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piston fit

jim wilson

Website User
Non-VOC Member
Do you need to fit compression plates with 8:1 omega pistons
if so what thickness
how about a 9:1 omega?
 
Last edited:

ogrilp400

Well Known and Active Website User
VOC Member
Compression Ratio

G'day Jim,
Only if you want to lower the compression ratio. Compression plates are used to raise the cylinder and head assembly and therefore make a larger combustion chamber volume and so lower the compression ratio.

Phelps.
 

Ian Savage

Well Known and Active Website User
VOC Member
Compression plates

Jim
Each 1/16" plate under the cylinder is worth about .5 off compression ratio
ie 8:1 piston with 1/8" plate equals 7:1
Ian

PS just a thought are you asking if the 'higher compression' 8:1 or 9:1 pistons need additional space to clear to the valves?
If so, no, the cutouts in the piston top give suitable clearance.
 

jim wilson

Website User
Non-VOC Member
that's the just of the question ian, i need to change pistons and the set i thought i needed were 8:1, which i got and had the cylinder honed to fit. turns out my old pistons are 7.4's. so when the new piston was fitted i noticed a rise in the deck heigth of the new piston not knowing if i need 1 or 2 C/P's in order to make the head fit.
Ps i would have thought that the new higher compression omegas would have had the same deck heigth as STD. 7.4 and the dome changed, so it appears otherwise.

so thanks guys for the answer.
 

timetraveller

Well Known and Active Website User
VOC Member
I would always put the heads on first with a little plasticine on the valve cutaways on top of the piston, enough to overlap the top side of the crescent. Assemble with cams timed correctly and tappets done up and then turn the engine over a few times. Dismantle and look at the plasticine, cutting through it with a sharp knife if required in order to look at the cross section of the plasticine. It depends upon what cams you are using and whether you are using standard sized valves. Also on whether someone has shortened the cylinder muff hight in the past. It could save a lot of time and grief later. It is not just that the valves can hit the flat in the cut aways but sometimes the cutaways are not large enough and you need to increase the radius with a scraper or sharp wood chisel. If plasticine is not a word you are familiar with it might be the same as 'play dough' but I am not sure.
 

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