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Chewed spark plug

M

Murraydali

Just got it firing!
Joined
Mar 30, 2012
Messages
9
Hi,

I have an '81 Lemans 2 with 88 mm barrels (iron liners) and pistons. I have owned it for about 6 months, but have put very few miles on it getting it ready for registration. The heads and barrels were recently off to replace the base gaskets, which the previous owner had installed wrongly so the oil return passages were blocked... (So it was smoking badly and fouling plugs). The bores, pistons and heads were all in sound condition and no evident problems. Once everything was buttoned up again it was running a treat.

I just replaced the oil fouled plugs with new NGK BP7ES - from researching this forum and others the consensus view seems to these are the best for the application. I did note the old plugs were NGK BP6HS. Prior to installing the new plugs I noted the reach of the plugs was an extra 4mm. This gave me pause for concern so I started and idled the bike, then revved it a few times before shutting down and pulling the plugs for inspection.

The left hand plug was damaged. The ground electrode had been biffed and was pushed against the central electrode - not badly chewed, but a couple of minor hits. I'm assuming the increased reach of the plug has put it into contact with valves (or piston?).

I re-installed the BP6HS plugs and the bike seems fine, with no ill effects.

I would welcome views from you why the BP7ES plugs clearly don't work in my bike. These plugs seem to be the accepted standard for this model.

Thanks
Paul
 
I don't know Dan. I'm assuming it was valves that biffed the plug, rather than the piston. Seemed the previous owner (who's unfortunately no longer around) knew enough to install different plugs...

Edit - actually valves doesn't make any sense Dan. It must be the piston..
 
That's weird.

Either the bike's squish is exceedingly tight - I run 40 thou which is quite tight and have never had a problem - usually when it is that tight valve clearance becomes the problem. Or non oem piston were fitted (The mark 2 came with nicasils - way better than iron bores) or for some reason the sparkplug holes were incorrectly machined - also unlikely.

I'd think about claying the piston to check what clearances you have.

If you want to use the 7ES you can get indexing washers at any hot rod shop.
 
Silly question, but the thread height and ground electrode height are the same between the two? It sure doesn't sound like it.
 
I think you'll find the E means extended reach, so the electrode protrudes further.

I suspect piston clearance, could be from different pistons or the barrels could have been machined. Stick what works for the bike. BTW I think 7 might be a little cool for a Tonti, I seem to recall using 6 in all mine, my LM1 uses the 6
 
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