Half moon seals lived 34 years!

Well, i get to join the club. After 34 years and 60,000 miles, my half moon seals are finally leaking at back of both cam covers. Only a weep, so not at all bad, but i think I’ll make this my winter project.

I’ve ordered the quality cam cover gaskets at $45 a pop. But @John_John1 is all out of aluminum half moon seals?! I hope he makes them soon, otherwise, is there an alternative?

While there, I see Kirby said its a good idea to replace cam cover bolts. Anybody know what’s best?

And I may as well replace the two banjo bolts at back of cams, correct? With John_Johns improvement parts.

Anything else while cam covers are off? Clean the tappets with a toothbrush? :wink:


If you modify your PCV system, as I did, you will create a slight vacuum inside the engine.
Your leaks might just vanish, on their own.

Wow, just thinking back, that was one of my first major jobs when I first bought my XJ-S. Major job for me anyway as I certainly was no mechanic and relied a lot on my Jaguar mechanics advice at the time. I would probably do things a little bit differently had I the opportunity to do over again but to tell you the truth I’ve had no troubles or leaks since.
I bought my 1988 XJ-S in 2007 (83000 miles) and not having a clue what I was in for. But working with my mechanic I just replaced the rubber half moon seals with the proper part # of the time. I would of held out if I knew they were available for the John John aluminum version. I wanted to also improve the banjo bolts but my mechanic convinced me if they’re not leaking to leave them alone more of a PITA job . So I did. I did clean up the crap out of everything and got a good set of gaskets and made sure everything seated properly. I didn’t change the cam cover bolts just cleaned them up prior to even knowing about JL’s or Kirby’s book.
Like I say it all worked out fine but I might have done a few of the things you’re talking about in hindsight.

Gary, I bought upgraded stainless banjos, and aluminum half moons, from Ron Kelnhofer back in about 2003. (Sadly Ron has passed on in the time since). I did the oil banjos myself, in situ, and it had me cussing a bit since it is indeed a small area in which to work. I was also replacing heater hoses (one gad developed a small split) so that gave me a little more space to do the banjos. They weren’t leaking but I liked the notion that flow could be improved with “improved” banjo bolts. I also replaced the coolant banjo bolt with one from him.

I left the half moons for the Jag mechanic that I use. He took care of it next time I had my car in for any work.

I have also more recently purchased items from John John. He does quality work also.

I’m working on a new batch of half-moon seals so they will soon be back in stock.
Link to my eBay listing: Jaguar V12 engine aluminium half-moon seals; E-Type; XJS; XJ12 | eBay


I’ve been reading through archives, and it seems there was a Jaguar TSB to replace the Cam Cover bolts along with the new gortex gaskets (which I have purchased). But those bolts are NLA, and if you can find them, people are asking $12 each! You need 24 of them.

Kirby has been mentioning over the last couple decades to simply use alloy steel socket head bolts, which are easier to get to I guess, along with a washer and lockwasher.

I’m a bit nervous using alloy steel into an aluminum head, would stainless steel be sufficient? It’s only 8 ft lbs, so I assume corrosion resistance is important, not strength. And is it OK to use antiseize here?


The original bolts were also alloy steel. No worries on that front.
The new bolts from Jaguar are simply flanged bolts with an external hex head. I bought originsl for one engine a couple of years ago, and yes, they were expensive. These bolts for McMaster Carr are the same length and flanged, just internal hex. Be sure to use then with the new style gaskets. You’ll need three packs of 10.

[Flanged Socket Head Screw, M6 x 1 mm Thread, 25 mm Length McMaster-Carr](Flanged Socket Head Screw, M6 x 1 mm Thread, 25 mm Length McMaster-Carr)


Stainless and alloy are both enough harder than aluminum that it doesn’t really matter which you use. If you screw it up you’re going to bugger the threads in the aluminum with either. So just don’t screw it up :grimacing:.
Definitely use anti-seize.
You might look and see if ARP has a nice 12 point stainless that would work.

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Thank you! So if they are flanged, does that mean no washers or lock washers?

I’ll write on my forehead 8 ft lbs max! That should keep me safe. :slight_smile:

No washers on the bolts, or sealant on the gaskets.

Ok, thanks, I’ll make a note - bolts no washers.

Yep, I read about the gortex gaskets needing no sealant anywhere, unless you use rubber half moon seals, which I’m not.

Now, I’ve read several recommendations over the years on what sealant to use for aluminum half moon seals between it and cam block side.

Any winners over the years as to which is the best to use?

Any sealant needed above half moon seals where the two gaps are?

The aluminum half moons fit very tightly in the heads. Only very light film of automotive silicone sealant is required. They’ll fit absolutely flush with the gasket sealing surface.


Thanks. I also read Jonas’s write up on Ebay…his advice is to use an anaerobic sealant, and then install the cam covers without gasket to help fit the half moon seals flush. Once they dry, then install cam covers with the new and improved gortex gaskets.

Jonas advice makes sense

If it factors into your manufacturing process, I need 3 sets please :slight_smile: My father and I have some maintenance to do.

Have just finished new batch

Let me know your PayPal email and I’ll send you the invoice for payment


@John_John : nice :slight_smile:

@gregma : half moon are easy to do when you have the cam covers out
I used the Loctite specific sealant, expensive, but supposed to do the job once (I still got plenty, but you’re too far so I can easily share)

while you’re in there, get the banjo bolts + washers from john_john

as you’ll have to take the inlet manifolds to get the cam covers out, it’ll be a good time to check for leaks on the side water rails , and replace the gaskets + clean around

top hat bushes, #11 here (you need 4 of those)

C3990 seem to be a few available

(get the real ones, you don’t want them to fail early

not sure you need the fancy stainless water pipes ? (I just brushed the old ones, and they came back fine

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Thanks. You’ve convinced me, water rails should be very easy with manifold out of way. I’ll splurge for SS rails, and make sure to get (4) oem top hat seals. How many gaskets are needed? I assume 4?

I’m definitely doing the two banjo bolts, got order from Jonas.

While i have cam covers off, i think im going to strip off the peeling paint, and have them powder coated blue!

Four gaskets per side. Be careful loosening the bolts securing the water manifolds as they might snap if not soaked with penetrating fluid first. While everything is apart, consider the “Lutz mod’” in the archives, to improve the water flow at the back of the engine. I did all that to mine.

Thanks for the info, I will make sure to get out my PB Blaster. I’ve gone through so many cans on this car! :rofl:

I’ve ordered everything for the rails - (2) Stainless Steel rails, (8) gaskets, and (4) top hat seals (C37990). Boy, the seals are expensive for OEM ($18 each) but I don’t want to do this again. As Kirby said earlier, install these seals DRY. Correct?

Do I need to drain all coolant from radiator? Or will removing thermostat hoses be sufficient?