Jaguar XJ-S. Manual - part 16

 
 

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CAM COVERS OFF?:  If you remove the cam covers for any reason that is not intended to involve further disassembly 
of the engine, it cannot be reiterated too many times that every bolt, nut, tool, or whatever that is in this vicinity should 
have a string tied to it with the other end tied to your finger.  If a metal part falls down into the timing cover while 
you’re fiddling around, see the section on timing cover removal on page 92.  Peter Smith suggests “I shove a big rag 
down the timing cover on each side to catch the bolts if they fall.  I find this to be a nerve-racking part of the job - if the 
damned things fall in it can really test your sense of humour!” 

The repair manuals provide complete instructions for timing chain disengagement and camshaft removal, except they 
neglect to remind you that you might want to measure the valve clearances before disconnecting the timing chain and 
removing the cams.  The assembly must be together to make the measurements, so measuring before disassembly will 
save you having to slap it back together to measure it later.  Of course, if you plan on reseating valves and the like, the 
measurements won’t do you any good anyway. 

You might be able to replace the timing chain without removing anything beyond the RH cam cover.  See page 94. 

 

CAM COVER DECORATION:  Some of us think those Jaguar decals on the cam covers are not in keeping with the 
general class of this automobile.  One suggestion is to remove the decals and attach some brass insignias with screws -- 
making sure not to cause a leak.  Might even find just the right key fob to use. 

A suggestion from Steve Averill:  “If you want to find something a little better looking that the Jaguar decal on the 
valve cover, why not either get it photo etched or alodyned?  That’d be more apropos than sticking on something that’d 
probably wind up looking “tacked on” & you could pick any pattern that appeals to you.” 

If you have them off anyway and have a milling machine at your disposal, you might consider skimming the top of the 
ribs, leaving the black paint in the grooves between the ribs.  It’ll really make it look snazzy. 

 

CAM COVER GASKET/HALF MOON SEAL:  This joint is one of the most notorious sources of oil leakage on the 
Jaguar V12, although it’s entirely possible much of that blame is misdirected; between the tappet block leaking (see 
page 70) and the banjo fittings leaking (see page 43), sometimes it’s hard to be sure just how much of the oil is actually 
coming from the cam cover gasket or the half moon seal. 

Many owners are surprised that the cam cover gasket is a thin piece of cardboard (or, later, a thin Gortex sandwich), 
expecting to see a thick layer of cork or some such.  Of course, the thick layer of cork is called for on engines with 
stamped steel valve covers, but the cam covers on the Jaguar V12 are a quality aluminum casting with precision 
machined mating surfaces.  A thin gasket should seal just fine.  Of course, the original cardboard gasket won’t, but it’s 
not because it isn’t thick enough; it’s because it’s a cheap cardboard gasket. 

Most of the newfangled gaskets introduced by Ford are only moderately expensive, but the Gortex sandwich cam cover 
gaskets are obscenely expensive.  Apparently as a result, some of the mail order places continue to stock the old paper 
versions.  There is irony in that, since Ford updated nearly all the gaskets wholesale whether they were notorious for 
leaking or not but the ones that probably needed the update the most were the cam cover gaskets. 

Also involved in this joint is the half moon seal, a semicircular rubber plug used at the back end of the tappet block to 
fill an opening made while machining the cam bearing journals.  This plug is set in the opening, the cam cover gasket is 
set on top of it, and the cam cover is installed to hold them both in place.  This is why I have listed the two items as a 
single section; it’s not really valid to talk about the cam cover gasket in one section and the half moon seal in another, 
as the sealing of this joint involves them both -- and the bolts as well, discussed below. 

Michael Neal reports that some of the aftermarket half moon seals shrink a lot after only a matter of months and create a 
leak, while the genuine Jaguar half moon seals don’t shrink enough to cause a leak if installed as he describes below. 

Technical Service Bulletin #12-35, which introduced the Gortex sandwich gaskets, specifies that no sealant be used on 
the cam cover gasket but that the half moon seal be coated with RTV sealant.  Based on experience using the new 
gaskets, Neal recommends the following procedure for the cam covers:  “After much trial and error and the monitoring 
of some cars for several years I have come up with some leak-proof techniques.  Only use silicone on the rear of the 

 
 

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gaskets.  Use Hylosil (British-made white silicone) to seal the end.  Use a lot.  Coat the half moon seal all the way 
around.  Also coat the ends of the gaskets on both sides and one inch toward the front.  Assemble and wipe off the 
excess silicone.  Let it set up overnight!  Over time, the half moon seal shrinks and the silicone retains the oil proof 
seal.”  This procedure makes sense, especially the part about coating the gasket on both sides; since the gasket is not 
compressed above the half moon seal, a key problem will be the seal between the gasket and the cam cover itself in this 
area, the other side of the gasket from the half moon seal. 

Neal’s selection of Hylosil probably also merits noting.  The ROM often calls for Hylomar for “dressing” gaskets, but 
it’s just that -- a dressing.  It won’t provide a primary seal by itself, it is only intended to help a gasket provide a primary 
seal -- a paper gasket that is, it shouldn’t be used on Gortex.  If Hylosil is unavailable, it might be wise to find 
something comparable, some silicone sealant that will form and maintain a seal on its own. 

