66 858 head with no spark plug boss

[Reg C.]

I am new to this forum so holiday greetings!
I have a 66 425 coupe with 858 heads dated 6/2/66 and 6/5/66 and my build date is late july. My question is: one head does not have spark plug bosses for spark plug shields. Can someone shed some light on this?

[Joe L.]

Re: 66 858 head with no spark plug boss

I am new to this forum so holiday greetings!
I have a 66 425 coupe with 858 heads dated 6/2/66 and 6/5/66 and my build date is late july. My question is: one head does not have spark plug bosses for spark plug shields. Can someone shed some light on this?

Reg-----


The successor head to the GM #3873858 was the GM #3904391. I believe the 3904391 did not have the bosses for the spark plug shields. Since your heads were manufactured very late in the 1966 model year, all I can figure is that some of the elements of the pattern which was used for your head without the bosses had been changed to the 3904391 configuration.

[Reg C.]

Re: 66 858 head with no spark plug boss

Merry Christmas Joe,
Is it possible that this head is the original head? Or a service head.
RC

[Craig S.]

Re: 66 858 head with no spark plug boss

Reg - since your dates are only a few days apart, I would guess you may have both original heads, one of which has the newer casting facets Joe mentions. I wonder how they assembled the car........one side with and one side without spark plug shields?........Craig

[Michael H.]

Re: 66 858 head with no spark plug boss

Reg - since your dates are only a few days apart, I would guess you may have both original heads, one of which has the newer casting facets Joe mentions. I wonder how they assembled the car........one side with and one side without spark plug shields?........Craig

I think the spark plug heat shields were eliminated in production a month or two before the date of these heads. I don't remember the exact time though.

I've always wondered how the spark plug wire ground terminal (with radio) was attached without the shield. Must have used the 67 style wires that attached to a valve cover bolt although there's no mention of that change in the AIM.

I agree, the heads are most likely the originals. Now we know the aprox date that this change occured.

[Joe L.]

Re: 66 858 head with no spark plug boss

Merry Christmas Joe,
Is it possible that this head is the original head? Or a service head.
RC

Reg-----


As Craig mentions and as I agree, based on the casting dates of the 2 heads and the build date of the car, I have no doubt that the heads are original.

[Reg C.]

Re: 66 858 head with no spark plug boss

Thank you for your input. I was not looking forward to finding aother matching head.
Reg