February 13, 20251 yr I'm a little confused as to what I should run for the ring gap. I'm using an aftermarket wossner forged piston, 71.44mm. Their online ring gap calculator gives the following suggestions: naturally aspirated: 0.29mm (0.0113 in) naturally aspirated racing: 0.32mm (0.0127 in) snow/marine: 0.36mm (0.0141 in) forced induction: 0.39mm (0.0155 in) heavy load/high heat: 0.43mm (0.0169 in) I'm guessing since it's air cooled I should go for 0.43mm (0.0169 in). I've never done this before so I wanted to ask people with more experience than me if this sounds about right. I'm not sure what the oem manual calls for or if it even applies since I'm using aftermarket piston and rings. I just got the case halfs together last night. It's been 15 years since this thing last ran and I'm looking forwards to it, the internals were all original when I took it apart.
February 13, 20251 yr Unless you are pinning the throttle and dumping the clutch, trials is one of the least demanding of engine applications. Maybe equivalent a lawn mower. The greater the possible temperature difference between the piston crown and the cylinder, the greater the ring end gap should be. A bigger gap is safer than a smaller gap. But a bigger gap will lose more combustion pressure / performance. The naturally aspirated spec should be fine. If you are a worrier, go with the naturally aspirated, racing spec. As with any engine, always let it come up to operating temperature before going wild with the throttle. The correct piston to cylinder clearance is equally important. Break it in according to Wossner's instructions.
February 13, 20251 yr I'd go by the Yamaha service manual and by my read 0.4mm ring gap is within spec 👍
February 13, 20251 yr The thing with the Japanese OEMs is that they sell rings you can just install without any attention. Wossner has chosen a more performance-oriented route.
February 14, 20251 yr Author As with any question I ask on forums I always find the answer right after posting the question. Found a .pdf of a service manual and it states top ring: minimum 0.3mm maximum 0.5mm second ring minimum 0.2mm maximum 0.4mm I feel much better now, I don't want to be doing this again for a long while. Thanks guys.
February 19, 20251 yr It's important to remember that forged pistons are heavier, have different expansion characteristics and are usually preferred for high performance apps, for their greater strength. You may notice more vibrations due to the weight, and you should plan for a longer break-in because of the different forging densities that cause somewhat uneven expansion.
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