It add's duration to all of the ports on the sleeve, easier and faster than doing it with dremel or hand tools.

Does it not increase volume of the combustion chamber? Higher RPM?

What thickness is that shim? 0.5 mm?

Yes, so it would help to know if they readjusted head shim to keep original combustion chamber size, so that way they gained higher Rpm's with port raising, and kept torque by reducing shims.

AFM

Yup, that makes sense. I guess Mr. JP was a little lazy or ran out of dremmel bits.

If you shim below sleeve to raise it that will also raise your deck height, if JP used same shims for deck height or removed some to keep the same deck height we dunno, just you might need ot machine your button some so you are able to get it low enough to have proper deck height and a shim between it and sleeve for a good seal. I'v enever ran w/o any head shims before but would be leary that it wouldn't seal good or the head might want to come loose.

Well, if you remove the .3mm shim from under the button and add it under the rim of the sleeve, there will be zero nett gain or loss to the combustion chamber volume.
Of course the ID required for the under sleeve shim would be different.

True. But like what others have mentioned, the exhaust height plus all the other ports are raised by 0.3 mm if 0.3 shim is added under the rim of the sleeve.

Higher all ports at the same time(not only exhaust height). I have tried this way before.

How different did it make the engine? Better or worse? Did you reduce the head clearance when you raised the piston sleeve? By the same amount? Thanks Dino.

Yes,reduce the head shims by the same amount. Engine will have better high end because you will have more than 126degree intake during and higher exhaust during....but not easy to adjusting. you can try...

I have also tried to lower the sleeve 0.2mm on my buggy engine for better low-end. that is another trick.

Yep, it will raise all the ports and I could only presume Mr JP knows what he's doing.
As to the overall effect, I have no idea.
I prefer to just buy 'em from the people that know what they're doing. I'm unlikely to catch up to their level of knowlege that separates a good engine from a blistering one.

It is depend how much you will put under sleeve shims. great amount will kill bottom end completely-engine will be for speed only ( realy fast on the end of long straight) but it will be realy slow on bottom end. I have tryed to raise sleeve on 21 Nova by 0.5 mm ( all timing raised by 11.5 degree)-it was no good!!!!!! Don't go there, it will be very high speed engine with no bottom end at all.

Yes.

Before you mod your engine,you need to think WHAT is not enough or WHAT is too much. If you don't have any idea,don't take risk. You can get the same result on gearing and pipe tuning. When it is not enough to you,then just think about mod it.

0.3 mm height change is 6 degree difference on 12 engine, it will not chnage a lot, it will defenetly add top speed in exchange of little bit bottom end-one of the modding for particular track (high flow tracks)