66 GTO head bolt problem


Asked by Jul 23, 2014 at 08:43 AM about the 1966 Pontiac GTO Coupe

Question type: Maintenance & Repair

I have a 66 GTO with a 67 GTO motor in it. I am in the process of switching from the 67 670 heads to 66 093 heads, I just want it to look more original under the hood. I have run into a problem with the center outside head bolt. It is a little too short and when installed there is not enough room for a socket to tighten the bolt. Is there a special head bolt for this area, one with a 5/8 head instead of 3/4 would work, or even allen.If so where can I get one. Thanks in advance for any help you can give me

2 Answers


Nice car- why worry about originality when it already has a different engine? sounds like you are just making more problems for yourself- I could see it if it was go fast stuff, but- whatever- I get all my nuts and bolts at Ace hardware- find one with the huge nut and bolt aisle-

Norm Davis

That bolt is indeed a special bolt with a stepped shaft and tall hex head on it. You can use a longer bolt and put a steel bushing under it to raise it above the casting. Or you can get the bolt from Ames Performance. On the flip side...you are downgrading the performance potential by using the older heads. The 66 093 heads have smaller valves (1.92/1.66) than the 067 (2.11/1.77). The older head also has a more restrictive valve guide area and won't flow as well as the 67 head. I wouldn't do it. With todays gas you're actually far better off with a set of 6x low compression heads. A mild port job, stainless valves, and Ram Air3 exhaust manifolds will give you enough HP on pump gas to make up for the loss in cc's. I understand the originality aspect - but most people will not see the difference in heads, and even hard-core Poncho enthusiasts will appreciate the head swap if it's done for a good reason. Good luck! It's a great car however you build it.

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