Hey all i have a 383 with the shortest plugs i can get in them. The engine is running about 10.7 to 1 compression. Flat top pistons, 462 heads milled .200 to lower the CC to about 62 and under high rpm and load i contacted my spark plug. It was the first time getting on my motor since breakin it in and it was my pasenger side third one back from the front. The rods are scat I beams and the engine is revving to just over 7 grand. IT didnt contact till 4000 and it sounded like m80's going off in my exhaust. I am going to try to put some spacers and pull them further out of the head but any other suggestions?

First verify that you have the proper short reach gasket type plugs. Then do you have standard tip or extended tip plugs? Standard tip should clear, but if those don't, then you will have to index your ground electrode positions.

Mark a line on the plug base in line with the ground strap electrode, do this on several plugs, then screw them in and find the plug that will have the mark facing up, away from the piston. You should probably do this to all your plugs to position the ground strap away from the piston.

You can buy a threaded ring to test your plugs in and you mark the correct index point for each cylinder on the ring. Then use the ring to figure out which plug works in which cylinder.

You can get side gap racing plugs, but they are hard to find and more expensive. Blown and fuel cars use them.
David

Ya its definetly contacting bad and i made sure it was the right plugs infact it is the shortest one the make in my application. So i was going to put washers around them to pull them out and that sounds good to to index my ground electrode. Do you think at 7 grand under full load i would have a problem if i was at 4 grand.

red,
What plugs are you using ? Like Dave said, if you get a set of the standard tip, not extended tip, plugs, they should easily clear. By the way, I think you meant to say you milled the heads .020" not .200" Spacers are not the best way to do it. Buy about 20 plugs and mark the electrode position on the insulator with a magic marker. Then keep trying them and you should be able to get a full set that will index the electrode to the top without spacers. I still dont think you should need to do this with a standard reach tip plug unless you have some huge domes on the pistons.

462 is about .007 per cc isn't it Bill?

Tim he must mean .020". That would be about right. At .200 he'd definitely have problems.

Dave

When you built the short block did you by chance use 6" rods? I'm not sure if it's possible but are you running the proper rod/piston combo? Just a thought.

Yes Dave, I'm not challenging anything. Just asking Bill if 462 heads are .007" per cc for milling calc purposes.

Yes. i beileve it is 200 thousandths on the mill. I believe the rods to be 5.7 when i measured them last it was 6 inches from end of rod to piston pin so that should be 5.7 and the deck has been milled too. And the deck clearance is 0 and there is a .043" quench. Like i said i have checked to make sure the spark plugs were the shortest i could get. I even checked with the local race shop and took the spark plugs up there. They ran the numbers and said i should use spacers to pull them away a little. I wanted to ask you guys before i did anything.

What make plugs are you running?....What's the part number?.....Is the reach something like 3/8"?.....Is the center electrode inside or above the plug?

I find it hard to believe that the heads were milled .200, what machine shop would do that and why? If they were then that's your problem. From what I understand the only way to get enough clearance for the piston would be to use a thicker head gasket and possibly the plug spacers.

Maybe the guy i bought them from were wrong maybe its only 20 thousandths. I do not have the part number on me but the plugs are shorty accel plugs for headers. The correct number for 462's and the electrodes are outside.

long shot but try a set of autolite racing plugs, and I'm not talking about the ones at walmart. the part number would be like AR133 for example. they have side gaped electrodes you can pick them up at advanced auto parts for $2.40 a piece so they are pretty cheap.

With flat top pistons, zero deck, and a .043 quench, there is no way the piston "should" be hitting a plug. Without pictures or actually being there it is kind of hard to diagnose. I would suspect detonation. How do you know the piston hit the plug? Did you pull the head? Do you have a picture of the plug? You might want to make sure you didn't pop a hole in the piston or crack it. Why would only one cyclinder hit and not the others? Don't take this post as me doubting you, I am just trying to approach the problem logically.

.200 off the head is a LOT. That's almost a 1/4" I don't think there is enough meat there for that.

