big bore installation

[adimassa]

Can any one clarify this will stock cylinder head bolt on big bore jug.not familiar with break in procedure on 2 stroke.

[Wheelman-111]

Greetings:

In a word, no. The combustion chamber volume is that space that remains above the piston and below the head. Its size is carefully carved to achieve a survivable pressure when all the volume contained in the cylinder is crammed up into the attic. If you increase the size of the piston without enlarging the space under the head, well... there's just too many molecules trying to occupy that relatively smaller space. Compression ratio jumps to the moon, and a hot glowing ball of fire burns a hole in your piston crown to let them escape. You can add a head gasket or two to make that much more room, but the gasket should be the same bore size as your new cylinder.

[Kevinstonge]

I fully agree with wheelman, however there may be a couple exceptions depending on what engine and bore kit. My polini bore on my variated AF05 calls to use the oem head.

[adimassa]

Thank you to those who responded.I used the same heads when building large displacement gas engines for autos. So since the dome is the same and head volume is the same, compression might be down a minor amount how ever displacement increase should be a win win. Just needed clarification on subject I will inspect gasket at jug and head when parts arrive. Not much on theory just don't want to do it twice/

[Wheelman-111]

Greetings:

Compression will be higher, not lower. The polini for SE05 is an exception, but most kits come with a cylinder head. Measure compression before ignition. More than 150 on pump gas is risky.

[maddog]

Never use a base gasket with an o- ring gasket, this will lower compression.