Need cylinder/piston info for 66cc Gt-5

[Tim Bailey]

I recently cooked a piston/cylinder on a what I think is a Gt-5 motor. I ordered a cylinder piston kit on FleaBay. Little did I know that there are two different rod lengths for these motors. My engine has a long rod but they sent me a piston for the short rod motor. Question, are the cylinders the same for both rod/piston set-up? I found a post from Jaguar that has port dimension/locations etc. and this was very helpful. I did find some differences between my old cylinder and the new.

 

[Steve Best]

No, there is a slight difference in the cylinder heights too.
The long piston 66cc that I have has a slightly longer cylinder too.
The difference can be made up with extra base gaskets or some filing if you must.

Steve

 

[tbailey4]

Thanks Steve, I noted that the new cylinder which is for the short rod is about 1mm taller than my original long rod cylinder. The transfer ports are a little different as well not sure of the effect that this will cause. Have you made a conversion like this before?

 

[Steve Best]

I have done many conversions on other 2 strokes, but not so much on the China Girl.
I have 2 different 66cc engines apart right now. The Grubee GT5A is pretty consistent with its spares. The 2012 black unnamed motor has slightly different intake, exhaust and transfer port configurations. It is an "angled fire" head but significantly different than the GT5A. All the parts mostly interchange, but you have to be careful. The piston has a taller height and longer pin and takes a 16mm bearing rather than the more common 12mm. Since all are nominally designed for a 38mm stroke, as long as the piston comes to the top of the cylinder, your port timing will be right. Shim or cut the cylinder, or swap pistons to get what you need. Height of piston skirt will make a difference in the intake timing but can be adjusted. Watch out for wrist pin bearing length. If it is too short it can shed roller pins. The bearing length must fit the piston.

All the cylinders I've looked at seem to be designed for a 42-43mm stroke, any one else notice this?

Steve