You are correct. But if you have "a good engine builder" mill larger pockets, You might as well have them put the whole thing together to ensure it works. Milling pockets will have to be tested with some clay first to make sure they work. The other option would be to get the correct cam & piston combination and put it together yourself. Personally, after running the 14:1 piston with the .425 lift cams for a while, I am going back to a 12:1 piston and A1A2 cams for thier quickness. The 14:1 has a lot of mass on top, provides good torque and horsepower, but it's not as quick revving at the 12:1 with the A1A2 set up. If Shmee can find a happy meduim without it costing too much in builder time, then go for it!

 

When you mill a piston how much compression is lost in the process. I know it will depend on how much you take off. Just trying to get a idea.

Thanks
JMiller

 

I would think the amount of compression lost when milling a piston for valve clearances would be minimal. The only for sure way to check the compression ratio would be to CC the head, measure deck height, and enter all the info into a Engine Size/Compression ratio calculator. You can find these java programs for free on the web.

 

Compression loss depends, like you said how much material you remove and how much compressed volume you have for example:
on a 650cc engine with a 60cc compressed vol =11.83:1 C/R
If you have to deepen all four valve reliefs 0.5mm and the V/R's have 36mm radius and 18mm width (symetrical) you would lose about 0.508cc.
This would give a C/R (based on the above volumes) of 11.742:1
Oregon is right, you would have to assemble the top end (with clay in the valve reliefs) roll through one valve cycle, with the cams degreed in then remove the head and measure the valve to piston clearance to know how much to remove.
Dynoray

 

Thank you for the responses.

JMiller

 

ya ditto!!! [img]i/expressions/face-icon-small-smile.gif[/img]

 

killer thread, great info.

I asked MrHP a while back if I could use his 14:1 piston with my a1/a2 cams if I stuck an extra base gasket on, knowing this would drop compression slightly but just curious...
His reply was that I should be able to use the 14:1 no problem with the a1/a2 cams, no need for an extra base gasket, but that he does recommend a clay test to be sure first.

Oregon, have you tested this combo, and know for certain?