Pontiac 400 heads???
 

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

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.

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 .

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

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

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.

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

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.

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

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.

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

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.

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


.

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

Did you drain the oil? Does it look metallic?

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

How were/are they adjusted?

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?

Does this look right? 😅
Its in TDC

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?

:confused:

Piston 1 and 6 is in TDC on the pic.
That's got to be the fault? 😅

I don't know what brand/model gear is mounted sadly.

If in your photo that’s truly TDC for number 1 then your Cam is way out of time (by a number of teeth ) When number 1 is at TDC compression ( both lifters for 1 on the heel of the cam) number 6 will be at TDC also, but that will be exh so you will see the exh lifter for number 6 opening .

Your dash 8 heads will only give you at best a 8 to 1 compression, especially if the pistons are the stock ones with a chamfer around there top.

So the earlier heads you have a 1974 head casting off a 350 or 400 would have been a 4X casting.

A head with a 75 casting date off of a 350 or 400 would have been a 5C casting unless it was cast in very late 75 and then it could be a 6X casting .

What is the number they have stamped on them.

Do any of these other heads have screw in rocker studs like your 6X heads do?

Yeah, I think that definitely is the fault with the timing 😁.

The two heads that was mounted was both 6X with secondary number 8.

The extra heads are different.. One is 6X, and the other is something I can't see, but it's not 6X..
I have attached a photo.

Would you think I could mount the old ones again even tho the compression is very low?

Thank you so much for all the help

That is a 75 5C head Mikael

The only heads I would bolt back on would be the pair that seemed to run without leaking coolant as in NOT being cracked .
That being said you still do not know if the valve springs they are set up with do not have excessive pressure .

It also seems that you don’t know what Cam is in the motor and in turn you do not know how much valve lift you have.

If I was to guess from your photos you have posted of the motors lifter valley and how much the lifters where up I would say that your Cam has less then .470” lift.

There’s a lot of if’s here that you still need to get around.
Are the heads cracked?

Are the all 16 valves in the heads sealing ?

Are all the lifters and Cam lobes good , saying they look good is not good enough as many folks have unfortunately found out.

There is a away to confirm that each lifter and Cam lobe are good and is a not too hard process that takes about a hour even if your slow and have to do it without a helper.

It will need to be degreed in for sure.
Take the cam bolt off and see if the cam gear has more than one keyway cut in it.
If not the cam gear is installed way out of time to the TDC.


:confused:

While most people talk about advancing or retarding their cam a few degrees this cam looks to be retarded 3 teeth. That is the major problem and needs to be fixed before worrying about combustion chamber size.

Stan

3 teeth!

So that’s like 17 degrees off from where it should be.

If some one has a cam gear can they count the number of teeth? I tried to count using the picture and got 48.

► (720 / 48) * 3 = 45 degree

Stan

I am very pleased the problem was found. Now I just need to correct it.
Do I "just" mount the dots in front of each other, or should I learn how to do cam phasing so they match properly?

I think I will send the X6-8 heads to a machine shop to do a cracking test and make sure they are 100%.
I will also figure out exactly which cam is mounted, so I know which springs I need.
(previously owner just told me they are a bit stronger than the originals).
I will also buy 16 new valves and make the shop mount them.

And maybe ask the shop to make the combustion chamber a bit smaller, so I get a bit more combustion. I don't know how much you can remove before it give issues tho.. Just thinking

The heads can be milled .060” but then the intake flange will also need to milled at least .045” and milling that amount might cost a lot, so check with your shop.

If there rebuilding the heads they might give you a better price on the milling.

Every .006” milled off of those heads removes 1 Cc from the chamber, so your 101 CC heads will be 10 CCs smaller.
This will get you a .5 more gain in your compression even though a valve job will sink the valves a bit.

Without a cam card or specs of cam, probably the best thing.

If reusing what's there, not hard to line up dot to dot.

:)

Before you pick or modify any heads you need to

1) Measure everything and calculate what your current CR is.
2) Find out what the specs are for the cam that is now installed in the engine. (If you pull off the cam gear are there any markings on the front of the cam>)
3) Tells us what octane gas you plan on running.

Stan

By the way how do the blocks cylinders look, any ridge on top?

Looks like the cam is installed off one tooth....
 

The BEST way to install and match them up is to have the dot on the CAM gear straight down and the dot on the CRANK gear straight up. That way they are close together and it's REALLY easy to match them up.

Once they are installed DOT-TO-DOT, rotate the crankshaft one full turn and the engine will be at Top Dead Center (TDC) on cylinder #1.

Good luck!

I will try to take of the cam gear and see if I can find any useful information besides it's a stage 1 cam.
As soon as I have some information, I will order new valves so I can match it up perfectly.

I am planning on running octane 100, as I live in Denmark and it's easy to come by.

The cylinders has been honed and there is new pistons mounted by the previous owner. One piston is pointing in the wrong direction, so I will take them all out to be sure everything looks good.