While using the upgraded Gortex gaskets, genuine Jaguar half moon seals, and gooping everything up properly with 
Hylosil may result in a fairly durable seal, these measures still fail to address the inherent problem at this joint.  In this 
author’s opinion as an engineer, a truly reliable seal in this assembly is only possible through a fundamental change in 
the way these parts work together.  A gasket -- any gasket -- will only seal properly when securely compressed between 
two hard surfaces.  Above the half moon seal, the cam cover gasket is not compressed; the rubber plug does not provide 
a hard surface for the cam cover to press the gasket against.  Instead, the rubber plug merely deforms a little, leaving the 
gasket locally uncompressed.  It’s no surprise that leaks are common.  The most common leak path may actually be 
between the gasket and the cam cover above the half moon seal, rather than around the half moon seal itself. 

The simplest solution, although not the first one conceived: omit the cam cover gasket.  Seal the entire kit 'n 
kaboodle with Loctite 518, including all the way around the rubber half moon seal, and bolt the cover on.  The 
rubber half moon seal will therefore be sealing against metal both top and bottom, and should work fine.  Likewise, 
Loctite 518 should provide a reliable seal in the joint between the machined aluminum tappet block and the 
machined aluminum cam cover. 

The first solution proposed was to somehow provide a rigid surface for the cam cover gasket to be compressed against. 
 One idea, suggested but as yet untried, is to drill a hole through the new rubber half moon seal and put a bolt through it 
with a washer and nut on the outside.  The head on the inside would need to be pretty broad and flat; perhaps a bolt 
intended for holding a toilet to the floor can be cut down to a suitable length.  After the plug is installed and the cam 
cover is securely bolted down, the nut could be tightened down to squeeze the plug from the sides (you might want to 
cut flats on the end of the bolt to be able to hold it still while tightening the nut).  Rubber is actually remarkably rigid in 
terms of hydrostatic compression, so squeezing the sides will make it expand forcibly in other directions and press very 
hard indeed on the tappet block and cam cover gasket, hopefully forming a secure seal.  This idea has an apparent 
advantage:  If the joint starts leaking later on, it may be possible to crank a little more on that nut and put a stop to it. 

John Napoli says, “I figured it was better to fill them than to play games with those silly seals.  Anyway, I filled the half 
moons up with Marine Tex.  I clamped wood on both sides, faced with wax paper, and positioned so that the goo stood 
proud of the mounting face.  After it set, I filed it down flush.  Seems to have worked.  I buy Marine Tex at marine 
supply stores.”  If you have the cam covers off anyway, you might want to go ahead and pull the camshafts and the 
tappet blocks so you can do this job on the bench.  Marine Tex dries hard, hard enough to serve as a base for 
compressing the cam cover gasket properly.  JB Weld might be an acceptable substitute, available in most auto supply 
houses.  Napoli points out that the only reason you would ever need this opening again is for boring out the cam 
bearing journals to an oversize, and if this is necessary then it won’t be difficult to use the boring machine to remove all 
the Marine Tex.  After completion of the reboring, you’ll probably want to install some more Marine Tex! 

Edwin Hyatt did this job:  “Used two old credit cards and a couple of miniature C clamps on the outside and inside of 
the U and piled the JB on.  Filed flat the next day.” 

Bob Gallivan:  “After you mix up the JB Weld use a hair drier to heat it up until it flows easily then fill the halfmoon 
spot, may even want to heat the tappet block in that area to help any air to escape.” 

The mold-in-place idea will probably only work with the tappet blocks out of the car.  When they are on the engine, 
they are held at an angle, so you’ll have trouble getting the JB Weld to stay in the recess while it sets up. 

 
 

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John Ashcroft went a slightly different route:  “Got some body filler and filled an egg cup to the top, pressed the old 
seals in to just under surface; when filler was hard, removed h/moons, perfect female mould.  Mixed up some JB weld, 
heated with heat gun and poured into mould.  Removed from mould after 24hrs. and they are perfect, even have the 
little groove for some Ultra Copper RTV, just have to dress the top so it is flush with tappet block.  Checked the fit with 
prussian blue, had to file just a fraction of what was left of the ridges.” 

This author went a more difficult but esthetically pleasing route and designed some aluminum plugs to replace the 
rubber half moon seals, and Ron Morse was kind enough to fab up a set for testing the idea.  Figure 3 shows the 
design. 

 

7/8" dia

1.0625" dia

.430"

3/16"

0.531"

1-1/4" dia

 

Figure 3 - Aluminum Half Moon Seal 

There are no tolerances shown on the drawing, so a description of which dimensions are critical is called for.  The 
1.0625” diameter fits very snugly in the opening, and the opening itself is rather precisely machined as a cam 
bearing journal diameter.  It might be acceptable to make this diameter a hair smaller, but it cannot be any larger or 
it won’t fit in the hole. 

The 0.531” height dimension is critical, since it locates the platform for the cam cover gasket to be compressed 
against.  This dimension should be held even if the 1.0625” dimension varies. 

