Break In Desired piston ring gap

[Wolfshoes]

A new Gasbike replacement engine block arrived with the cylinder needing to be attached to the lower casing. After assembly, it appears to start up ok with a downhill start and a lot of pedal assist and after a lot of effort, idle for a few minutes at mid range RPMs at full throttle. My guess is that the rings are too tight and are expanding faster than the cylinder under power. This would explain why the the engine will not keep running since it takes all the power at best to maintain idle. With all the talk being that the gap has been traditionally been too wide, I have not seen in print a recommendation of how to inspect and measure, when the engine is assembled, the piston ring gap width and how to file the odd shape around the non rotating pin inserted in the cylinder without going too far. With the odd shape, I could believe the only measurement may be a visual micrometer, but any standard during assembly would be better than risking a tear down to file it later, and perhaps broken rings. There must be a standard in general for piston rings gap of that size in a two stroke.

Since you asked, the engine piston (which was factory preinstalled in the cylinder dry) and wrist pin were reinstalled using Marvel's Mystery Oil. I was going to run it right out of the gate using 100:1 Opti 2. Since the engine was so tight I hedged my bets to keep from breaking the rings by running about 1 oz per gal of Mystery Oil in the Opti-2 mix. The previous engine was broken in this way for the first gal of fuel used.

 

[crassius]

not sure what you mean by 'engine was so tight' - does piston have a number stamped on top? these numbers mean an oversized piston - about 20 micrometers per number

I'd be more inclined to suspect cylinder to have different deck height or port map than old cylinder, unless you can feel that it is hard to turn the motor.

 

[Wolfshoes]

I have 2 new 66 Gasbike pistons that have the arrows pointing opposite directions relative to the ring pegs. There is a recommendation on the other forum site that the ring pegs are to be on the INTAKE side. Is this correct?

 

[crassius]

yes, no matter how piston is marked, the locating pins are always on intake side

 

[Street Ryderz]

A new Gasbike replacement engine block arrived with the cylinder needing to be attached to the lower casing. After assembly, it appears to start up ok with a downhill start and a lot of pedal assist and after a lot of effort, idle for a few minutes at mid range RPMs at full throttle. My guess is that the rings are too tight and are expanding faster than the cylinder under power. This would explain why the the engine will not keep running since it takes all the power at best to maintain idle. With all the talk being that the gap has been traditionally been too wide, I have not seen in print a recommendation of how to inspect and measure, when the engine is assembled, the piston ring gap width and how to file the odd shape around the non rotating pin inserted in the cylinder without going too far. With the odd shape, I could believe the only measurement may be a visual micrometer, but any standard during assembly would be better than risking a tear down to file it later, and perhaps broken rings. There must be a standard in general for piston rings gap of that size in a two stroke.

Since you asked, the engine piston (which was factory preinstalled in the cylinder dry) and wrist pin were reinstalled using Marvel's Mystery Oil. I was going to run it right out of the gate using 100:1 Opti 2. Since the engine was so tight I hedged my bets to keep from breaking the rings by running about 1 oz per gal of Mystery Oil in the Opti-2 mix. The previous engine was broken in this way for the first gal of fuel used.

There is a proper way to check the ring gap you simply place the ring in the bore and use a feeler gage to messure the gap.When you get used to seeing the proper gap it's not hard to tell when it's too big.

 

[Wolfshoes]

The pins were located properly and ring gap was the usual grand canyon. Judging by the chrome polish pattern on the cylinder, the lower wall was a bit egg shaped. I changed the cylinder with a replacement part, also from Gasbike, and the engine is running better. Their replacement cylinder was shorter allowing the piston to hit the head so a second head gasket was installed for now to add clearance. The head may be a good candidate for a squish band mill job and then go back to one head gasket. Gasbike's replacement piston assy does not fit their engines. Not even close. Thank you for your help in getting the engine up and going.