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Discussion Starter #1
I was able to find the head numbers. My best quess is 16. It is pretty worn with age. The only other number it could be is 18. I noticed that 16 did come on a 400 in 1968, and an 18 came on a 350 in 1968. I really think it is a 16, I had a few other people look. What I thought was strange is when I changed the spark plugs they were a 5/8 plug. The book said it should have been a 13/16 plug. From what the book said 71 and older use 13/16 and 72 and newer use 5/8. Also I did check the block and it is a 400. Would heads from a 350 fit on a 400?
Thank you to Randy and Lars for helping me locate the head numbers.
matt
 

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Any head will bolt onto any block, at least for the 350's and 400's anyway. But that doesn't mean that the combination will work well. The same with the intake manifolds. And there has been a whole lot of mixing and matching out there so unless you are sure that the parts you have came on the block then I wouldn't assume that's the case.

And there are oddball heads out there with numbers that don't fit the list. I am running 061 heads on the 350 in my '69 LeMans conv. According to the chart they would be from a '67 2bbl 400, but I originally bought the engine they came on in back in 1982 from a guy who wrecked his GTO, and they have 2.11/1.77 valves so that makes no sense... (by the way to get big valve heads to work on a 350 with a high-lift cam you have to add chamfers at the top of the cylinders for valve relief.)

I have never seen a pre-'72 head that is supposed to use the smaller plugs so that seems suspect. Are you sure that someone just didn't have the right plugs in there?

Given the small plugs I don't think that there is a sure way of telling what you have without checking the valve size and cc's. If you are satisfied with the way that the engine runs then don't worry about it. If you plan on performance modifications then you should probably pull them and have them checked out anyway.

Walt
 

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Discussion Starter #3
walt
the plugs in there were r45ts ac delco. I just replaced them with the same.
i haven't had the car very long, and i am trying to figure out what heads i have. With the casting being 16 and the date code being e158 and the spark plugs being 5/8 I am confussed. Performance is o.k. but i plan on do mods this winter. I do know that the guy i bought it from replaced the motor in 1985. I was hoping that the heads were from the orignal motor. I know the block isn't. It is a 1977 block.
thanks
matt
 

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Matt, I am no expert but I did build my share of Pontiac musclecars through the 80's and have done plenty of mixing and matching myself. With a 16 code on the exhaust ports the '68 heads sound right, but the small plugs really do bother me.

The '77 block is fine, although I always try to keep 60's parts in 60's cars just because. But 70's heads have a much lower compression ratio and definitely don't belong in a classic goat. The old heads sometimes disappear during engine swaps because they are valuable. The LeMans convertible that I have now is a rare "Sport" version and came with the 325 HP 350. The previous owner (clue-less) was very proud that he had put late 70's "Trans Am" heads on it when he had the engine rebuilt. I figure that the head swap was the shop's idea, and I am sure they made some good money on the old heads. Needless to say, the 350 with big-chambered 400 heads was a real dog and tossing them was one of the first things I did.

Which goes back to your situation. If you have the original heads on the car then the C/R will be very high for today's gas and it should want to ping and run-on especially if you try putting regular in it. This also puts quite a load on the starter motor when the engine is hot which is hard not to notice. The 70's compression is much lower which will make the engine much happier on pump gas, but this also kills power especially at higher RPMs. If you have the original heads and a reasonable cam it should pull strong through 5,000 RPM. If power quickly dies back over 4,000 RPM that's not a good sign. So how the engine actually runs should provide some clues.

By the way if your 400 had the '68 350 heads then the C/R would end up at almost 12:1 which would create big problems, so this is highly unlikely.

Other things to look for- although the 70's heads will bolt up the heat crossover passages don't always match up correctly with the 60's intake manifolds. If you have the original 60's intake and an exhaust leak at the crossover that's also not a good sign. Let's see, another thing worth doing is pulling a valve cover to see if the heads have screw-in studs. To be honest I don't remember if the 16's came with them and there were plenty of excellent '60s heads that didn't, but its another thing to add to the ID list.

