Bore Center Distance

Can anyone tell what the distance between the bore centers on an 850 engine should be? (not a Mk3, if that makes a difference).

I am fitting a Nourish crank into the 850 cases and when fitted to the crank the distance between the centerlines of the rods is 3.50". The bore centers in the block are at 3.44".

Since the rods are offset by 0.030" in their bores, there might not be enough axial clearance at the big and little ends to allow the pistons to be centered in the bores.

Thanks ~ Gary

The spec is 3.436-3.438 for crank and barrels.

In reality they are all over the place. jim

Thank you Jim, It looks like my block is pretty accurate then, but the crank is a bit off. Actually after measuring everything relevant it looks like there just might be enough clearance between the little end and piston to accommodate the offset.

comnoz said:

The spec is 3.436-3.438 for crank and barrels.

Click to expand...

:lol:

comnoz said:

In reality they are all over the place. jim

Click to expand...

Entertain us a bit here. What values have you seen, even if it is from recollection. I think this would be some good insight to our beloved Nortons. Someone made the statement years ago that a Norton big twin was put together like a Dagwood Sandwich.

Dances with Shrapnel said:

comnoz said:

The spec is 3.436-3.438 for crank and barrels.

Click to expand...

:lol:

comnoz said:

In reality they are all over the place. jim

Click to expand...

Entertain us a bit here. What values have you seen, even if it is from recollection. I think this would be some good insight to our beloved Nortons. Someone made the statement years ago that a Norton big twin was put together like a Dagwood Sandwich.

Click to expand...

I would say .030 off is pretty common. {total] .060 off would worry me a bit as it is likely to cause off center wear to the bore. Jim

The worst thing I have seen was when I sent a crank to a local grinder and had it ground .010.

He put the crank between centers and tightened them up good which bowed the crank while he ground it.

When I put the crank in my motor with topless pistons to check the center, I found the small ends of the rod were about 1/8th inch closer together at TDC than they were at BDC. OUCH((&$$&(* Jim

Jump rope at rest.

Dances with Shrapnel said:

Jump rope at rest.

Click to expand...

Yeah, maybe it would have straightened out at 9000 rpm. :twisted: