Motorcycle Repair: piston rings, piston head, piston kit

Question
Hi, I have a 1971 honda sl 100 I am working on and i got it running but one of my rings are cracked. the engine was bored out, and i looked on the piston head for the size of the new piston and all it says is 150cc. Now my question comes in where I dont know what size rings to buy for the engine. Do I buy rings from another bike that has a 150 engine or do I buy a bored out size rings for the sl 100 I got, and if I do buy for the sl 100 what size rings would I buy? Also, I had it running fine before I broke a piston and I was running the engine straight of itself but then when I turn it off one day and tried to start it back up again I realized I didnt have a spark so I put a battery to it and know it has one. But, what I am asking is do you know why that probably happened? Thank You Stephen

Answer
Stephen, I will try to understand all of your question, but the short answer to your piston ring problem is to have a machine shop accurately measure the bore size of the cylinder and piston. There is no 150cc engine of that era.

Big bore pistons for 100s are hard to come by, as they use a smaller 14mm wrist pin. SL125s use a 15mm pin size and there are more pistons available for that size engine. Either way, the cases would have to be bored for the larger sleeve size of a big bore engine.

So, from my perspective, you could have a 100cc engine and someone reamed out the wrist pin hole or someone could have installed a 125 crankshaft or whole engine assembly in the frame.

http://www.powroll.com/P_HONDA_VINTAGE_100-125_EARLY.htm

If the piston is 61mm, it could be an early CB750 piston or some kind of after-market piston kit. You will have to consider both the bore size, as well as the ring thickness, when you are searching for replacement rings.

In regards to your engine running w/o a battery. Normally, these bikes all have battery powered ignition systems. The charging system uses a permanent magnet rotor, which generates electricity across the stator field coils anytime the engine is turning over. If someone rewired the charging system to put the output of stator into the rectifier, then back to the coils, the engine will make a spark, often enough to run the motor. If the system is run like that, unregulated by having a battery in the circuit, the rectifier can overheat and fail. You may now have a total loss system, instead of a fully functioning charging system. I don't have enough details to make a more accurate appraisal of this situation.

I do know that the early 100-125cc cylinder heads had very weak camshaft bearing surfaces and that the cams get loose in the head, which affects the ignition timing, due to variable point gaps at different engine speeds. This can cause piston seizure or failure, if the timing overadvances due to the wear between camshaft and cylinder head.

Bill Silver