With flat top pistons, zero deck, and a .043 quench, there is no way the piston "should" be hitting a plug. Without pictures or actually being there it is kind of hard to diagnose. I would suspect detonation. How do you know the piston hit the plug? Did you pull the head? Do you have a picture of the plug? You might want to make sure you didn't pop a hole in the piston or crack it. Why would only one cyclinder hit and not the others?

Thats what i thought. How in the hell could it hit. Only one cylinder did and thats what boggles me. The engine has 200 miles on it. And i can rev it up all day long but it didnt do it till a load on it and the car is so loud i can't hear if it is detnoating. Its got about 10.7 to 1 DCR and 8.5 SCR so i am running 97+ octane in it. I am sure there is no hole in the piston I do not even want to begin to consider that because IT didnt happen. I know it hit the plug because you can see where the piston contacted it and actually bent the elctrode up and pushed it over the tip, it contacted on the bottom left corner so maybe it was the fact that it was pointing down and got to close under load when the pistons began to expand a little and the rods stretch.

The heads have been off two times in 200 miles I am not ready to pull them again till i get money to get my AFR's. After re-gapping the plug she fired right up and held idle fine. The pistons are forged so i doubt they got hurt maybe some dings in it but thats fine. I do not think detonation could do it when considering how the tip of the elctorode was malled and actually flattened by the piston hitting it not a big spot but if you looked close you could see it.

I think i am going to try to position the elctrodes up and space the plugs out a little because i do not want to risk doing that again. Or at least space that one out. DOES ANYONE KNOW WHY ONLY ONE PLUG WOULD CONTACT?

I talked to summit and they said my valve to piston clearance is not high enough.

462 is about .007 per cc isn't it Bill?

Tim,
6 or 7 is pretty close

Rad,
The Accel shorty plugs are extended tip plugs. You need to use a "regular" (not extended tip) plug. Other possibility is the head is machined wrong in that one clyinder ?

Thats what i thought. How in the hell could it hit. Only one cylinder did and thats what boggles me. The engine has 200 miles on it. And i can rev it up all day long but it didnt do it till a load on it and the car is so loud i can't hear if it is detnoating. Its got about 10.7 to 1 DCR and 8.5 SCR so i am running 97+ octane in it. I am sure there is no hole in the piston I do not even want to begin to consider that because IT didnt happen. I know it hit the plug because you can see where the piston contacted it and actually bent the elctrode up and pushed it over the tip, it contacted on the bottom left corner so maybe it was the fact that it was pointing down and got to close under load when the pistons began to expand a little and the rods stretch.

The heads have been off two times in 200 miles I am not ready to pull them again till i get money to get my AFR's. After re-gapping the plug she fired right up and held idle fine. The pistons are forged so i doubt they got hurt maybe some dings in it but thats fine. I do not think detonation could do it when considering how the tip of the elctorode was malled and actually flattened by the piston hitting it not a big spot but if you looked close you could see it.

I think i am going to try to position the elctrodes up and space the plugs out a little because i do not want to risk doing that again. Or at least space that one out. DOES ANYONE KNOW WHY ONLY ONE PLUG WOULD CONTACT?

I talked to summit and they said my valve to piston clearance is not high enough.

I'm thinking detonation now as well as some of the others and detonation can and will destroy spark plugs and it can occur in 1 cylinder only. I can't even begin to tell you how many sets of plugs my N20 car went thru before we finally got it tuned correctly.

Your valve to piston clearance is a seperate issue than a piston hitting a plug. Valves open and close and depedning on the cam those times vary. The spark plug does not move. I would expect the piston to hit the plug at all RPM's. If you were running aluminum rods I could see that big of a change but, not with steel rods. One cylinder can't be machined that much different or the heads wouldn't seal.

Since you said it only happened at 7000+ RPM that makes me wonder if your crank might have been flexing. the only thing that would make since is if that one crank "journal/throw" has more stroke than the rest. Still doesn't add up since you stated all holes are set at zero deck. With flat top pistons I just can't see how it is that close.Did you compare the plugs from other cylinders, could it be you got one plug that was mis-labeled (actually longer than it should be).

I would still put money on it that detonation is the culprit, it's the only thing that makes sense. Pull the other plugs and see if you have any silver/grey/black specks on your other plugs. As mentioned above detonation can occur in one cylinder and not the others or be worse in one than the others. What do you have your timing set at?

