[455Grandville]

Putting my daughters T/A up for hibernation out of the winter weather soon.

I'm going to freshen up the engine; probably not a full rebuild since the stock 455 uses no oil. It's a factory assembled '71 455 engine, and I have a few 400 6X heads (not the 350 smaller cc heads) and a few 455 4 x heads which already have hardened seats.

I can machine the heads myself and I think the 4Xs have pressed in studs. Aside from smaller 6x cc chambers which head would be best. Not sure if I want compression too high (with the 6x heads) because 93 octane is surprisingly not at every station.

Which heads would be best ?

[PunchT37]

6x. Better head than the 4x. The compression will not be too high with the #8's.

[steve25]

The 6X heads have a bigge, different shapedr and a tad better flowing Intake port, both heads suck in stock form on the exh side in terms of flow needs for a 400 no less a bigger CID motor.
In re-worked form the 6X heads have more potential on the EXh side in terms of getting the center ports airflow balanced with the end EXH ports.
The 4X heads you have with the seats install on the EXh side would be what I would use for now for no lead fuel as long as the seats where installed to not restrict down the EXH bowl diameter.

[tpssonic]

Shameless plug:

http://grandrapids.craigslist.org/pts/4138546077.html

[hurryinhoosier62]

6X-8s would be my choice. With 101cc chambers, you should have an 87 octane friendly
engine.

[screamingchief]

I'd use the 6X-8's.

Sweating details on a stock shortblock like that is a waste of time,as it's not gonna amount to anything significant.

And no way I'd opt for a pressed stud head over a screw in stud head...

Plus the 455 4X heads offer nothing better than the current heads,no gain in CR or such.

All the 4X heads might have over an earlier 455 head is maybe the 2.11" intake valve,and I would'nt even hold my breath on that one,as every pressed stud 4X head I've personally seen was also a small valve head as well.

4X heads are transition heads,lots of oddball things about them.

Plus they seem to be far more prone to cracking as well,so if you do use them,be absolutely sure to mag them as well.

I'll take a 5C or a 6X over a 4X anyday.

Any port differences between them can easily be dealt with if deemed absolutely necessary.

FWIW

Bret P.

[Blued and Painted]

I wondered why there was oil in the void aria above the exhaust crossover on my 4x-3h heads. Sure enough they were cracked clean through the crankcase aria into the exhaust seat and bowl.

From what i've read the 6x are the "best of the best" big chamber iron heads.

[455Grandville]

Originally Posted by hurryinhoosier62

6X-8s would be my choice. With 101cc chambers, you should have an 87 octane friendly
engine.

The 6X-8 heads on my GV's 455 was 98 cc before I milled 'em; you'll get some variance.

I believe I calculated the compression at 9.3 : 1

However the difference between the old 7.6 : 1 200 horse 455 (with less than 25K orig miles) is night & day

[gtofreek]

i would use the 6X. Every set of 4X heads I have seen have been cracked. have yet to see a cracked 6x head and I have worked on more 6x heads than 4x.

[455Grandville]

Originally Posted by gtofreek

i would use the 6X. Every set of 4X heads I have seen have been cracked. have yet to see a cracked 6x head and I have worked on more 6x heads than 4x.

I'm not too familiar with 4x heads; I have a stack of 455s with 4X's

Where do they crack ? Between the seats like '75 & later Chebby SBC "light" heads ?

[hurryinhoosier62]

Originally Posted by 455Grandville

The 6X-8 heads on my GV's 455 was 98 cc before I milled 'em; you'll get some variance.

I believe I calculated the compression at 9.3 : 1

However the difference between the old 7.6 : 1 200 horse 455 (with less than 25K orig miles) is night & day

My 6X-4s were 98cc before we milled them to 92cc. I suspect the valve seats had been sunk by a previous valve job. If I remember correctly, both the sets of 6X-8s under my work bench are 100-102cc. I know my 7K3s are 96cc.

[gtofreek]

Originally Posted by 455Grandville

I'm not too familiar with 4x heads; I have a stack of 455s with 4X's

Where do they crack ? Between the seats like '75 & later Chebby SBC "light" heads ?

They were all cracked in the center exhaust seats, but not in between the seats, but rather in the short turn and 180° from there.

[Grand73Am]

Does that apply to all 4X heads? I have a set of 4X-7h heads, from a 400 engine, dated May 1974. With 98 cc chambers, I thought they'd work well on a 455.

[steve25]

If the 4x heads had seats installed, one would conclude that they had already been checked for cracks.

[Cliff R]

All Pontiac/GM head offerings from 1973 and up will have induction hardened seats.

All of them should be crack checked around the seats. Most common to see them cracked in Chevy and Pontiac engines on the center two exhaust where the greatest heat is.

7M5 heads are about the worst for this deal from what I've seen here. Haven't found one set to date that weren't cracked.

4X heads aren't nearly as bad as 7M5's, but I've quite a few cracked in the center exhaust seats.

The 455 4X heads are "doorstops" unless you plan on using a turbo or supercharger....IMHO.

Any 400 4X or 6X-8 offering would be a nice upgrade for a 455, and work fine on any grade of pump fuel with the right camshaft.

I wouldn't stick my nose up at the 400 4X heads with the small valves, even if they have pressed in studs in them. Working with virgin material any competent machine shop can upgrade them with the larger valves. All the valves will be full height in the chambers for improved low lift numbers, and they can either heli-coil them for screw in studs, or thread them for 1/2" studs while they are doing the work.....Cliff

[screamingchief]

Originally Posted by steve25

If the 4x heads had seats installed, one would conclude that they had already been checked for cracks.

I did'nt read his comments as him saying that the 4X heads had hardened seats installed in them,rather that they came with induction hardened seats already vs. the '71 455 heads that would not have had the hardened seats in them as SOP.

FWIW

Bret P.

[gtofreek]

Originally Posted by screamingchief

I did'nt read his comments as him saying that the 4X heads had hardened seats installed in them,rather that they came with induction hardened seats already vs. the '71 455 heads that would not have had the hardened seats in them as SOP.

FWIW

Bret P.

That's how I understood it too.

[steve25]

I dont know, maybe he can clear up that "already have hard seats" statment in his first posting?