I'm helping a member set up an engine. This is from a PM... Maybe some of this will help the rest of you understand what is happening inside these scoots (2t's).
I can help more if you need it...
"Yes the crank 1/2's can very easily twist on the con rod pin. You have to support only the side you are working on. Let the other side just hang in space.
On some of the better bearing sets there will be a thin band of Nylon on the outer race. The area that gets pressed into the case. That goes toward the inside IF you have it.
The nylon cage itself does not matter which side. Actually the Nylon cage is only quieter than the standard steel or the better bronze cage. Some people think they are better but in truth they are not. Bronze will outlast the engine cases. The problem with them is the hot/cool cycles. They get brittle, and fall apart. Just like plastic in the sun.
If you plan to keep the bike for 4-5 years then I would consider getting better bearings. If you want to sell it sooner... no issue.
I wouldn't use the toaster oven either. It will melt the cages. I would use a pot of boiling water or a pre heated oven.
The toaster oven goes full power untill it reaches temp. then it throttles down. You need at least an hour in a pre heated oven or same time in a pot of boiling water. Water hits 212* f, and no hotter. It's the safe bet
BUT you will only gain perhaps 0.0002-3" on expansion. Thats why I suggest to be ready with the press. You will need it.
Temp on the piston... Melting point is somewhere near 1200 - 1400* f depending on the alloy
Normal combustion temp is around 16-1800 * f. 400 is the magic number that many use for exterior head temp. I NEVER use this number because you can close the throttle on a long downhill, and see 5-600*f in heat soak, even tho the cylinder temp is cooling.
Here's a good article on this...
http://www.contactmagazine.com/Issue54/EngineBasics.htmlThe Ignition timing on the Zuma begins around 15*BTDC just like all the other scooters, and slopes straight down to 0* at 12krpm. What this "curve does is gradually push the cylinders heat out into the pipe.
The ONLY chance of melting a piston is from Pre-Ignition (carbon/sharp edges, like a piston edge or spark plug hole, port edge, casting marks on the piston dome OR lean mixtures creating pre Ignition.
Detonation is controlled by the squish band. The band forces burning gasses into the center of the flame front to prevent runaway combustion. Listen close for crackling in the pipe. That crackle is detonation. You need more cool fuel mix to stop it and or a tighter squish depending on the head.
A relatively shallow or flat across head is more prone to detonation, and needs a tighter squish. A bell shaped head or high dome head needs less squish.
If you have or can find a large ball grinder or make one from sandpaper, and a golf ball, it pays to hollow the center of the head. You'll lose a few compression ratio points but lowering the jug to just a note paper gasket (if you can) will gain ax rpm, and reduce the chance of detonation.
It's a f***ing can of worms. One little thing messes with everything else. Even the wrong rollers can mess up a perfect cylinder... I hate scooters.
