QuestionHi, my piston blew on my sl125. It overheated and a part of the piston broke up. There is no other damage luckily. What would cause it to overheat this much? The oil smelt horrible.
Also, after looking for a piston I found that the Honda CB and CG std sizes are different to the sl. My piston measures 55.94mm diameter and the CG and cb models are 56.5mm. Does this sound right? Thanks very much.
AnswerJohn, Honda's first version 125s were 56.mm (122cc), then switched to the 56.5mm sizes for a 124cc displacement around 1976.
Honda CB500F pistons are now being substituted for 125 pistons in the 56mm sizes. If you find some XL125 pistons in the right size, you can get some increase in compression.
Piston damage, such as yours can be traced to incorrect ignition timing, lean fuel mixtures, lack of oil, primarily.
The early engines with the one-piece cylinder head often had worn camshaft/bearing surfaces, which allows the cam to wobble around which affects the ignition timing at the end of the camshaft. Any side play in the camshaft will be equal to a change in the ignition point gap, which causes a change in the spark timing.
Bill Silver
