8 replies to this topic

[Harvmck]

Hi all,

First post here…

I am in the early stages of building a 1098cc engine which is overboard +60

After a dry build to work out compression ratio I have found my pistons (which are 8cc dished) sit proud of the A+ block I am using…. Pistons 1 and 4 are 0.004” out and pistons 2 and 3 are 0.008” out. Measuring across the pin and taking the average.

Is this acceptable? Could using a thicker 0.040” 1275 head gasket work?

I’m using a 1098 crank and a 12g202 cylinder head
Pistons being used: https://www.somerfor...cr-060-1098cc-a

[Ethel]

I can't see it being much of  an issue. If you have compression ratios & balancing to adjust, you may find some skimming serves an extra purpose.

[nicklouse]

Pistons are often proud of the block when there is a need to increase the CR but the head can’t be skimmed any more. But you need to check for clearance. And see what thickness of gasket suits as they are not all the same. You don’t want a 1275 gasket as you are not using a suitable head.

[eric67]

Considering it is only 4 or 8 thou I would remove that amount by rubbing the piston crown on wet and dry paper taped to a surface table or plate glass. I doubt that it would have any effect on the volume of the dish but this could be re-measured after removal if necessary.

 

Be careful if you do this this to hold the pistons evenly on the paper and check progress regularly to ensure they remain square. The width of the top band will be a good reference and don’t go mad as it won’t take much doing.

[Cooperman]

It will not be a problem so long as the chamber volumes give the correct CR.

[Harvmck]

Thanks all, compression ratio works out at around 9.25. Will find the thickest compressed gasket and go with it

[DeadSquare]

Thanks all, compression ratio works out at around 9.25. Will find the thickest compressed gasket and go with it

What make of gasket did you use and how thick was it when you had compressed it ?

[mini13]

I'd think about swapping piston and rod combinations to try to even up the protrusions, also double and tripple check that its not a measumrent error... also check the stroke incase it is coming from there.

the protrusions not an issue as long as the fire ring is no where near the piston, bear in mind the rod can stretch a little at high rpm, and the crank can flex, so that can take up clearance... but you should have the thickness of the headcgasket to play with so you should be safe,

[Tones61]

a compressed bk450 is 38/40 thou ,

same for a small bore one 

Edited by Tones61, 04 March 2024 - 01:04 PM.