Jaguar XJ-S. Manual - part 17

 
 

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If you need M6 socket head or flange bolts, finding them locally may be more troublesome -- at least here in the US.  
Craig Sawyers says, “Well, here in the UK, the challenge is to find non-metric SS screws!”  Again, going with slightly 
longer screws than the OEM 25mm items -- perhaps 30mm -- might be a good idea. 

Dave Oxenreider says, “I found a whole slew of 18-8 stainless steel socket head cap screws in both metric and English 
in the McMaster-Carr catalog.  Averaging about $25 per box of 100.”  Joe Bialy opted for non-stainless (no good 
reason for stainless here, actually):  “Part #91290A332 for $7.34”  See page 708. 

Another source would be Barnhill Bolt; see page 707. 

If you have a local Grainger outlet (page 708), the Grainger catalog doesn’t seem to list any suitable M6 socket head 
screws but they carry “button head socket screws” which use a smaller Allen wrench but will work.  The box of 100 
screws 30mm long is stock number 3L184 and costs less than $20. 

On the other hand, if your cam covers need M6 screws, it might not need screws with unusual heads.  The typical 
generic locally-available M6 screw has a 10mm hex head.  It appears that a 10mm socket will actually fit in the recess, 
making the search for socket heads or flange bolts unnecessary. 

An idea that might work with either 7/16” or 10mm hex head screws:  If you can find some sleeves with a length of 
perhaps 10mm, 1/2”, or 15mm, you could use them together with screws that are proportionately longer (40mm, 1-3/4”, 
or 45mm) which would raise the location of the hex head enough to enable you to get a socket on it with less trouble.  
A cursory inspection of this author’s engine indicates you certainly wouldn’t want to go any longer than 15mm -- you’d 
start interfering with the bottom of the intake manifold.  There is a spot at each end of each manifold where a bolt hole 
boss is provided (holds fuel pressure regulator at the front, other stuff at rear) and it might actually help to grind away a 
bit at the bottom of this boss.  No reason not to, although it might not be that helpful either.  The bigger problem is with 
the center runner of both the front three and the rear three runners on each bank, which go right over a cam cover bolt.  
Any taller than 15mm would most assuredly cause interference here. 

If you can avoid interference, having the heads up higher might actually make it possible to retorque these bolts without 
pulling the manifolds off!  It wouldn’t be easy by any stretch, but it might be possible. 

With the socket head cap screws, the author used split ring lock washers and flat washers.  As noted on page 26, flat 
washers are always recommended when a bolt head or nut sits on aluminum.  (Note:  TSB 12-35 specifically prohibits 
the use of washers under the special Jaguar bolts in this location.  Put that down as a fundamental disagreement 
between their engineers and this engineer -- and note that this engineer’s cam cover bolts haven’t come loose and the 
gaskets don’t leak, claims which several other owners report cannot be made about the official Jaguar assembly 
procedure.)  This author found some very suitable flat washers on a bubble card in a Wal-Mart:  They are size 12, also 
known as 3/16”.  If you ask for 1/4” flat washers, you get washers that fit so loosely on the screw that it looks like a 
socket head might pull through the hole!  Also, the OD of the standard 1/4” washers are bigger than the flat area on the 
cam covers, so you’d have to cut them down to get them in.  Conversely, these #12 washers fit perfectly on a 1/4” bolt 
and are the correct OD for the cam cover flats.  Make sure you find cheap #12 washers; better quality ones might have 
closer tolerances and not fit a 1/4” bolt. 

Should you use lock washers?  This author thinks so; after all, they can’t hurt.  Craig Sawyers replaced his cam cover 
gaskets with new paper gaskets, and they shortly began to leak; when he went back in there, he found the bolts loose.  
Why?  Well, maybe it’s because the paper gaskets compress and relax the tension on the bolts, and therefore switching 
to the Gortex sandwich gasket is the fix; or it may be because these bolts are simply too short to stay tight without lock 
washers (see page 26) or some other retention scheme.  The situation is rife with opinions, but note that it’s really hard 
to retorque these fasteners without removing the intake manifolds.  This is not a place to save money or weight; if it 
might help avoid problems, do it. 

You can buy enough flat washers and lock washers to do this job for less than two bucks total. 

Another type washer that will work on the cam covers is the 1/4” spiral groove washer used in a couple other places on 
the V12 -- if you can find a supply of them.  These washers serve as both a flat washer and lock washer in one.  Also, 
the 6mm wavy spring washers mentioned on page 26 as a substitute for the spiral groove washers will work nicely. 

 
 

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There is one other benefit to going with longer screws with spacers.  Longer screws have more “stretch” when 
tightened to a particular torque, and therefore hold tension better and are less likely to vibrate loose.  It might still be a 
good idea to include lock washers, but providing yet another scheme to help keep tension on those cam cover gaskets 
can’t hurt.  The spacers might omit the need for flat washers, depending on the wall thickness of the spacers and 
therefore how well they distribute load onto the aluminum face. 

As always, be sure to use anti-seize compound on the threads when assembling. 

TSB 12-35 specifies that the cam cover bolts be torqued to 9.5-11.5 N-m (7-8.5 ft-lb).  They didn’t mention whether 
this was for the later metric bolts or the earlier SAE bolts, but hopefully it won’t make much difference.  It’s also 
probably a good spec whether you’re using their special bolts or some of the generic substitutes described above.  It’s 
not really a place where torque is critical, however; just tightening until they feel tight should be good enough. 

 

TIMING CHAIN TENSIONER:  To replace the tensioner requires removal of the timing cover which would put this 
section under “Engine Work -- Front”.  However, the problems occur when retracting the tensioner, which is usually 
done when removing the cams or the heads -- so it’s here in the “Engine Work -- Top” section. 

