Back in 1982 I was doing my usual "rebuild the bike the day before the Enduro" thing..
Registered YZ125J, my first MX bike, and I was 18 years old and just getting into the swing of having to regularly put in new rings, pistons etc..
Anyway, I was only going to put a new ring on the existing piston, so Saturday afternoon I fitted the ring, put it all back together and tried to start her up..
No go – it wouldn’t start. It had been running perfectly earlier that day. After trying everything I could think of, I rang Rob Assink (Gaythorne Yamaha owner and tuning guru) at home, and after explaining the situation, he asked what I had checked. After I told him it had a new ring, but seemed to have low compression, he told me that the ring was probably stuck in the ring groove. I hadn’t cleaned it out before putting the new ring on.
So I went upstairs to have dinner, vowing to pull it apart again straight after dinner. One thing led to another and it was about 8pm before I went back down stairs.
Sure enough, the ring was stuck in the groove. So off with the piston (I’d left the piston on first time through), clean the ring groove, fit the ring, on with the piston, on with the barrel, reconnect the cooling system, put all the plastics back together etc.
But the bloody thing still wouldn't start! Again I checked everything I could think of... But still no luck...
In a bit of a panic at the thought of missing a round of the Queensland Yamaha 6 Day Series, I again rang Rob Assink. It was only when he finally answered the phone sounding half asleep that I realised it was after 1am on a Sunday morning...
To Rob's credit, he took the time to ask what I'd done this time, what I'd checked this time etc... Then he told me to pull the pipe off the bike and shine a torch into the motor...
And THAT is how I learnt what the arrow on the top of a piston is for.............