how much should i cut my piston?

[Peter]

hello guys,
i m mocking up and measuring deck hight and i get 0.35 mm for cyl 1 and 0.16 for cyl 2
barrels and shims seem to be equal, cause i exchanged them a few times and readings keep the same..
so.. how much is the tolerance for DHr?
would it be OK if I cut this one piston by 0.2mm?
I didnt measure nr 3 and 4 yet...
will do this the next hours

Thanks, Peter

[louisb]

Check your rods, then match your longest rods with your shortest pistons. Try to equal it out first that way.

--louis

[Roman]

I always try to deck the cylinder seating surface so it is absolutely parallel to the crank.
What is important is the height of the cylinders. The two cylinders on the same side should be equal in length or at least within 0.05 mm to get a flat surface for the head to ride on.
I wouldn't cut the piston. Then you are very likely to get uneven weight and you have to compensate that under the piston that weakens it.
I also grind my own cylinder shims to exact thickness.
Try to measure the deck with at least one ring installed, that centers the piston in the cylinder.

[Peter]

thanks for the advice guys,
the two cyls are equal in length thats for sure.. i used a straight ruler and looked towards the light , but no light passed through.
i could see this way that the seating surface is parallel ( well sortof
i didnt use rings, but i will try this as well
didnt have time to measure nr 3 and 4 , i spent more time measuring 1 and 2
another thing i saw was that a case saver is sticking out of the seating surface; not easy to see, but i could feel it with a straight edge that i put flat on the sealing surface..
i ll have to sort that out first...

[Peter]

hello guys,
case saver problem is solved thanks to good advice from Steve,
and i measured again: also nr 3 and 4:
1: 0.34
2: 0.14
3: 0.12
4: 0.12

tried to switch pistons , and put

[Peter]

...whoops..

...put a thinner spacer under cylinder 1 and i got 0.19 if i remember right...
Still not good...
So what do you guys think that the wisest thing to do is?
will cutting 0.2 mm off the top make it so bad?
I thought piston cutting was pretty normal, whith all of the "good" parts today?
i know i ll have to balance the pistons again, but thats no problem i think..?
i remove the weight under the pin boss, not on top...

what do you guys think?

[Harry/FDK]

Hi Peter, i'm going thru the same thing and its driving me nuts. Once you sand of the "hairs"of the shims it's a different story on thickness. I just ordered 3 sets of two thicknesses i need, then i will sand them down and try to match as close as possible.

Roman, do you have machinery to grind them to specs or do you sand them down.

Thanks,
Harry

[Peter]

Hi Harry,
i played with different shims, but then the barrels dont sit equally,
so i allways end up with a difference in DH...

with thicker shims on one side i got 0.19 for the two cylinders and on he other side with thinner shims i got 0.20 and 0.05;

so i think i will ask a machine shop to take 0.15mm from the one with 0.05 and end up with 0.2 mm DH.

Thanks, Peter