Half moon seals lived 34 years!

I love John John’s work, and I think his half moon seals are spectacular: that said, for the longest time I was able to make the slightly hardened rubber ones work by a thin swipe of silicone on all the surfaces. It made them completely leakfree.

I did that on the Jeepster when I put its engine in, and they’ve been completely leak free.

From his photo, he just cranked out enough for 80 V12s! Within a couple years, will all Jaguar V12s have his half moon seals forever? :wink:

1 Like

Yes, the seals don’t need oily lubrication. All the coolant? No, as long as it is below where you are working! Kinda hard to guess!
When everything is apart you may find some corrosion on the castings where the seals reside. After some scraping, I used JB Weld to smooth out the surface of the casting. No leaks so far.

You can get the coolant right below where you want it if you siphon thru the heater hose…

I have also made replacement stainless steel blanking plates as mine where rusted. Also integrated throttle spring bracket into the blanking plate.


I have siphoned (small tube) from the radiator bleed screw hole. Works okay. Just takes a bit longer

1 Like

I’m going to have to stop reading this post now. It’s reminding me of all the stuff i should go back in and redo, with proper half moons and water rail Lutz upgrade. I always get torn between upgrade or if it ain’t broken leave it. ???


I have the McMaster Carr bolts in hand. My previous bolts look fine, so perhaps by 1988 Jaguar switched to better bolts? I like these new ones better anyways. And a little easier getting a hex on them rather than a socket.

The rubber half moons, if done right, should last a long time. I would just wait for something else to happen, for an excuse. The half moon seals leaking is only reason why I’m tackling banjo bolts and water rails.

I’m not bothering with Lutz change, there was talk a while ago debating it…when I’m not sure, I just stick to Jaguar’s original design.


Question. My aftermarket coolant senders in the rails are I think brass, or ?, as I’m getting a bit of bluish galvanic corrosion where they meet. Or could it be I used a copper based anti-seize? While I have rails off, I will easily clean things up. Is there something I can put on the threads to prevent this from happening again?

Another question, I have (8) gaskets for the rails/heads. I see there are (4) blanking plates in the middle. If I remove these to clean up, do they need gaskets? Or not? Should I let them be?

I’d pull the blanking plates and clean up. Just my preference as I like to detail stuff. IIRC they require gaskets just like the water rail heads.

I think the banking plates have gaskets.

Seems that I saw in the last several days that Jonas has milled blanking plates in addition to the multitude of half moons

I would recommend to pull them off. When I removed mine they had quite a bit of corrosion so I made stainless steel ones. I made them slightly thicker so I could drill them and install additional overheat temperature switches. I used VW 110c overheat switches and wired them in series with temperature sensor so if one is activated then temp gauge would jump to H
I still have few extra sets if someone would like to upgrade :wink:


In addition to taking care of cam oil banjo bolts and water rails while I have cam covers and intake manifolds off, I am planning on a PCV system mod based on what Kirby mentions in book and what Dave has successfully done. I will introduce a thread once I start work on all of this in mid-December.

Pretty simple really -

  1. PCV valve now goes into “pig snout”, with hose going up to same air intake rail for vacuum. Block off old hole in B bank air housing.
  2. Tap A bank cam cover with something like this (3/8" ID fuel hose barb 1/4" NPT threads), with hose going to A bank air housing. Secure with nut, and Blue loctite. This will be for outside air into crankcase.
    Found one in blue anodized aluminum, to match my cam covers when I powder coat them blue :slight_smile:

I will have to wait to get cam cover off to decide exactly where to drill, but most probably on interior sidewall, between cylinder 3 and 4? (Thanks Dave!), or more preferably between 5 and 6, if I can get hose over there.

1 Like

Take some pictures Greg when you’re all done!

1 Like

Maybe some photos along the way also. Thanks

Don’t forget the most important thing- a refrigerator magnet. (I have a few spares.)

You mean to block off B air bank housing?

1 Like