Why is there a hole in the jackshaft cover under the throttle pedestal?

In my attempt to swap an EFI throttle pedestal over to a carb v12, I’ve discovered there is a 1.5in hole under the pedestal on the v12 engine. What is this hole for? It was removed even in the pre-HE EFI cars.

Would it be possible to swap the later jackshaft cover (say pre-HE XJS) to a carb V12 and still use the original carb distributor?

This is rumor and hearsay… originally the engine was to have two distributors, each running six cylinder. Now would be a nice time to hear a debunking story.

I can’t find any difference in the covers other than what you note and an odd boss here or there.
The hole you mentioned may be a leftover of the very early twin distributor setup prior to opus pointless coming along. There was also additional jack shaft bearing support at the rear.

I can only go by what l have which is the design and background of the V12 booklet by Walter Hassan.
It makes mention of Opus due to the problems encounted using twin 6 cylinder distributors on the early quad cam version.

The second hole boss takes the shape of the throttle pedestal. It can’t be for a second distributor. The casting was redesigned at some point if there were once two distributor holes, because now there is one distributor boss and hole, and one throttle pedestal boss and a second hole of no obvious use.

They could always have stopped machining the second hole if it no longer had a purpose.

So it must have a purpose in assembly. What that would be I cannot imagine.

Perhaps Machining holes you don’t need is the basis for the Leyland toilet flushing logo.

Dear Vikram, EDIT: I’m not We’re ignoring your real question… so here it is again …

“ Would it be possible to swap the later jackshaft cover (say pre-HE XJS) to a carb V12 and still use the original carb distributor?”

I thought l did answer but short answer is from carby block through to HE block the covers are interchangable. I don’t have a 6 litre block handy but can’t see any reason it would be any different other than the provision for the revised breather system at the rear. I know nothing of non distributor motors.

My apologies, you did. I missed the real question and your real answer in my haste to respond to the topic title question.


Like the P3 Rolls… and most V12 Ferraris. I did not know that was a plan for the Jaguar engine.

Until a viable non points ignition system came along a single 12 cylinder distributor was not capable.
The figures used in Hassan’s booklet quote a V12 @ 6000 RPM requires 600 sparks per second and considered 400 sparks per second as max for make and break points system.
They did have issues with the synchronisation of the two distributors in the development.

This thread prompted a refresh by way of some long stashed away jag stuff.
This one is interesting, from promo “genisis of the jaguar v12”, check out the tail end of, and the length of the jack shaft.

I learn something new everyday! Very cool, I figured it was a throw away due to the Brico FI venture going toes up late in the game.

Some type of trigger off of the shaft for the FI in other words.

Thanks for posting.