GT-5, 66cc Cylinder info needed

[tbailey4]

I recently cooked a piston/cylinder on a 66cc engine. I did not realize that there are two different pistons for the 66cc engine. Apparently, and please add to my knowledge when you see gaps, there is a short connecting rod motor (Pk-80) and a long connecting rod motor (Gt-5). Not aware of this difference I bought a cylinder piston kit on FleBay. Of course the piston is for the short rod and I need the piston for the long rod. I measured the old cylinder vs new cylinder and there are some minor differences. I found a a post from Jaguar that listed the dimensions for the ports measured from the top of the cylinder and used this for reference. My questions are, should the cylinder port arrangements be the same for both pistons? In other words, can I just buy the correct piston and use the cylinder I just bought? Note: The new cylinder is a full mm larger when measuring the distance from base gasket surface to head gasket surface and this has me concerned. Thanks in advance for any knowledge shared.

 

[butre]

you can use it, but you'll be down on compression. use a Teflon head gasket to bring most of it back

 

[45u]

Yep you have a type A and a type B piston. Best to use the correct one.

 

[Tim Bailey]

I opted to remove the base gasket which is about 1mm. I measured the ports on both old and new cylinder to see where the differences were. I also used some dimensions that Jaguar had once posted. One major difference was the transfer ports. On the old cylinder, the top of the transfer ports was relatively flat, on the new cylinder the top was sloped by about 2.5 mm. By eliminating the base gasket this brought the top of exhaust and bottom of intake close to the original cylinder (and Jaguars numbers). It also brought the transfer port Average Dimension close as well. Started right up, idles better than before and has acceptable power. It should be noted that I had a hard time getting the motor with original cylinder to run right. No low end, no idle, and I had to run it very lean to get top end out of it. Another note: I have a twin bike with stock FleaBay motor that has great low end and pretty good top end. Same pipe, same intake, same carb. Its nice to have two bikes to really see how changes effect the performance. Cheers.


Measured compression is at about 150 psi.

 

[Deleted member 12676]

at that pressure it will run too hot which can warp the head and lose power as the piston gets too hot.
also the extra combustion pressure can cause the cylinder plating to flake off.
Put on another head gasket (best to make your own out of hi temp gasket material) to lessen the compression

 

[Tim Bailey]

I'll double check the compression. I normally warm the engine up, install a comp gage with flexible hose and pedal the bike to cranking speed. I notice pressure is dependent (somewhat) on my peddling speed. Is this the correct method? What is the desirable pressure? Thanks

 

[Deleted member 12676]

I don't recommend any more than 130psi