Small block head part# help

[Jerry G.]

I purchased a set of heads that carry the casting # 3991492 and cast date 3F17. I think these are standard service replacement heads but there is something wrong here. My Chevy Power books says these heads should take a 14mm flat seal 3/8 reach plug. When I pit an AC 44 in them the electrode top doesn't protrude into the combustion chamber and is in fact down inside two or three threads. I measured the thread length and it appears to be 1/2 inch. The spark plug seat does not appear any different from a set of 461 heads which do take the 3/8 reach flat seal. I would also like to know the CC of the combustion chamber. These are not off road heads because they still have the coolant crossover hole in them. Any ideas? Jerry

[Joe L.]

Re: Small block head part# help

I purchased a set of heads that carry the casting # 3991492 and cast date 3F17. I think these are standard service replacement heads but there is something wrong here. My Chevy Power books says these heads should take a 14mm flat seal 3/8 reach plug. When I pit an AC 44 in them the electrode top doesn't protrude into the combustion chamber and is in fact down inside two or three threads. I measured the thread length and it appears to be 1/2 inch. The spark plug seat does not appear any different from a set of 461 heads which do take the 3/8 reach flat seal. I would also like to know the CC of the combustion chamber. These are not off road heads because they still have the coolant crossover hole in them. Any ideas? Jerry

Jerry------


The combustion chambers in the 3991492 head are 64 cc, nominal, assuming that the heads are un-cut.

The 3991492 casting was machined into heads using BOTH gasketed and tapered seat spark plugs.

The 3991492 head was not used for any PRODUCTION applications I know of. It was used for SERVICE heads for many years, though.

[Jerry G.]

Re: Small block head part# help

Thanks Joe. Does that mean I can use both,either, a tapered plug and a straight plug in this head? Do you know what straight plug has the reach needed for this head? This will be used in Vintage Road Racing and the old stright design looks more "vintagy" Jerry

[Joe L.]

Re: Small block head part# help

Thanks Joe. Does that mean I can use both,either, a tapered plug and a straight plug in this head? Do you know what straight plug has the reach needed for this head? This will be used in Vintage Road Racing and the old stright design looks more "vintagy" Jerry

Jerry-----


I did not mean that the heads were machined so that they would be compatible with both type plugs. I meant that some of the 3991492 castings were machined for gasketed spark plugs and some were machined for tapered seat spark plugs. Both are, of course, 14mm. I recently posted photos of both types of seats in another thread but I don't recall which one.

However, the spark plug counterbore in heads machined for the tapered seat style plugs is not large enough for installation of a gasketed-type spark plug. At least, it's not large enough in all the examples I've seen. So, if these heads were machined for tapered seat plugs, I don't think you'd be able to install the gasketed type plugs, at all.

Are you sure that you're using the correct REACH gasketed type spark plugs?

[Clem Z.]

Re: Small block head part# help

Jerry-----


I did not mean that the heads were machined so that they would be compatible with both type plugs. I meant that some of the 3991492 castings were machined for gasketed spark plugs and some were machined for tapered seat spark plugs. Both are, of course, 14mm. I recently posted photos of both types of seats in another thread but I don't recall which one.

However, the spark plug counterbore in heads machined for the tapered seat style plugs is not large enough for installation of a gasketed-type spark plug. At least, it's not large enough in all the examples I've seen. So, if these heads were machined for tapered seat plugs, I don't think you'd be able to install the gasketed type plugs, at all.

Are you sure that you're using the correct REACH gasketed type spark plugs?

correct joe as i just checked some heads i have and the counter bore is not large enough for gasketed type plugs so you must use tapered seat ones.

[Jerry G.]

Re: Small block head part# help

I must have a set that was machined for straight seal not tapered. I just installed a set of straight NGK7 and it goes in and seals fine. I have a set of later 492 and they will only accept a tapered plug. Now my problem is the plugs manufacterer indicated they have a 1/2 inch reach but don't appear to go into the combustion chamber far enough. They look to be about one thread short at proper torque . I measured the reach and I measure it to be 13/32 not 1/2. Is there a plug part number or AC number called out for this head? Jerry

[Clem Z.]

Re: Small block head part# help

I must have a set that was machined for straight seal not tapered. I just installed a set of straight NGK7 and it goes in and seals fine. I have a set of later 492 and they will only accept a tapered plug. Now my problem is the plugs manufacterer indicated they have a 1/2 inch reach but don't appear to go into the combustion chamber far enough. They look to be about one thread short at proper torque . I measured the reach and I measure it to be 13/32 not 1/2. Is there a plug part number or AC number called out for this head? Jerry

here you go. http://webcache.googleusercontent.co...&ct=clnk&gl=us

[Jerry G.]

Re: Small block head part# help

OK, well it looks like I need the TT design which is 1/2" reach FULLY THREADED. I didn't realize there were 1`/2 plugs that are not fully threaded. Seems kind of strange. Jerry