My timing is set at about 18 degress initial and i believe some where around 30 total. The engine did it at 4000. And the summit tech line said it was my valve to piston clearance because my pistons have been largely recessed for my valves and nothing for my spark plug. Now that i think about it though that would make sense.

I took the car up to 5 grand before under load with higher octane in it and it was fine but i wasnt using as much octane becuase it seemed to like it more but that was definetly at idle and lower rpm and now under full load. When it closed the gap it was under full load and lower octane. Although i still am not sure how it could only detnoate in one cylinder. My exhaust is to loud to be able to tell if it is detonating. The first thing i will do i guess is get it tuned and the timing set in on a dyno or at the shop and ahve my carb adjusted right too although i think its pretty close to dialed in everywhere.

My timing is set at about 18 degress initial and i believe some where around 30 total. The engine did it at 4000. And the summit tech line said it was my valve to piston clearance because my pistons have been largely recessed for my valves and nothing for my piston

Further proof Summit techs are on crack...;)
Seriously, sorry about your luck. As Royce pointed out it would be tough to diagnose without pics or being there, but something is up. If the pistons are truely zero deck (you did check them all right?), unless those heads had been milled WAY down I can't see how they would hit your plug, barring a mechanical issue. I have seen a motor with close tolerances spin a rod bearing and smack a plug (usually a dome piston), but on a flat top its real unlikely, again unless those heads are milled WAYYY down. Your piston would smack the head before the valve. Are you 100% sure it was the piston hitting the plug?
If you can't be sure what caused it I would pull the head and investigate, there will be tell tale marks for sure.
V to P clearance has nothing to do with what you are seeing, it is a combination of valve lift piston height in the hole, and cam duration. All of which could be additional problems to what you are seeing if the head is messed up, but have nothing to do with smacking a plug.

I had the same problem when I put new aluminum heads on my 302. The first time I spun the motor past 5,000 rpm, the pistons hit the plugs. I now run Champion C63CX plugs with the cut-back electrodes. As has been mentioned, if you don't want to use expensive plugs, you could index a standard set of plugs so the electrode is always away from the piston. (That's probably why only 1 plug hit the piston - due to the orientation of the plug electrode to the piston). It's definitely hard to understand how the plug is hitting a flat top piston though.

Well here is some new news that might help address the problem. I pulled out my reciepts and cross checked everything and ran all my deminsions and found my DCR is 11.6 to 1 and my S
CR is 13 to 1. I was running higher octane but not that high in it. I think things should be ok since i only got on it twice but i will buy the shorter plugs definetly up my octane but what should i be running in this case???

When i buy my new AFR heads with 75 cc chambers the compression ratio will still be 8.63 for the DCR will this be pump gas friendly or something like 97 octane?

From what i understand the engine would be reaching extreme detonation and could have easily closed my gap.

Well here is some new news that might help address the problem. I pulled out my reciepts and cross checked everything and ran all my deminsions and found my DCR is 11.6 to 1 and my S
CR is 13 to 1. I was running higher octane but not that high in it. I think things should be ok since i only got on it twice but i will buy the shorter plugs definetly up my octane but what should i be running in this case???

When i buy my new AFR heads with 75 cc chambers the compression ratio will still be 8.63 for the DCR will this be pump gas friendly or something like 97 octane?

From what i understand the engine would be reaching extreme detonation and could have easily closed my gap.

At 8.63 CR you should be able to run 93 octane pump gas all day long. Just out of curiousity does your block have 2 bolt mains or 4 bolt mains? If it has 2 bolt mains I'm wondering if you aren't experiencing a little crank whip?

no its 4 bolt main

You need to take it apart and measure it to know for sure what is going on. That is the only way you will know.

With the confirmed higher compression, I think you found your problem. Time for race gas, I won't give an octane recommendation. You might want to call VP, Torco, Trick, etc.. and see what they recommend. Your engine builder should also have info for you.

I would still pull the other plugs and check them for signs of detonation. At this point I would actually change them all and make sure they are the proper heat range for the compression you are running.