[Mikael Nielsen]

Hello everyone.

I hope someone can help me to understand the different heads you can get on the 400 engine.

It all started with I bought a Trans Am from 1979, with a 400 block from 1973. Previously owner could not get it running due to low combustion pressure (40psi), so now it's my turn 😅.
He says I just need to swap the heads to the original ones that I also got with the car and buy new valves.
So I have two different heads, but I don't know which ones to mount, and with what valves/valves springes.

The heads mounted now has the stamps:
6X GM7

The other heads have two different stamps on each head:
6X GM2 and 60 gm4

The 6X GM7 has single springs mounted, but I don't know if the is correct?

I hope someone has some kind of knowledge they would like to share, as I am quite confused.

Regards Mikael

[BILL BOWMAN1]

6x has around 96-100 ccs. Compression will be low maybe 8:1 ish, but still should start and drive. I’m sure you have an issue somewhere else.

[steve25]

Your TA was born with 6X heads of which there were two different chamber sizes, a -4 ( small chamber ) and -8 ( large chamber.
Your TA would have had the -4 and this thumbnail I posted is where you find out that info on the head.

With compression that low either that head has burnt valves ( may need exh seats installed also ) and or it overheated , cracked the head and then blew out the head gasket.

If I recall right the 6XO head(s) are very large chamber California emissions heads.

Do not reuse any of the stock valves, get all 16 new ones .

I would get the heads fully checked out to see if there still usable..

Crower 68407 springs are a good replacement for stock length valves and they are a duel round wire spring like the originals.

Pay no attention to the GM2 or 7 as it’s
meaningless to head ID.

I hope your 73 block is ok, because that’s a good block .

[Stan Weiss]

You need to find out what is wrong.

Have you done a cranking compression test on the engine?

If so were all of the cylinders the same around 40 psi? If not please do a cranking compression test and post results.

Do you know if the previous owner played around with the cam / cam phasing?

Stan

[Mikael Nielsen]

Already learning so much. Thank you greatly!

He said he had installed a sports cam, nothing crazy but not stock either. Would I still buy stock length valves for that, or do I need longer ones?
I still assume it's 2.11'' for the intake and 1.77 for the exhaust.

I will take the heads to a machine shop to fix the surface, valve seats and pressure test them to make sure they are good.

All 8 cylinders has low compression yes.
He mounted new pistons, honed the block, new cam, new lifters and new rockerarms.. Then he apparently ran it for 20 minutes and then it died 😅.
It should be noted he doesn't really know much about engines and is quite inexperienced (as me..)

Thanks again everyone!

[Stan Weiss]

Originally Posted by Mikael Nielsen

Already learning so much. Thank you greatly!

He said he had installed a sports cam, nothing crazy but not stock either. Would I still buy stock length valves for that, or do I need longer ones?
I still assume it's 2.11'' for the intake and 1.77 for the exhaust.

I will take the heads to a machine shop to fix the surface, valve seats and pressure test them to make sure they are good.

All 8 cylinders has low compression yes.
He mounted new pistons, honed the block, new cam, new lifters and new rockerarms.. Then he apparently ran it for 20 minutes and then it died 😅.
It should be noted he doesn't really know much about engines and is quite inexperienced (as me..)

Thanks again everyone!

Check the cam phasing and also check if the cam went flat.

Stan

[steve25]

Stan. I can’t see having only 40 psi of cylinder pressure can take place no matter how far out the Cam phasing might be.

I also don’t think this fellow right now would know how to check Cam phasing .

If that cam and new lifters went flat then there’s likely a good amount of metal grit in the motor and that’s not good for rings , pistons , cylinder walls, oil pump and Bearings.

Since you don’t know what cam got put in the motor its Going to need to come out to

1) determine its condition.

2) check how much lift it has so you then knowe what needs to be checked out and done to the valvetrain.

The machine shop needs to crack test the heads, not pressure test them.

[77 TRASHCAN]

Just a guess. If the previous owner's engine experience was minimal, my guess would be the timing chain installed incorrectly, maybe way off...

[steve25]

Please ask yourself this folks .
The OP stated that the motor ran for 20 minutes.

If the Cam was that far off from the start to only have 40 psi across the board , then how did the #1 even start in the first place ?

And if the Cam had enough lift and was that far off to have only 40 psi then a valve might have been hitting a piston.

Possibly the Cam might have been off enough to make for excessive cranking which wiped out the Cam even though the previous owner did get it to start.

[PunchT37]

Or, it it a higher lift cam that they followed the book and tightened to 20 ft lbs. Cam most prolly round.

[Mikael Nielsen]

So.. Now I have taken apart most of the engine and is down to the lifters.

I am waiting for the pulley tool to remove the harmonic balancer, so I can check the timing of the cam and see if it is installed wrong.

As you can see on the photos, the cam looks alright. I will have to wait for the tool to see how it's aligned.

[BILL BOWMAN1]

Pontiac balancers are a slip fit. You don’t need a puller.

[PontiacHO]

Is the compression gauge confirmed as being correct? If it ran and now it doesn’t, was the needle and seat causing a flooding situation and washed down the cylinder walls? It seems there’s more to check before a disassembly should occur however that is happening.

[Steve C.]

https://forums.maxperformanceinc.com...d.php?t=381050


.

[Mikael Nielsen]

Is it possible that the low compression could be due to the cam not lifting the valves enough, to get a good compression?

Just wondering

[BILL BOWMAN1]

Did you drain the oil? Does it look metallic?

[PunchT37]

Originally Posted by Mikael Nielsen

Is it possible that the low compression could be due to the cam not lifting the valves enough, to get a good compression?

Just wondering

Or, tied to my earlier post, the valves are adjusted too tight and holding them open causing low compression.

How were/are they adjusted?

[Mikael Nielsen]

I have just drained some of the oil, and it looks fine.

When the time comes and I figure out the problem, I have a question:

I have justed checked both set of heads, and they all have the number 8 on them..

The two I pulled of are from 1977, and the ones I got with the engine are from 74 and 75..

Does it matter which ones I mount, or do they all fit?

[Mikael Nielsen]

Does this look right? 😅
Its in TDC

[johnta1]

Does not look good? But maybe pic distortion.

Turn the crank until the top gear mark is at bottom and the crank gear mark is at top, all in line. See where the piston top is then?

What brand of chain/gears are they?