There are few design features of the Jaguar V12 as poorly conceived as the material the arch of the timing chain 
tensioner is made of.  If you manage, on your first try, to retract the tensioner and reengage it successfully without 
breaking this arch, you should consider yourself lucky.  The arch apparently works well when new but gets brittle with 
age or heat, and an old one can be broken very easily.  Michael Neal, who works on Jaguars every day, says “I’ve 
become so paranoid of old tensioners, my failure rate has been very high as of late.  I’m not too surprised though, most 
of the cars have been approaching ten years old.  I’ve been as careful as possible, knowing exactly what I was doing, 
and have watched the damn thing crack on one of the pivots.”  Obviously, if you have the timing cover off for other 
reasons and the tensioner is intact but has a few years on it, it may be advisable to replace it anyway. 

The problems may have to do with the V12’s proclivity towards overheating.  Craig Sawyers says, “The V12 chain 
tensioner is, according to the technical article that was published when the V12 was launched in the early '70s, made 
from molydisulphide-loaded nylon.  Just looking up the spec for something else, I found that Nylatron GS is just such a 
material.  All the characteristics that are listed makes this a sensible choice for the tensioner blade, apart from a 
continuous working temperature of 100°C maximum, with maximum non-continuous temp of 145°C. 

“I'll wager that embrittlement of the blade is due to a combination of the oil environment and overheating.  I have only 
the evidence of one piece of information:  My engine had never overheated, and the blade, although grooved where the 
chain had worn it, was still flexible after 160,000 miles.” 

Other engines have arched timing chain tensioners in which the arch itself is made of spring steel.  It would seem a 
simple matter to design a replacement arch for the Jaguar V12 tensioner made of spring steel, thereby eliminating the 
fracture problems permanently; if noise or wear is a concern, the spring steel arch could be faced with Teflon or some 
such.  However, despite more than two decades of trouble with this piece of crap, apparently neither Jaguar nor any 
aftermarket companies have opted to offer an improved part. 

You can see what this tensioner looks like at 

 

 

http://www.jag-lovers.org/xj-s/book/tensioner.html

 

For those without access to the www I will attempt a description of the tensioner, since it is well hidden and its 
configuration may not be apparent until it is too late.  Basically, the tensioner consists of a plastic arch that the timing 
chain slides over top of on its way from the crankshaft sprocket to the right side cam sprocket.  This is the slack side of 
the chain, so it’s the proper place to put the tensioner.  Between the legs of this arch is a spring that pulls the legs 
together, making the arch steeper and taller and therefore taking up slack in the chain.  The end of the arch nearest the 
crankshaft is located by a pin on the block, while the end near the cam sprocket has a steel foot attached that is free to 
slide up and down on the inside of the housing as the arch changes shape. 

Alongside the spring is a latching assembly that consists of a rod that is attached near the pinned end of the arch and 
passes through a hole in a rocking latch attached to the steel foot on the other end of the arch.  The aforementioned 
spring is actually attached to this rocking latch, slightly off center, so that it not only applies tension to the chain but 

 
 

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also causes the latch to rock in the CW direction (as viewed from the front of the car facing rearward).  This assembly 
serves two purposes:  First, the rod going through the hole in the rocking latch makes a very effective one-way lock; the 
rod can easily slide in the direction that allows the tensioner to take up slack, but it cannot back up and allow more 
slack.  Second, when the mechanic has manually retracted the tensioner, a step at the end of the rod provides a catch for 
holding the tensioner in retracted mode while working on the car. 

When removing the camshafts or the heads, the tension on the chain must be disengaged.  However, the chain is not 
removed; the sprockets are simply unbolted from the camshafts and supported in place by special brackets provided for 
the purpose.  The heads are removed in this state, leaving the chain and sprockets hanging there in mid-air. 

Before proceeding, please reread the warning on having the cam covers off on page 59.  Also, don’t do anything until 
you’ve read through this entire section and the following section on disaster avoidance and understood them. 

To retract the tensioner requires a pair of tools collectively known as Jaguar tool JD.50.  One tool is inserted from the 
top of the engine where the cam cover has been removed, underneath the cam sprocket, and hooks onto a hole in the 
steel foot at the top end of the tensioner; this tool is for pulling the tensioner back to the disengaged position, and has a 
handle shaped to rest against the top of the sprocket for leverage.  The other tool is inserted through an access hole in 
the front of the timing cover and is used to manipulate the rocking latch.  The rocking latch has a slot in it, 3/16” wide 
by a little over 1/2” long and roughly parallel to the chain line, for inserting this tool. 

 
 

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TRIM HOOK AS SHOWN

1/8"

1/4"

3/16"

 

Figure 4 - Improvised JD.50 Tools 

Improvising both tools is fairly easy.  For the first tool, a sturdy hook is required; a standard battery hold-down bolt 
may be a good place to start, but you’ll need to trim the hook as shown in Figure 4 in order for it to fit the hole in the 
tensioner properly.  Robert Louis Woodling says, “I used a long hanger for a peg board...”  Adding a lever that rests on 
top of the sprocket isn’t too hard, you can even make it out of wood; just drill a 1/2” hole near the center of a 1” x 2” 
board about a foot long, insert the battery hold-down bolt through it, and put a fender washer and a wing nut on the end. 
 The hole needs to be about 1/2” to allow the battery hold-down bolt to tilt relative to the board. 

Alternatively, Thomas E. Alberts says “I tried the lever style tool like the manuals show and didn’t like it much.  I made 
one with a long 3/8” bolt (~12in) and a steel tube (~1.5in dia).  I formed a hook by grinding away most of the bolt head, 
padded one end of the tube and put it against the valve cover mating surface and with a washer and a nut on the