1940(ish) M20, iron head. Recently purchased, no history known (i.e. whether it was in use in its current configuration, or has had a rebuild and parked).
It currently has a Champion L29YC plug installed. 1/2"/12.7mm reach.
If I gap the plug, install and crank the engine over then the plug gap closes up. I've verified it's contacting the piston by using a white paint pen on the plug which upon removal shows witness marks.
What might cause this? Everything I've read suggests the plug reach is correct. Significant head skim perhaps?
What are my next steps? Install a shorter reach plug (seem to be available down to 10.9mm) or pull the top end?
Thanks all. After reviewing pictures/the bike I now see it's over the valve, d'oh! I have no experience with these sidevalve engines. I assume there's no adjustment on valve lift then.
Thanks for confirming re the head skim, good shout I'll try a copper washer first. I see you can buy 1.2mm thick copper headgaskets on eBay which may help?
A 1mm solid head gasket would be the correct one to maintain the squish area inside the head to its correct dimensions...Spacing the plug is the way to go as long as it's not dissappearing up the threaded hole it screws into when fully tightened....Shrouding the plug too much will detrimentally affect combustion...
The only way you can have more valve lift than standard is if someone has fitted an alternative cam or modified the original.....Ian
Hi Iain,
Your problem is simple. The L29YC is an extended nose type, where the centre contact stands out further and thus the earth contact as well. Champion part numbers with a Y in them denote extended nose. The C denotes copper core. You need to go back to a conventional style plug, simple as that!
I wish I'd checked your replies a few days ago! I've just bought and installed a new plug and realised the 'extended projection' of the old one after comparing. D'oh. All sorted now, running loads better and without any copper washers/valve contact.
