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Spark Plug Mystery

1148 Views 8 Replies 5 Participants Last post by  QuadRacr57
I adjusted my valves today and decided to change the spark plug while I was at the shop buying valve shims. I brought in the old on, a CR7HS and the manual says to use a CR8E. The "7" is a lot shorter than the "8", so I'm wondering which one I should use? The motor is completely built, and I just bought the quad, so not sure why the guy before me had this shorter plug in. I tried the longer one, and manually turned the crankshaft and I didn't hear it hit the plug, and I stuck a wire down the spark plug hole and measured to see if the longer plug would hit the piston and it didn't seem like it but I'm scared to start her up. There's a bad camera phone pic, but at least you can see the length difference. Any help is appreciated


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i would be scared too, the high comp piston probably goes up further and hits the stock plug
I don't think the piston has anything to do with it. I'm running a 14:1 piston and I have the stock plug. When you say you have a "built" motor? what exactly do you mean? Is it a stock block with hi-comp piston and head work, or is it an aftermarket jug and set up? I might be able to see where an aftermarket kit was installed, calling for a different plug.
Okay, I used the wrong wording, it is not completely built with an aftermarket block. It looks like it is the stock block, but it is bored out, 14:1 piston, ported and polished head, titanium valves, HotCams. But I wouldn't think any of this would affect the plug length. Any ideas?
Something isn't right here.

How did it run with that shorter plug in it? If that is "truely" the correct stock plug on the left when threaded completely into the head just the very tip of that plug sticks out into the combustion chamber..... so if the longer one barely sticks out (like it should) then the shorter one was about a 1/2" up in the threads and barely igniting any fuel.

Who did you get the motor from? Maybe he was a yahoo and just grabbed a plug out of his tool box and said.... well it fits.
Yea he was kind of a "yahoo", did everything half-arsed. So as long as the plug threads don't stick out past the head, it should be fine? If so, I'll take some more precise measurements to be sure. And the quad seemed to run great while it had the shorter plug in there. But who knows, maybes it's got more to give using the longer plug and completely igniting the fuel. And that is the stock plug on the left, thats what the factory specified. So as long as only the ground on the plug sticks past the head, that should be fine?
Stock head and jug, then yes you need to run the CR8E. Again, this also assuming that there is nothing really special about the motor that might cause the piston to sit up a little higher. Couple of ways to VERIFY that this is correct.

A. take the head off and insert the CR8E plug and inspect the clearances.
B. Use a gun borescope and look into the hole for the plug. Why? Because if the hole has black threads at the bottom of the head that go up a ways (looks like the plug threads) then it is a good bet that the plug that was in there, was not the correct one and did not extend far enough into the head.

I can see how the shorrt plug would still work, but I can also see that the short plug is really black, meaning that it probably isn't igniting all of the fuel properly.
Oh good idea! I have a fiber optic bore scope that I can use to look around in there. I'll give that a try today after work. Thanks everyone!
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