Three bearing engine - centre bearing

[Stephen Barker]

What should the diameter be for the centre bearing on a three bearing crankshaft when machined for + 40 shells ?

[Stuart Giles]

1.71" original diameter is 1.75"

[Chris KC]

What should the diameter be for the centre bearing on a three bearing crankshaft when machined for + 40 shells ?

At the risk of sounding sarcastic they would be - 40 shells.
Mine is 1.7500" at standard, but it's a Reliant crank and I have a slight suspicion they were toleranced marginally differently from Austin ones. May be better to check with someone who sounds more confident.

[Zetomagneto]

What should the diameter be for the centre bearing on a three bearing crankshaft when machined for + 40 shells ?

At the risk of sounding sarcastic they would be - 40 shells.
Mine is 1.7500" at standard, but it's a Reliant crank and I have a slight suspicion they were toleranced marginally differently from Austin ones. May be better to check with someone who sounds more confident.

Stuart sounds confident, looks like the right answer to me, 1.750" - .040" = 1.710".
Yes, the shells will be - .040".

[Colin Reed]

Hi Stephen,
My advise would be fit the new shells in the block and have who ever is doing the Grinding to measure,
Reason for this is they are mass produced and there will be tolerances, also the housing could be out of round.
Every ones feel with a Micrometer is different especially going from inside to outside.
Do not assume the shells and housing are correct Check

Colin
NZ

[Stephen Barker]

Hi Stephen,
My advise would be fit the new shells in the block and have who ever is doing the Grinding to measure,
Reason for this is they are mass produced and there will be tolerances, also the housing could be out of round.
Every ones feel with a Micrometer is different especially going from inside to outside.
Do not assume the shells and housing are correct Check

Colin
NZ

Thank you all for your replies.
I have checked the crankshaft, it is 1.71 but when torquing the centre bearing with the - 40 shells, crankshaft spins easily up to 46lbs torque, but when torqued to 50lbs it almost locks up and gets really tight. Any reasons?

‘With the shells just held together by hand round the crank, should the ends of the shells meet each other because I have a 30 thou gap. This would allow the shells to slip round slightly in the caps which can’t be right,.’

[Stuart Giles]

Your apparent binding problem is quite likely to be caused by the centre main being misaligned WRT the front an rear mains, this isn't an uncommon problem on 3 bearing A7 motors. Have you got the block bolted down to the crankcase when you have this issue? that is likely to make a difference.

If you just want to check the shells and crank journal for clearance, put the shells in the centre main housing halves and bolt the two halves up either with the crank out of the motor, or without the front or rear main bearing in place.

[Chris KC]

Hi Stephen,
My advise would be fit the new shells in the block and have who ever is doing the Grinding to measure,
Reason for this is they are mass produced and there will be tolerances, also the housing could be out of round.
Every ones feel with a Micrometer is different especially going from inside to outside.
Do not assume the shells and housing are correct Check

Colin
NZ

Thank you all for your replies.
I have checked the crankshaft, it is 1.71 but when torquing the centre bearing with the - 40 shells, crankshaft spins easily up to 46lbs torque, but when torqued to 50lbs it almost locks up and gets really tight. Any reasons?

‘With the shells just held together by hand round the crank, should the ends of the shells meet each other because I have a 30 thou gap. This would allow the shells to slip round slightly in the caps which can’t be right,.’

Do them up 'wrist tight', then give the side of the housing a tap with a ball-pein hammer before torqueing up to snap the shells into alignment.

I hesitate to comment on your torque but mine (as I mentioned, a pressure fed Reliant crank conversion) are done up to 30lbft. I think it's a different structure so maybe you're right.

[Stephen Barker]

Have put some shims between the centre main clamp bolts and torqued the bearing to 50lbs and the crankshaft rotates reasonably free now, will put the block on and see what difference that makes.

[Chris KC]

Stephen,

As an external observer it's a little difficult to assess your position on account of the slender amount of information you have shared. If you are newly building up an engine there's more than a little chance that the C/M is out of line with the front and rear mains. Shimming can correct this but a far better solution is to fabricate a new C/M housing and have it line bored. As observed above, this should be done with the major bits properly assembled so it doesn't pull out of line when built up. My own engine was built in this manner by Stuart Rolt in the 80's and has been completely reliable. He used to bore out the new housings with a bloody great reamer mounted in a lathe.

Even thus corrected though, a new set of shells will often bind if not administered the aforementioned tap or two on assembly, I've had the same issue myself. Built up two or three times and went tight on torque application; once everything seated into place properly it runs freely. (I had this tip myself from Stuart when I made the same enquiry to him years ago).

A parting thought, do make sure the housing is assembled the right way around, front should be marked.

If the car has been running reliably prior to whatever you are doing now I would think very hard before shimming out the C/M.