#2 piston hits spark plug - Alfa Romeo Bulletin Board & Forums
 
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post #1 of 3 (permalink) Old 07-25-2010, 07:33 PM Thread Starter
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#2 piston hits spark plug

The #2 piston on my 1967 1600 hit the spark plug, and grounded the electrode. It did it on 2 new plugs also. I put another washer on the plug for the #2 cylinder, and it seems to have given it enough clearance. I really don't want pull the motor in the summer, especially since I'm registered for the Tour de Marque at the Glen. The other pistons look fine. #2 however is black with a nice little divot where the spark plug hit. Looking at the other 3 from the spark plug holes they are nice and clean. You can even read the numbers on them. I'm hoping it's a worn piston pin bushing, and it doesn't hit the exhaust valve. There isn't a knock that would indicate a lower end bearing. Has anyone else had a similar problem? Will I cause more damage by driving it carefully (no drivin' it like I stole it) till winter?
I posted in the engines forum, and I posted it here also, so it gets the most coverage. If there's any way I can get the #2 piston, and liner out with the motor still in the car? Can the oil pan clear the support, and oil pump for removal? I know, lotta questions. Just trying to figure out how to salvage the season.
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post #2 of 3 (permalink) Old 07-25-2010, 08:09 PM
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Not piston, but valve. There's like a year of clearance between the piston crown and plug unless you've got a really stretched rod or the head has been decked to death. (with the affected cylinder at TDC on the compression stroke, look down in the plug hole and you'll see just how far away it really is)

Really high lift cams (11.5~12mm and up) can also create the issue as they're shoving the valve way out there into the area where the plug protrudes. Oversized valves will do it too as 'bigger' literally means 'closer' AFA the valve head to spark plug distance.

If you look closely at the 'divot' on the plug, I think you'll find that it is actually an arc shape pretty much identical to the diameter of the intake valve head.

Get some indexing washer for your plugs, (available at most speed shops), and using them set the plugs so the ground electrode is aligned fore to aft in the chamber when fully torqued. If that's not possible, set so the open side of the electrode is pointed more toward the intake valve as that's the one big enough to reach over and slap the gap shut.

Or, get plugs with less protrusion.
EG: NGK B7ES (short tip) instead of BP7ES (protruding tip)




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Last edited by Tifosi; 07-25-2010 at 08:41 PM.
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post #3 of 3 (permalink) Old 07-25-2010, 08:51 PM
Yup, happened to me just as Darren described. Big valves + big lift = shaved electrode.

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