Is it possible to mill your heads to bump up the compression? If so, how many thousandths? Cam belt problems? Just curious about an alternative to breaking the engine down futher. I appreciate your suggestions and guidance. Later, Dan
All of my feedback on this post is general content, not Duc specific. I've never had a Duc head in my hands, but I've milled a few heads.
Milling a head is a reasonably good way to get more compression.
On a 900, milling off 0.035" will raise your compression about 1 point.
Cams will be more retarded the more you mill off, but this can be rectified with offset keys or adjustable sprockets.
You may run out of tensioning pulley adjustment.
Valve to piston clearance will be reduced.
Valve to head clearance will be reduced.
It does reduce the strength and stiffness of the head. Usually that's not a problem on air-cooled heads. It can be significant on water cooled heads, depending on how thick the deck is.
It's not reversible. Once it's milled, the material is gone. If you get a set of higher compression pistons, you can mill them down
if they're too high. Or bin 'em and get a different set.
actually, you're better served to mill the lip on the top of the cylinder. but, go too far and you'll have issues with the o-rings. DEFINITELY figure out your squish first. actually, i care more about valve to piston clearance... also, once you do this, you may need to run the ngk D8EVX plug, not the DP8EVX-9. the D8E_ family doesn't have a projected insulator and i think has a lower absolute height.