73.5 mm ofset bore please help
Turbo Mini Forums

Shout out to all you big bore engine builders.

When getting blocked bored to 73.5 (1380) spec and it is ofset bored.

What will the gap be between the two middle cylinders at its smallest point.

Will the gap between the middle two be smaller than the gap between 1 -2 and 3-4 even when ofset bored.

What will the gaps be when not odffset bored.



Also if you have a similar spec block what gap between cylinders do you have.

any was your blog ofset bored.


Many Thanks

David

Yes the gap will be smaller

And not offset bored - Just dont go there!!

Do the math, offset boring varies between engine builders. I offset my 74mm block 10 thou per pairs of cylinders, that basicaly moved pairs of cylinders 20 thou further apart, so the center wll will be 20 thou (0.5mm) thicker, then if it was not(but still thinner than the other two walls), irespective of bore size. If you have a standard block you can meaure bore centers and then work everything out from there as to whether the block you have has or has not been offset bored.

Piss poor if its 73.5mm and has not

Edited by Sprocket on 24th Jul, 2008.

On my block, I only went to 73mm (1360) and although Vizard says offsetting isn't required at 73mm, I did what Sprocket says - I measured it all very carefully first, including measurements of a new BK450 gasket and I then worked out the optimum position of each bore.
I drew it out and just told the machine shop to offset to my drawing.
The end result is the centre gap is still slightly smaller than the outer two gaps (but nowhere near as bad as if I had not offset) but if you look at a new BK450, you will see that the fire rings are closer at the middle gap anyway. The end result in my case is the best possible match to the gasket.

EDIT - add pics.
Overall

Outer

Centre


I hate to think what the centre would have been like without offsetting, and this is only 73mm.

And, no, I haven't machined the pistons for an adequate dish yet, I'm still working that out.....

Edited by Rod S on 24th Jul, 2008.

Schrödinger's cat - so which one am I ???

You want bto see that middle bore wall with 74mm pistons! IIRC its only 2.4mm thick, the fire ring of the AF460 gasket is flush with the bores!