any access to a lathe? trim the crown of the piston... but 4mm is heaps! down to the top ring land! that cant be right? if you turn it over without the head, how far does the piston stick out? just remove that much...
might only need a trim... a bit of modelling clay helps to check clearances
gah! its 4mm matey!
these are metric motors and should be measured as such! gripe over... it might actually be 3.5...?
thats a lot. and well, shimming up the cylinder is probably the only answer. if turning the piston is out of the question. this can be tricky. it has to be 4mm( or 3.5?
) ali plate cut precisely where needed and dead flat.
using anything "squishy" at that thickness is a no no. ali or steel plate. repeat after me. ali or steel plate. no matter how hard it is when you squeeze it, you are not a running engine! it is made from ali and steel for a reason!
you can do it if you have a fret saw file drill etc, combined with top notch metal fabrication skills,but its hard to not warp it. get it machined...made of cash? could get fancy with CAD and get it laser cut in various thicknesses...and materials! not terribly expensive and saves a lot of frustrations.
if the pistons 4mm higher, shimming the head alone will reduce your port timings to something ridiculous where it wont rev over 3000rpm or so... regardless of your comp ratio which will have increased anyway by having reduced the exhaust port timing...(if you measure comp ratios from the ex port closure)
you have to get the edge of the new piston to the same point in the new cylinder as where the old piston and cylinder were at TDC. stands to reason. um...iunno! id be getting a new one by now...
gawd im lazy...