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Mocking up the deck heights for the rebuilt engine today I noted that one of the secondary spark plugs was hitting the piston top. I've noted before that the secondary spark plugs protrude above the deck in the head, and one is apparently tall enough to hit the piston. I pulled out the old pistons and, sure enough, there's a clear impact mark in the same place on one of them.

Those plug holes are also quite uneven. They clearly come in at different heights in the head and the seats must be different depths because the plugs also protrude different amounts into the chamber. Seems like whoever did that job did not keep very tight tolerances.

Anyone else noticed the same thing?

I run 8-1 compression and 1x plug on my lower front cylinder hits the piston and closes the gap if I use the autolite plugs. I changed to a NGK slightly shorter plug and now is ok. Sonex would have fixed it for me if I sent the head back but never bothered. They were MoFoCo heads. Runs just fine with the different plugs

Yes I too have had issues with the MoFo heads purchased from Sonex. Not only did I wait 6 weeks for them, One of the plugs NKG plugs would not clear and all of the tapped spark plug holes are sloppy. I probably would not have accepted these heads if I received them in a timely matter, but being grounded for such an extended time I made do. However, as a consumer, when I accepted these inferior parts, I should expect the same quality time and time again.

Ok, so this is endemic. I don't think my heads are Mofoco, though, they are about 10 years old and I think they're CB.

I already use NGK plugs, so that's not going to help. Looking at how they come in, it's kind of random. If the electrode ends up pointing towards the piston when the plug is tightened, it will protrude much higher, given the angle of the plug. The one that does hit is facing that way. Another one, that does not hit, probably would if the plug was tightened another 1/3 turn.

I need to flycut the piston tops by about 0.020 to get the deck heights right (in lieu of getting another, very thin, barrel shim) anyway, so I'm thinking I'll mill a small dish in the pistons right where the spark plug protrudes. It only needs to be maybe 0.030" deep to give adequate clearance, so I don't think there's any risk of not leaving sufficient material.

I would be very surprised if some speed shop somewhere didn't offer shims to put under the spark plugs. Not for clearance, but to align the electrode for best flame travel. If I understand this thread correctly, if the plug is in a certain orientation, it won't touch.

Evinrude's Etec motors had a certain orientation that was to be met +or- 30deg. They used taper seat plugs, so it was a brain-teaser as to which plug went into which hole. You marked them, installed them, then tried to match the best plug to a cylinder. 20 plugs on hand made the job easier.

CB then MoFoCo and now Empi. I'm going to the shop and check my Empi heads now.

Area 51% wrote:I would be very surprised if some speed shop somewhere didn't offer shims to put under the spark plugs. Not for clearance, but to align the electrode for best flame travel. If I understand this thread correctly, if the plug is in a certain orientation, it won't touch.

The problem is that it depends on how much you compress the washer, which likely isn't even reproducible if you remove and replace the same plug. Taper seats probably are better for that.

Use a second plug washer?