I’m getting a hot Flash
While stripping his 1954 BSA Golden Flash, Pete Whichello found the left-side piston had been getting very hot – so hot that the crown had started to collapse and was cracked across the middle!
Says Pete: “The magneto that was fitted was 8° out from side to side, so I split the difference, making this side 4° retarded – would that be enough to do it? The centre electrode burned off the plug once on this side, too. I wondered if the compression is too high, although the pistons – numbered ‘11431’ – don’t look very domed.”
A piston has to get extremely hot to begin melting, and I doubt 4° of retard would cause that in normal riding. Hepolite listed compression ratios of 6.5/ 7.25/ 8 /8.5/ 9 and 10.5 to 1, but a shallow combustion chamber design meant that even the 10.5:1 didn’t look that extreme. The standard ratio for Pete’s bike was 6.5:1 (and actually had a dished crown); 11431 is 8:1, quite high for this model. It’s usual to retard ignition slightly for high compression (the flame has less far to travel), so although the raised compression may have increased temperature it should have compensated a bit for the retarded timing.
But I’m interested in this plug electrode burning away. That happened on my Gold Star when I fitted a plug one-grade softer (‘hotter’) than standard. Also I’ve had vintage side-valves that overheated and dramatically lost power if they were not fitted with racing-grade plugs; they became ‘glow plugs’ wreaking havoc with ignition timing. What if Pete’s plugs were standard A10 grade and didn’t like the increased compression, the extra retard on the affected side being enough to cause this preignition? Pete’s sorted the magneto now, but if he can’t get original lowcomp pistons, I would suggest a harder (‘colder’) plug grade.
‘A PISTON HAS TO GET REALLY HOT TO BEGIN MELTING’