[70.5_firebird]

I just bought a 1975 pontiac 400, but it has mismatched heads on it.
The block has a build date of A275 - Jan 27, 75.
1 head is a 6x, build date B245 - Feb 24, 75.
1 head is a 5c, build date J304 - Oct 30, 74.
From what I can see online, both heads are 2.11/1.66, 7.6cr and 98 cc's.
Is it ok to run different heads like this? Is it possible it actually left the factory like this?
Also,when I rebuild the engine, would I be better off using a set of 11 heads off a 1970 350 that I have?
The 11 heads were used on 70 400's as well, 1.96/1.66 with 8.6 cr listed on 70 400's.

Thanks for any help.
Mike

[1971WARBIRD]

as i understand it the 5c and 6x are almost the same,like haveing a 13,16,62,or 12 head mixed together.as long as the chambers are the same size should be okay.

i'd say very unlikely that it came from the factory that way,the 6x and block are pretty far apart,but maybe.the 5c,that would be just reaching to far back on the shelf.

do a compression check to make sure they are both the same as there were both small and large chamber 5c and 6x heads.

years ago i bought a set of heads at my friends yard,it was a fresh built motor that the guy could never get to run right so he junked it.71 400 XX block(i think)with a 71 96head on one side and a 68 16 on the other side.10+ comp on one side and 8.2ish on the other.never found out for sure what shop did it but have a very good idea,the same one that ruined my pistons.

[smagnotti]

The 5c head has air injection bosses in the exh ports, so yes ,its unlikly that the motor left its birth place that way.

[Matt Meaney]

Originally Posted by 1971WARBIRD

years ago i bought a set of heads at my friends yard,it was a fresh built motor that the guy could never get to run right so he junked it.71 400 XX block(i think)with a 71 96head on one side and a 68 16 on the other side.10+ comp on one side and 8.2ish on the other.never found out for sure what shop did it but have a very good idea,the same one that ruined my pistons.

now thats funny! years ago, a friend of mine bought a car with a 400 that had one 7K3 and one 62. that thing ran like a champ. never would have believed it if i didn't drive it myself.

[1971WARBIRD]

it was your normal to big a cam and carb with stock exhaust manifolds.saddly the shop help to make another person think pontiacs were junk.all i remember is Bill said the guy ended up with a small block chevy in his bird.after seeing the heads i was afraid of the shortblock.


i wonder if your friend's had a mild cam?that might have made it runable.

[Will]

There are two different versions that are common for both the 5C and 6X heads. There is the 5C-4 and 5C-8 and 6X-4 and 6X-8. If both heads have the same secondary stamping (4 or 8) then they are identical for all practical purposes and no problem running them together. Here's how you find the secondary stamping:



The #11 heads will probably offer a better compression ratio due to smaller chambers. For performance work they will need to be converted to screw in rocker studs and larger valves. The ports will also need minor work to flow the same as the larger valve heads. Any good shop with a modern Serdi or Sunnen head machine can do this work for you without a whole lot of extra cost.

Hope that helps.

[70.5_firebird]

Thanks for the replies guys.
The secondary stamping is 8 on both heads, so I guess it's all good. It's supposed to be a good running engine, but it was pulled months before I picked it up,seeing the different heads made me wonder but the price was right.

One more question, before I pull heads and intake off this 350, any way to try and track down the origanal car it came from? The guy I bought it from thought it was from a convertible parisienne I think, any way to track the current owner? If I'm lucky, his original motor and trans might be worth enough to build this 400.

Thanks again.
Mike