Big Valve Military head?

I picked this up a few years back , was told it was a military application head , that has big valves !
Looking at the waterways it fits a XJ6 block .
Anyone know anything about them ?

Some UK military vehicles had them fitted, such as

British magazine Practical Classics rebuilt one of these military units about 20 years ago and raffled it off.
The whole engine was painted in that light green colour.
The military engine had some minor upgrades.

I remember the article. Was it dry sump?

Everything, including distributor and plug wires were shielded against water ingress and maybe electrical interference.

One row of tappet guides (exhaust IIRC) is larger but the valves are XJ6 EFI size - i.e 1.625" ex and 1.875 inl. It would be good to get exact measures.

The key valve gear difference is a mechanism to ensure continuoys slow but positive rotation of valves. This was to protect against excess wear at high temps and revs in the battlefield, when/ if forced to use crummy fuel.

By rotating fractionally each stroke, the red-hot exh valve material was always coming down against fresh seat material and this tends to reduce/even-out seat erosion that could eventually lead to a burnt valve or seat. It would be nice to get exact valve head diameters and a look under the cams.

The reason the race guys like thos heads is the bigger tappet guides alloq for bigger cam lobes without striking the edge, but cams that aggressive aren’t road user friendly.

Thanks for that Peter , not sure if I will ever use it , I don’t like the XJ6 Blocks , but if I did use it the extra water ways would be blocked up !

I vaguely recalled discussing this before, and tried out our spiffy search feature:

I’m taking my info from Roger Bywater’s work IIRC. I think the vast majority of XK valves rotate anyhow, because nothing is perfect and any off-centre tappet wiping by the cam lobe would tend to rotate the tappet & valve. I have seen plenty of valve stems worn asymmetrically, however, which shows the same stem edge stays rubbing the guide

I’m just doing a story on a J6 engine replacement and am hoping to buy the discarded engine to see exactly what (if anything!) is different in the cam area. It all looks disarmingly standard under the cams in the few areas you can see in the photos. Maybe it was just a bit of extra offset but that’s not what I recall (or mis-remembered!)

As I understood it (?), the impetus for valve rotation is twisting of the valve spring ends, relative to each other, as the spring is compressed/expanded. This is a relatively weak effect that depends partly on inertia. So the valves rotate more at high RPM, and if certain wear occurs, they can stop rotating permanently.

So valve rotation doesn’t necessarily depend on tappet rotation. We’ve discussed the latter as well, but my memory is weak. I think we agreed that there is little or no offset of the cam lobe from the tappet centreline? But, as Galileo said, it still moves!

Ah! You’re right - I think the spring had more to do with it. Would certainlt transmit more torque via spring retainer and collet grip than the polished pad face of the tappet turning the polished valve tip.

Well I asked him to quote for me buying his old J60 so maybe we’ll see.

Petep

Hi Ian,

Rob Beere Racing used the military spec engines to build the engines for the Revival Motorsport D-Type (RAM) series in the UK in the early 2000’s. The inlet valves are 1.8" and 1.625" exhaust valves. There are some modifications that are needed for a stock XK block as Pete has suggested. Exactly what is needed would be best asked of Rob Beere.

Best
Keith

Thanks for the feedback all :+1:

Just as an aside, we had a hard working Nissan mini bus running between Kaduna and Minna in Nigeria clocking up incredible mileages…watched the local mechanics take the head and sump off one day for rings and a valve grind…one guy sitting there in the dust slamming valves straight up and down with the grinding paste and rubber sucker…rotate it I suggested…no boss he replied this is the way the valves work, all I can say is that when the bus was a up and running again, it seemed as good as ever…as far as XK valves go, I have only ever seen wear on one side of the stem only…

Just like British bikes with hemi heads.

Question: unlike British hemi pushrod engines, where the wiping action of rocker tip across the end of the valve gives side thrust, how does an XK valve, hidden below a bucket tappet, get any side thrust at all?

Pete

Is the wear always on one side (relative to the head, e.g. always outboard)?

I can’t recall whether outboard or inboard as always examined the valves after they were out, interesting point Pete made though, is it possible that the tiny amount of rock the bucket might produce under load would transmit to the valve ??or is it a lubrication anomaly ??

Hahaha–good point that you wouldn’t know the orientation of the valve stem wear after you remove the valve. It was a stupid question on my part. But I think such wear wouldn’t occur were the valves rotating. The “passive rotation” of valves due to spring twisting is not robust. BTW, I read that a key to this is design of the split retainers. Due to differing inertial forces on valves (whole valve moves) and springs (only half moves, in effect), the valve “unloads” from the retainer as it heads upward, allowing the spring and valve be briefly decoupled. The spring twists briefly while the valve does not. This causes the 20 RPM or so net valve rotation while the springs are twisting/relaxing 2000 times/minute.

But if anything stops the valve–uneven wear for example, it won’t rotate again because the passive rotation force is so weak. That’s why positive valve rotation is an advantage.

IIRC the valve stems are a different size, specific to the military heads. Please dont ask how I know this.