Walt
 

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Matt -
Just because the heads had the new "peanut plugs" in them doesn't mean they are the correct plugs. Pull a plug out and check the spark plug seat. You can visually tell if the seat is for a flat gasket (large plug) or a taper seat (peanut plug) application. A pre-71 head does not use a peanut plug, but I see them installed all the time.
Lars
 

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Discussion Starter #6 (Edited)
Thanks again Walt and Lars!

Walt the car seems to run and start normal. When hot and I try to restart it, it seems to act like it is out of gas in the float bowl. (would the sound I should be listening for, sound like the timing is to far advanced or would it start like it was out of gas for about 5 seconds,if it had high compression) The guy I bought it from said he ran 87, since I bought it I have ran 91, not that it probably makes a difference. It has never pinged or ran on. Since it is a 77 block would the pistons lower the compression? Also is there a way to do a compression test and cross reference the number to tell the compression?
 

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Discussion Starter #7
Hey Lars,

I had a mechanic look at my head spark plug seat and he said that I did have the wrong spark plugs. That I needed the bigger plugs. he also said the casting number was 16.
thanks
happy matt
 

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Man I have been out of this for a long time, except for playing around with my LeMans a bit during the past few years. So I am reaching back into memory. I just started watching this board when I decided to buy a new GTO. I'm glad Lars jumped in, maybe others with more recent experience can jump in.

But just for fun I'll give you my thoughts.

I agree with Lars that the small plugs should thread in. I can't imagine why anyone would do this but then the big plugs were still the "standard" when I was working on Pontiac engines. I suggest that you stop by a machine shop with some old-timers, they should be able to take a look at the seats and tell you right away which plugs are correct and answer your head question for good.

All of my 60's high-perf 400's didn't like pump gas much when they took the lead out, even the high-octane stuff. Unless something has changed in gas formulation I can't imagine running 87 octane in an old 10+ C/R engine. Also all of them really worked the starter motor especially when hot (sounds like too much timing advance). I installed high-torque starters and the biggest batteries I could find in order to keep them cranking through the summer months.

You can't tell compression ratio with a compression tester because the cam duration/timing affects the reading. And I never worked on a Pontiac engine with dished pistons and didn't even consider that your engine could have been rebuilt with them.

The bottom line is that you can probably resolve the head question by figuring out which plugs really belong. But the only way to tell what else you have is by pulling things apart.

Walt
 

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I bought some new plugs for my '73 455 and the NAPA guy tried to give me the little plugs. When I showed him what I wanted he went back and got the larger ones. I wonder if both sized plugs are being listed in some of the parts catalogs?
 

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Matt -
As Walt says, you can't tell comp ratio from a compression test - cam overlap/duration has a bigger effect on cranking pressure than the C/R.

From your other post regarding your block ID, and from the results of your spark plug hole configuration inspection, you have a 1977 400 block with a set of 1969 GTO large-valve heads on it. This is a nice combination. Although the #16 heads were advertised at 72cc with a compression ratio of 10.75:1, I have found in measuring these heads that the actual chamber volume is 75 cc, and that the block deck height (always more than "0") and gasket thickness will produce an actual comp ratio a full point lower than advertised if you have stock flat-top pistons. This will give you a comp ratio of about 9.75:1 if you run the numbers. If you pull a plug out and bring the piston up close to TDC, you can see the top of the piston with a small flashlight - verify that it's a flat-top and not a dished piston (stock Pontiac engines used flat-tops - Pontiac varied comp ratio through the cylinder head chamber volume).

The 9.75:1 comp ratio is street drivable on premium pump gas as long as you don't get the timing too far advanced. Start out at 36 degrees total and back off from there if you notice any detonation.

DaveH said:
I bought some new plugs for my '73 455 and the NAPA guy tried to give me the little plugs. When I showed him what I wanted he went back and got the larger ones. I wonder if both sized plugs are being listed in some of the parts catalogs?
1971 is the last year for the full-sized plugs. The '73 uses the taper-seat "peanut" plugs if it has the '73 heads on it.

Lars
 

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Lars,
You're right! It actually has '70 heads on it - no wonder I got the wrong plugs :rolleyes: - should have though of that!
 
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