Head on a C4 1986
Thread Starter
Intermediate
Joined: May 2008
Posts: 42
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From: Oklahoma City OK
Head on a C4 1986
How can I find out what type of head I have on my 86? VIN #1G1YY0789G5117332 there has been some work on the motor before I got it. No idea if the drilled out the holes or change pistons so in real need of help on this one. Trying to change the spark plugs and its going to tell me if I am setting them at .34 or .45.
Drifting
Joined: Sep 2004
Posts: 1,638
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How can I find out what type of head I have on my 86? VIN #1G1YY0789G5117332 there has been some work on the motor before I got it. No idea if the drilled out the holes or change pistons so in real need of help on this one. Trying to change the spark plugs and its going to tell me if I am setting them at .34 or .45.
Your RPO codes in the boxes lids behind the seat may help - in the stickies is the list of what those codes ment!
Carl Johansson
Team Owner
Joined: Dec 1999
Posts: 34,873
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From: Lacey WA RVN 68-69
NCM Sinkhole Donor
Early '86 coupe heads are cast iron so a magnet will stick to them. All '86 convertibles and later coupes had aluminum heads.
The plug gap will be .035 or .045 depending on the head.
The plug gap will be .035 or .045 depending on the head.
Le Mans Master
Joined: Feb 1999
Posts: 9,936
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From: South Carolina
St. Jude Donor '03 & '05
Just put a magnet on any part of the head. Under the valve cover, by the exhaust, up front by the alternator, air pump. Anywhere as the whole cylinder head is madeof cast iron.
Le Mans Master
Joined: Mar 2004
Posts: 5,107
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From: Dublin CA
The magnet will tell you cast iron or aluminum. 
Since you are not the orignal owner, there is no way of knowing if it is a aftermarket head unless you pull it. If you don't need to pull it ....don't. The front of the head may be the easiest place to try the magnet. It doesn't have to be a large manget.
You can gap your plugs at .35 up to the .45 without a problem. The GM distributor will handle the larger gap without a problem. If you find that you have aluminum heads, BE SURE to use anti-seze on the plug threads and be careful when you start them. You don't want to cross thread aluminum.
Since you are not the orignal owner, there is no way of knowing if it is a aftermarket head unless you pull it. If you don't need to pull it ....don't. The front of the head may be the easiest place to try the magnet. It doesn't have to be a large manget.You can gap your plugs at .35 up to the .45 without a problem. The GM distributor will handle the larger gap without a problem. If you find that you have aluminum heads, BE SURE to use anti-seze on the plug threads and be careful when you start them. You don't want to cross thread aluminum.
Thread Starter
Intermediate
Joined: May 2008
Posts: 42
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From: Oklahoma City OK
So if it sticks then its cast iron right and I don’t need to see the piston head for this. I was thinking that I had to get the magnet on top of the piston.
Le Mans Master
Joined: Feb 1999
Posts: 9,936
Likes: 38
From: South Carolina
St. Jude Donor '03 & '05
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Thread Starter
Intermediate
Joined: May 2008
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From: Oklahoma City OK
I will not be able to go off of that, only for the fact that the factory motor has be fixed/replace. Not the last owner but the owner before him: took the vette in for an oil change, shop failed to due it right and left the plug out. Result was a blown motor and a new motor and/or new parts were used to fix the damage, but some kind of up grade was made.
Team Owner
Joined: Dec 1999
Posts: 34,873
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From: Lacey WA RVN 68-69
NCM Sinkhole Donor
If the motor has factory aluminum heads, it will have centerbolt valve covers. If the motor has been rebuilt or replaced, it's entirely possible that aftermarket heads could have been installed. Most aftermarket aluminum heads use perimeter valve cover bolts so the only real test for factory iron heads is a magnet.
In any event, when installing new plugs, gap them at the factory settings for the type of cylinder head material. If they are aluminum heads (regardless of OEM or aftermarket), set the plugs at .045. If the heads are iron, set them at .035.
In any event, when installing new plugs, gap them at the factory settings for the type of cylinder head material. If they are aluminum heads (regardless of OEM or aftermarket), set the plugs at .045. If the heads are iron, set them at .035.