A piston being on the rod wrong is not the end of world, but if that rod is not on the crank journal right with the off set of the bearing facing the crank cheek then that’s an issue!

I have now aligned the crank and the cam as good as possible, and is now trying to turn the piston.

While removing, I noticed the upper rod bearing having some small scratches. Are these okay? Or do I need to replace all of them?
The previously owner replaced them all, but I am not sure if it's damaged.

Picture attached

When you take the heads to the shop,have them pressure test them beside magg.Tom

From your picture of the bearing it shows that there is or was a lot of fine grit in that motor and the fact that each bearing near its ends show no ware means that the rod bore is out of round to some degree.

That condition can lead to spun rod bearings and potentially a rod failure which 99 % of the time takes the block out.

How many rod bearings look that way?

Before you take the crank out to look at those bearings check the crank end play and note that down.

Its a good thing that it didn’t start!!

That is a lot of bearing damage if it only ran for a few minutes. As one of the other guys said, that indicates there was contamination of the oiling system when the engine was built. Check some more bearings for the same condition. Any bearing with that much damage should be replaced. If I saw that in an engine that was removed from the car, I would fully disassemble the engine and clean the block's oiling passages. That level of bearing damage is unacceptable for a motor you want to live a long time.

Also be aware you cannot just turn the rod around to fix the orientation of the piston. The bearing chamfer must face the outside of the rod journal. Usually this means having a machine shop press out the wrist pin and correct the orientation of piston to rod. Is this the only piston that is backwards? The notch or arrow on the top of the piston must face the front of the engine, and the chamfer on the big end of the con rod must face the outside cheek of the rod journal. The prevents the rod bearing from contacting the radius at the journal's edges where the crankshaft journal meets the edge of the crank throw.

Are you in the US or somewhere else? In the US, its fairly easy to find other heads. The 6X-8 heads are very low compression. I would find a set of 5C-4, 5C-9, or 6X-4, or 96 heads to use instead. If you're stuck with the 6X-8, then have them milled about .050" and check intake fitment.

Not a Pontiac but the basics are the same:

https://www.youtube.com/watch?v=F6hQ6W8jobs

Perfect!
I didn't expect this to be a complete rebuild, but here we are I guess 😅.

Tommorow I will take out all of the pistons, to check the bearings and their orientation.
Then also do a check of the crank play and check all of the main bearings.

I am pretty stuck with the X6-8 heads, as parts are difficult to come by in Europe..
The plan is to get the machine shop to remove 0.060 from the heads and remove 0.045 of the intake manifold as per Steves recommendation.

As I said before.
Measure everything and calculate your current CR.
Measure / Identify what your cam specs are

BEFORE YOU DO ANYTHING TO THE HEAD

Stan

Post pics of the crankshaft journals. Its possible the bearing damage is due to rough crank journals that were not properly machined the last time the engine was rebuilt. Condition of the cam bearings can help tell the tale. If they are in good shape, the crank was rough. If the cam bearings are all chewed up, then debris in the oil is more likely.

I have now removed all the rod bearings and pistons. It's almost the same on all of them.

I measured the crank play before I removed one of the main bearings. It showed 0.0078 inches.
I have attached a picture of the bearing and crankshaft. I guess I have to order new main bearings also with new bolts..

I could not find any information on the cam, so I measured the height of the intake and exhaust cam to 0.275 inches.

I have calculated the CR value as good as I can to 7.72:1
I have attached a photo of the data I have used.

Thank you everybody.

I can see the pictures were pretty bad. I will take new ones tomorrow. I hope I don't need to machine the crank also.

I made a mistake on the calculation, due to the wrong type of piston I chose. The new value is 7.27:1

So not really better..

That crank absolutely needs to be polished or maybe turned to the next undersize. A quick check is if you can catch/feel your fingernail drag on any of those grooves on the journal, the crank must be machined.

Those journal surfaces are not serviceable as they are. The brown wear mark between the rods bearings indicates long term wear. This crank was not machined properly the last time the engine was built. I would say the bearings are also too tight, but without knowing the condition of the journals beforehand, or knowing if the oil was contaminated, it hard to say why the bearings were moving metal the way they are. The contact patch looks too long on the main bearings, so I suspect the main bolt torques were incorrect. What size are those bearings? Are they marked STD or some undersize on the back side of the shell?

The correct way to figure bearing clearance is to mount new bearings into the main caps and torque them without the crank in place. Measure the internal diameter of the bearing inserts with a dial bore gauge. Use those measurements to determine the exact diameters to machine each crankshaft journal to achieve the desired oil clearance of .0025". That's the way the pros do it. The easy way is to just turn the crank journals to the next undersize, but the actual clearances can be wrong/scattershot doing it that way if the block is out of spec.

Your next job is finding a very good machine shop close to you. Do not order any parts before bringing that crank to a machine shop.