GT40 Head Swap Project

[octanejunkie]

So I got this bright idea, let's grab a set of GT40 heads from a junkyard and throw them on my buddy's 67 mustang - seemed much cheaper than buying heads, at the time at least...

Went to the junkyards and found a stack of GT40 heads, some GT40Ps too, but mostly F3ZE-AA casting heads. Now from doing some research I know these are 60-63cc Cobra heads circa 1993, but these came off a '96 explorer, so the valve train is not HP oriented.


Long story short, the heads were quite dirty/rusty so I took them to my local engine shop. $200 later I got them back shot blasted, cleaned, mag'd, all valves and guides checked, surfaced, painted, new valve stem seals, etc - they look new now and everything checked out fine with one exception, the springs, retainers and keepers.


The exhaust valves have rotator caps and lower keeper grooves than the intake valves and the springs are different between intake and exhaust. Not really a big deal either since the new cam will dictate what springs we will use and the installed heights can be trimmed with shims. Installed height on EXH valves without roto caps is 1.710 and the installed height on the INT valves is 1.790, so .090 shim stack on the INT side and a .010 (or .015) shim on the EXH side and we will have a standard 1.700 installed height.

On to the rockers. Since these are pedestal heads we have two basic choices, run pedestal rockers or machine for studs and guideplates. Not sure which way we will go yet, but obviously there are many options so we could go either way.

Now my biggest concern, chamber size and compression ratio. Since we are not sure what is in the 67 mustang now, engine was replaced in 2000 with minimal paperwork, I am concerned that we may actually end up loosing power if the heads on the engine now are smaller chamber than the GT40 heads. Obviously I need to pull a casting number off the installed heads and look them up. Next concern is the CR. Since we are also not sure what pistons are in the motor and what the deck height is a huge part of this project is on hold till we pull the heads, cc them and measure the piston to deck clearance.

I guess my question here is, would it be conceivable that a "stock" 302 rebuild would have smaller chamber heads than 60-63cc and what would a "standard" deck height and piston volume be on a stock rebuild from a generic engine rebuilder?

I know NO ONE here can answer these questions exactly, but a ballpark based on practical experience wouldn't hurt.

We might get busy with it this weekend, at least pulling old heads and CC'ing them vs. new heads and measuring deck height - so then we will know for sure.

Either way I am holding off on ordering head gaskets until we have all the facts, just in case we need thicker or thinner gaskets to tune CR and quench.

We are going to do the heads only first to see what difference the GT40 heads will provide over the installed heads, but ultimately we are going to do a cam swap too.

This is all somewhat temporary until we can source a good 351W candidate block for a fresh buildup, but the 100K mile 302 still has over 150 psi cranking pressure so the motor still has some life in it.

I am looking for comments and suggestions from those that may have done a similar swap and I will do my best to keep this thread updated wtih our progress.

[Sokoloff36]

what your prolly gonna have to do is like you said cc the chambers of the heads that youll be using. then cc the cylinder too. thats how your going to find your compression ratio. I hope this answers your question.

[86lxnotch]

i would not worry bout gasket thickness to change comp ratio. only reason for diff thickness gaskets is to get the proper quench number

[Sokoloff36]

Originally Posted by 86lxnotch

i would not worry bout gasket thickness to change comp ratio. only reason for diff thickness gaskets is to get the proper quench number

actually people do put gaskets with a different thickness on them for a change is compression ratio. now i will agree it wont change the ratio a drastic amount but it sure will make a difference.

[86lxnotch]

i know it will change it slightly. what i am saying is that IS NOT why they make diff thickness gaskets. if you have the wrong quench area you have a higher chance of detonation which we all know is bad.