"X" heads

Will Reeve · Post by **Will Reeve** » Sun Oct 24, 2010 2:46 pm

Guys,
We got my spare heads I was given with the car off the shelf and noticed that they were different! One has a X stamped after the part number and has a noticeably larger volume, the valves are closer together with less land in-between! The the same SD1 size inlet and exhaust valves are in both heads. Normal SD1 on the right.
Any idea of the application for the X head?

ChrisJC · Post by **ChrisJC** » Sun Oct 24, 2010 3:42 pm

Are the actual valve centres different?

I've seen a similar variation before, where on the head on the left, the valve seat is cut further into the head, so the valves sit further into the head too.

Chris.

CastleMGBV8 · Post by **CastleMGBV8** » Sun Oct 24, 2010 3:54 pm

Will,

From the pic the head on the left appearsd to have had the seats cut for larger valves which would explain why the standard valves appear to be recessed.

The usual upgrade to stage 3 is 1.63" Inlet. 1.4" Exhaust.

Can you measure the seat sizes?

Kevin.

mgbv8 · Post by **mgbv8** » Mon Oct 25, 2010 9:47 am

the ones on the right are P6 heads I think. The ones on the left are later heads with smaller chambers. I've got a few pairs of heads like this where the cutout around the valve seems excessive.?
They are the same heads but with different sized chambers yes??

Darkspeed · Post by **Darkspeed** » Mon Oct 25, 2010 10:17 pm

Best way of checking is with the casting reference - P6 heads have Birmal and subsequent heads have the various ERC HRC references as detailed in the identification thread sticky.

If Birmal P6 castings then the x may be reference that the rebuilder has fitted the larger SD1 valves

Andrew

DEVONMAN · Post by **DEVONMAN** » Thu Oct 28, 2010 7:33 pm

Darkspeed wrote:Best way of checking is with the casting reference - P6 heads have Birmal and subsequent heads have the various ERC HRC references as detailed in the identification thread sticky.

If Birmal P6 castings then the x may be reference that the rebuilder has fitted the larger SD1 valves

Andrew

Also P6 heads would have a short thread in the spark plug hole.
Regards Denis

Will Reeve · Post by **Will Reeve** » Thu Oct 28, 2010 7:42 pm

I'll take some more photographs and some measurements. The valves do look like they are set further back in the seats for some reason.

Flying Phil · Post by **Flying Phil** » Thu Nov 11, 2010 1:07 pm

Years ago I visited "Rovertec" in Fleckney and Ray (Who is now V8 Developments I believe) showed me some X heads and he said that they were better for tuning -( as there were thicker port walls for more grinding to give better flow?)