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Join Date: May 2008
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Spark plugs / Detonation

Is it possible for the spark plug to hit the piston? Looking down the plug hole with a mirror and flash light there is a small shinny spot on the piston in line with the hole. This is a '74 CIS motor, I've checked the photos I shot of the installed pistons and the high side of piston is to the left. Could the shinny spot be detonation? I'm in Cal. and using Chevron premium. Plug gaps are all good at 0.028"

Can the plug be checked with a Ohms meter? Setting the meter to ring a sound, 4 plugs will sound out two don't. Plugs are NGK BP7ES.

The engine has about 1500 miles from rebuild this is the first time removing the plugs. Engine runs good with a little smoke at start up most times.

Cold compression checked at 135 - 140 psi.
Old 07-03-2011, 05:43 PM
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The CIS piston is concave on the spark plug side of the piston...unless the piston is upside down? Then there is other problems.
Bruce
Old 07-03-2011, 06:47 PM
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The pistons are in correct, I can see the AE mark on top which is in the cavity. So I assume it would be impossible for the plug to touch the piston. It is hard to tell exactly what the "spot" I see is.
Old 07-04-2011, 08:24 AM
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The light from your light source? Somehow carbon has yet to adhere to this spot, which is closest to where the spark is and the spark kernel is hottest?

If you rebuilt this motor to stock, and accounted for any machining on either the cylinders or the head during reassembly, there would be no reason to expect detonation. And even less to expect plugs hitting pistons. Because the compression isn't that high on these motors, and there is plenty of clearance.

What prompted you to look in there? I am doubful that you can see anything useful this way, at least with the engine in the car. Maybe on an engine stand. But if you could see the AE mark, perhaps your eyes are especially good. Or did you use one of those fiber optic extensions you can attach to what otherwise would be a flashlight, so you can poke the light source down the hole without having the flashlight obstruct your eye's view?

If you are worried, see if you can borrow a fiberoptic bore scope. With that you can really get a decent view.
Old 07-04-2011, 11:29 AM
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I'm sure nothing is hitting the pistons, it just caught my attention seeing what looked like a bare spot. Just using a mirror and flash light shining off the mirror checking for debris's around the plug hole.
Old 07-04-2011, 12:32 PM
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Old 07-04-2011, 12:32 PM
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