells, they looked odd, a dull grey colour, not bright metal that i was expecting. Looking at the shells they were marked as STD ( and original Jaguar parts) , looking at the journals they looked in very good condition, but when i measuring the journals they are coming out being 10 thou under....

To me it looks like the crank has just been reground and the engine rebuilt with the old shells. Or is this "normal" for this engine?

Ive only driven the car about 70 miles before deciding to rebuild (due to a poor tensioner)
Oil pressure was good

Im assuming that this in error and very poor mechanicing by the previous owners

[jagwit]

Welcome to the forum!!

I've just replaced the big end bearings on a V12 I'm working on and although they bearings seemed VERY new to me, they were also "dark grey" in colour with even darker patches here and there. I believe this engine was "re-built" and then stood for very long not being driven.

It certainly does not sound right to me that a 10th crank gets std bearings. I would get 10th bearings before rebuilding.

[MarekH]

If you have no confidence in where you are at, then Plastigauge them all and find out where you stand with the whole thing. Then take a view as to how to proceed.

kind regards
Marek

[jagwit]

then Plastigauge them

What is this Marek?

[pjhicks2]

Hi - Little egg on Face, looks like my digital Micrometer is out, Dug out my old mechanical one plus I plastiguaged 2 mains and a couple of big ends. They agree that the crank is NOT 10 thou under, Plastiguage tells me that there is about 1.5 thou clearance - Happy boy... But lesson learnt, dont rely on "new" digital technology, The guys who built the engines didn't have them .


many thanks to all those who helped.

Pete

Jag V12
Triumph TR6
1963 Mini ....

[MarekH]

I kind of thought that's where it'd end up, but didn't want to wade in and say it must have been measured wrongly - I think you'd have noticed if the clearance really were 10thou.

Plastigauge is a clever plasticene product a bit like a sticking plaster which flattens out like a pancake when it is pressed between two surfaces.

You assemble the bearings etc with a strip of Plastigauge inserted in the relevant bearing journal and this squashes the plastic. When you disassemble again, you compare how wide the strip has now become against the chart that comes with it and that tells you how thickness of the plastic. This tells you what the gap was. It's a very neat way of putting a micrometer into where one won't fit and means you don't have any error bars due to maths or measurements adding and subtracting between different components or verniers.

kind regards
Marek

[Whitact]

Slightly off topic, but is it possible to regrind these cranks satisfactorily?
The book says no due to the hardening process used on the journals but replacement shells are available in various over sizes. Has any one had this done and put significant miles on the engine subsequently?
Cheers

[pjhicks2]

HI

I know that the crank has been hardened and in theory regrinding should not be needed, but within the service manual - in the specifications section it talks about the regrind limit - it goes up to 40 thou... well through the hardening....


Peter

[Hugo]

I wouldn't go to less than -.020" without getting the crank re-nitrided. Simple process.