Hi Guys
i have an old chain saw two stroke engine which is a 33 cc short block engine with a walbro carb and it truns an 18*8 prop at about 7600 RPM but i want more power from it so i decided to increase its piston diameter to 40mm (stock one have a 36 mm diameter of piston) but i am not sure that will it work and increase its Rpm and power and do any one have done this before?and is it possible?
i also want to know that how to alter the port timing of that engine and if you guys have some links about such type of alteration then let me know


waiting for your reply
Abdul Samad

If you can find the cylinder piston/wrist pin that fits crank case and connecting rod, go for it, but don't aim fo more rpm. Displacement is torque. With higher rpm you are liable to run into vibration problems if the piston is heavier.

Hi
thanks for your reply also want to inform you that i also have two brand new pistons along with rings and wrist pins one of them is 40 mm and the other is 42 mm but the wall thickness along with the old piston is 5mm and if i put a 40 mm piston then it goes to 3mm wall thichness and a displacement of 42cc and with 42mm piston it goes to 2 mm wall thichnessand the displacement is 47 cc so please suggest me the best piston to replace with


waiting for your suggestions
Abdul Samad

It sounds like you plan to bore out the existing cylinder for the larger piston? That may get into a lot of things like strength and heat transfer, etc. Maybe you can get a piston and cylinder from a larger cousin of the same engine. I think some manufacturers do that to increase their engine sizes, just put a new jug and piston on the same crankcase. Just a thought.

Good luck.

Richard

A lot depends on cylinder fastening.
If it has a cylinder foot flange and is fastened with short bolts, the increased combustion pressure will act on a decreased (60%!!!) cylinder wall.
I would not go for the largest piston. There just is not enough meat to work with.

You cannot bore these cylinders.

Some are hard chrome plated and others have a hard hot metal sprayed in them to act as a cylinder wall

you can, if it's cast iron.
you also can if the cylinder is a high silicon alloy and you apply a new Nicasil layer and final grinding to spec.
Been there in both cases in the previous century.
Given enough material, you also can put in a meehanite liner.