I'm in the process of having my build done. I got some referrals from some members and found what I thought to be a reputable builder. One of the things he is doing is milling the heads to up the compression. When I shared that I immediately received feed back that I should have used different pistons vs letting them mill the heads.

I noticed that lots of the recommended builders, that people use, mill the heads on their builds. So....can any of you proffesional guys share why you choose to mill over replacing pistons ???

My basic understanding of why a builder might do it is: It allows for being able to increase compression in a more precise way and, and that by milling the head you increase compression but can still use a flat top piston, domed pistons to increase compression are more likely to interfere with air flow in the chamber and ping. Or something like that

I've already decided but am interested in learning why ?

Thanks

 

If you use adjustable push rods you're ok. Milling heads with stock push rods will cause a lot of problems. Bent push rods, bent valves and or holes in the top of your pistons.

 

More than just pushrods, you change angle of one the intake track, I've had to shorten intakes to get them to fit in the bottom side, although I do agree with better flame travel across a flat top, I guess its all in how you wanna tighten up the squish, reguardless, you're achieving the same thing either way ,

 

Excellent point!

 

I understand that milling heads requires you make other changes to make them work.
My question is why do legitimate builders choose to do it. I'm sure they do it because they believe it to be a better choice. Why ?
I mean if you walk in and tell a builder money is not an object. I want it built right and this is what I want to use the bike why would they mill heads and not use pistons ? I can't believe that guys who's livelyhood depends on their reputations would choose to mill heads when the could just slap in some pistons ???

 

Like they said...But hopefully in planer terms...If you mill your heads you will change almost all of the the angles, and some of the distances, for everything critical that bolts up to the head.. The vaulve covers will remain the same... But the intake and exhaust manifolds may even have to be machined... valve lash will change. I dont reccomend it. put the bigger piston in and suffer those conquences....

 

my heads were milled to get rid of a slight ridge in the head surface from the mfr. at no point am i worried about my top end. readjusted the pushrods to compensate for the difference.

and its not just slapping in pistons. a good engine builder will plan the motor out to ensure all the components work well with each other

 

you can use thinner head gaskets to accomplish the same thing

 

Raising compression is most often done by using a thinner head gasket like a Cometic .030 vs HD stock .050. Its a common method used when not doing a big bore job and with stock heads. It can be done with big bore jobs as well.

Domed pistons are often used with big bore jobs where the builder wants a larger jump in compression like from stock 9.2:1 to 10.5:1 but with stock or slightly modified heads. Shaped dome pistons raise compression and can alter the flame travel to help evacuate the cylinger better for exhaust flow.

Milling heads is often used to get the chamber CC down to a specific number on blue printed competition motors. This is usually followed by chamber work to get the gas mix to flow in a certain way to max out the combustion process or to aid in the flow of exhaust gasses. Often when chamber & head work is done the builder will use flat top pistons set at a low squish number...ie: all the work is in the heads.

There is no specific correct way to raise compression. ie- gaskets, pistons or milling, beause each mehtod will do it to some extent. Each builder has their method based on their experiences for the specific build at hand. Sometimes all three are used to get a certain HP/TQ number at a wanted RPM range.

Just my general opinion based on my past motor builds.....others will differ.

 

I would go with swapping pistons to get the compression, you can put in forged pistons which will be more durable then the factory cast pistons.
If you really wanna do it the best way I'd say get a big bore kit with forged pistons. Now you're increasing the displacement as well as compression.
Hope this helps