The .430” width is necessary to fit around the boss on the tappet block.  This dimension can be made larger, but if 
made smaller it can make the part difficult to install.  Since the width of the boss was not held to tight tolerances by 
Jaguar, if possible you should measure it before having this plug made and widen that .430” dimension even more if 
necessary. 

All the other dimensions are wide tolerance, almost anything close will work. 

Of course, once you have replaced the rubber plugs with the aluminum, how do you seal the aluminum?  The 
original design intent was to use a 1/8” cross section Viton O-ring cut in half.  Unfortunately, this really doesn’t 
work well; the O-ring halves cause the plugs to pop up while you’re trying to finagle the cam cover and gasket into 
place, and if you just allow the cover to press the plug back into place you won’t even know whether or not the O-
ring ended up positioned properly until you start the car up. 

Forget the O-ring idea.  Seal this plug using a similar method as that used for sealing the rear main bearing cap to 
the block:  Hold the plug in place and inject a silicone sealant into one end of the opening until it comes out the 
other. 

 
 

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Or, apply a coating of Loctite 500-series sealant to the curved mating surfaces before installing the plug.  If you’ve 
had the tappet blocks off, you should have a suitable sealant on hand from sealing them to the heads (see page 70); 
if it’s good enough to seal that location, it certainly should seal this one.  If this sealing method is chosen, you’d be 
better off omitting the O-ring groove from the plug design, perhaps saving a bit on the machining cost. 

Or, you could consider installing the plugs with JB Weld or Marine Tex.  Getting them out later might prove to be a 
challenge, but they shouldn’t ever need to come out.  Again, omitting the O-ring groove might be a good idea. 

If you don’t want to make these aluminum plugs, Ron Kelnhofer (page 716) has some for sale. 

It has been suggested that it may be simplest to just heliarc weld the openings in the tappet block shut and machine 
the gasket surface.  There is no way I would recommend this, though; the distortion likely to result from such 
welding would surely compromise the seal between the tappet block and the head as well as play havoc with the 
alignment and concentricity of the cam bearing journals and tappet holes. 

Once you have provided a hard surface to compress the cam cover gaskets against, forget about gooping the Gortex 
gaskets as Neal recommended with the rubber seals.  Gortex gaskets are supposed to be installed dry, and now they 
should be. 

 

CAM COVER BOLTS:  Once we’ve addressed the shortcomings of the half moon seal, we still have other problems to 
deal with.  Neal says,  “New bolts are also necessary.  Unfortunately these parts are only carried by the dealer.” 

Before ordering bolts, determine whether your engine requires SAE bolts or metric bolts.  Judging from the 1988 
Jaguar Parts Catalogue, the change from SAE to metric took place with engine number 8S24175.  The part number for 
the new metric cam cover bolts is FS-106251/J. 

The replacement of the bolts may not be as much due to the excellence of the new bolts as to the lousiness of the old 
bolts.  The original bolts -- both SAE and metric -- have a “triangulated” thread that presumably helps it to drive into 
the aluminum housing, but probably is also very effective at boogering the aluminum threads up.  Martyn Sandbrook 
says, “I recognise these as "taptite" screws.  We used to use these in the telecoms industry way back in the seventies.  
They are like selftappers but they "form" a thread rather than cut one and are supposed to be better.  You don't see them 
these days.  I thought they were junk when I first came across them.  Seems strange to be using them in a leak-critical 
area as they only grip on the outer lobes once the thread has been formed and won't take much tightening torque.” 

If you’d rather not pay Jaguar prices here and seek other sources for bolts, note that 7/16” hex heads will not do, you 
can’t get a wrench or socket on it; either 3/8” (or smaller) hex head, socket head (allen wrench drive) or Torx drive 
bolts will be necessary.  And you might want to look at the access before you select bolt head styles, since you may 
want to retorque them a while after replacement.  Of course, buying el cheapo grade-zilch bolts (like most slotted head 
screws) is not a good idea; not only will they rust pretty badly, they may not hold enough tension to keep the cam cover 
gasket from leaking. 

An alternative bolt that would work on SAE threaded engines was found at a Home Depot in their “specialty fasteners” 
rack.  It is called a “serrated flange bolt”; it has a 3/8” hex with a washer face and a row of serrations around the bottom 
of that washer face.  The biggest problem will be finding enough of them to do the job; often such racks in hardware 
stores only contain a half dozen or so. 

Another alternative SAE bolt:  This author bought a box of 1-1/4” “alloy steel” 1/4”-20 “socket head cap screws” from 
a local industrial fastener supplier -- less than $20 for a box of 100.  These fasteners are jet black.  Strength is not a 
problem, alloy steel cap screws are stronger than Grade 8. 

The original SAE bolts were 1” long, so replacing them with 1-1/4” bolts provides enough length for some washers 
under the heads (and perhaps for thicker gaskets, if you find something non-OEM).  There’s no problem with bolts 
being too long, since the end just comes out the back side of the flange on the tappet block.  Using the longer screws 
also makes use of all the threads available in the tappet block, including the ones at the bottom end that haven’t been 
worn by the lousy OEM screws.