i recently had the backplate off on my picco .28 and i forgot to put the screws back in and when i yanked on the pull start the backplate spun and put a dent in the piston and it wouldnt fit through the sleeve right...so i figured what do i have to lose and i cut on it a bit with a dremel. i made a smooth and clean cut and now it fits through the sleeve like it did before..and im wondering if this will affect the performance on the engine at all. thx.

It most probably will affect the engine. The engine might run a risk of disintergrating or if not, you will have extreme loss of power. I would just buy a new piston and sleeve, or even a new engine (almost the same price).

How much piston material did you remove? Put the piston at TDC and look at the bottom edge of the sleeve for any air gap between the bottom of the piston skirt and lower edge of the sleeve.
You might just have SPI'd your engine (S.P.I.= Sub Piston Induction )
SPI is an old racers trick...it might not make any difference, or...??? There was a long thread a while back about improving mileage or something, that entailed cutting the bottom of the piston on the E. side...but how that might work with your engine and pipe is the question.

If you didn't need to remove much material, and didn't bend the rod then I'd say try running it and see if it still runs o.k....take it easy at first just in case.

i just cut about 3mm into the skirt and a 3mm width of the skirt..i checked and there is no air gap. its a perfect fit.

well proptop i just got done runing the engine..its on a savage twin x2 picco .28 with 18cb and the one with the cut piston will still flip it over..so hopefully it will be fine, and who knows maybe better fuel economy

the thing about the SPI, is you have to have it big a noughf to work well, but not to big. if its too small then the engine will not idle for a long time, but too big you will lose power.

theres a thread that has more info on this, somwhere on here.

well today when i ran the engine it seemed like it was loading up with fuel to fast to burn it all...tomorrow im gonna check if i can even see the notch i cut throw the exhaust port..if i cant see it then the SPI shoudnt matter, right?

SPI is a no no unless you want to mix exhaust gas with fresh air while there's that great big hole through the crankshaft still open trying to get as much clean air into the crankcase as possible. Don't do it unless you're using a reed valved engine with an open exhaust . Regardless of the wild claims made in that other thread.