Originally Posted by West03
Ok, now explain that in english
I wish I could - it makes sense in my head!! But I'll try anyway:
**important to the people that might crap on me for this: this is just how I see engines today, and its only my opinion**
The bit of the head button that sits inside the sleeve has it's own length (i've never measured it, but Im guessing between .12's they would be very close to each other from different manufacturers, and the same for .21's)
Say for example, universally they can all sit 6mm inside the sleeve, for example. The shims change the depth/how far it sits inside the sleeve. Not entirely true for this next bit, but the exhaust port - being the last port closed/first port open, more or less determines how much collected air/fuel mix can be compressed.
*thinks about what to explain next*
Because of crank/port timing and new technology and whatnot, more and more engines today can cram more air/fuel mix into a single stroke and make more power, but then there comes a limit to how much you can compress before you start having problems (undershimming for example).
So to be able to control the amount of compression made, and to be able to adjust the ignition timing, shims are there so you can advance timing or retard it - almost like how you change the spark timing through the dizzy on a full scale car (12-8 deg BTDC)
Something like that!