[65goatman]

Has anyone used the 6X-4 heads on a 389 motor using 389 pistons and mild cam without having problems? And if you did, what parts did you use that were stock and what if any modifications did you make? Currently, my motor is at the engine rebuilders and waiting for my decision on heads and pistons to use. Regarding heads, I have a set of 092s and 093s to use, but want to be able to drive the car using pump gas. I will keep the heads for resale value.

[65goatman]

Has anyone used the 6X-4 heads on a 389 motor using 389 pistons and mild cam without having problems? And if you did, what parts did you use that were stock and what if any modifications did you make? Currently, my motor is at the engine rebuilders and waiting for my decision on heads and pistons to use. Regarding heads, I have a set of 092s and 093s to use, but want to be able to drive the car using pump gas. I will keep the heads for resale value.

[Blackcat]

Currently running 5C's on a 389 .40 in my PU.
Watch the valve clearance. Nothing bigger than a HO or ram 3 for a cam.
If your timing chain lets go figure on New valves at the least. (had a new chain break after only 2k miles.) It took out 7 valves.

[b-man]

I would keep the original 093 heads on your '66 GTO Tri-Power engine. Run a set of dished pistons to lower the compression. A 15cc dish will get the compression down close to 9:1, enabling you to run 91 octane premium unleaded fuel.

A few gallons of leaded racing fuel from time to time will be enough to coat the valves and seats and keep them happy.


If you decide to use the 6X-4 heads you'll end up with between 8 and 8.5:1 compression depending on bore size and how much you mill the heads. You'll have to mill them pretty heavily (about .060) to get close to 8.5:1.

In my opinion it would be worth the extra $$ to have a set of pistons custom-made (Ross, SRP) to your final bore size with the 15 cc dish and valve reliefs properly located.

When you open the hood the engine will look 'right' with the 093 heads. Your cam choice won't be limited by a set of pistons with valve reliefs that don't match your heads.

[Skip Fix]

The 6X chambers are so deep I would really think they should be fine without any valve reliefs for a mild cam. They are .200 deeper than a RAIV chamber putting the valves that much further from the piston! The valves are in a different location/angle than your earlier ones. It will drop the compression. But they did use them on all the 76 and up 400s.

But a dish and some pocket ported factory heads will work too.Probably just throw some TRW 2262 in a lathe and cut a 2.8 x.100 dish and be fine.Some aftermarket dishes wil be lighter though.