Jaguar XJ-S. Manual - part 21

 
 

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On the bottom of the heads where they come in contact with the gasket, there are several openings for coolant to flow 
from the block through holes in the gasket and into the head.  One row is round openings, the other row is oblong holes. 
 Several of these holes may be rimmed with casting flashing; apparently the core meets the form right at the hole, and 
there was little effort to clean away the flashing at the factory.  When the head is off, take a Dremel and clear away this 
flashing to ensure the holes are fully open.  Might not be a big deal, but couldn't hurt. 

With the Jaguar V12, with just the heads off you might be able to accomplish several things that would require opening 
the bottom end on other engines.  The first possibility of note is that you can reseal the liners to the block.  Use the liner 
retainers (or some reasonable facsimiles) to hold 11 liners in place.  Turn the crank until the piston in the one 
unrestrained cylinder is at the bottom of its stroke, and then continue turning the crank while encouraging the liner to 
come up with the piston.  When the piston is at TDC but still at the bottom of the liner, the ledge on the side of the liner 
that sits on the block will be higher than the head surface.  You can then carefully clean the surface on the liner and the 
block, apply new sealant, and slide the liner back into place.  David Johnson actually used this method to reseal a liner 
that had accidentally come loose, and ended up knocking a few more loose just to make sure they were all properly 
sealed.  In fact, it might not be a bad idea to turn the crank a little with no retainers in place, just to see if any liners are 
loose enough to move.  Or, succumb to the temptation to reseal them all whether they need it or not.  See page 101 for 
notes on sealants to use here. 

Now we move on into theoretical, since as of this writing the following ideas have not been tried and reported back on; 
attempt at your own risk.  The next suggestion is that you may be able to replace the liners from above.  This is a bit 
harder than the resealing idea, since it requires taking each liner completely out.  The challenge is getting the piston 
rings back into the liners when reassembling, since even with the piston at TDC you’re working in too tight a place to 
use a conventional ring compressor.  However, you may actually be able to finger the rings into place one at a time.  
Squeeze the top ring, slide the liner down on it, and move on to the second ring.  John Napoli suggests you might even 
go ahead and cut a small taper into the bottom end of the liner before installation to make it easier to get the rings in.  
Or, you might be able to fashion a homemade ring compressor that will work in this space, perhaps from a hose clamp 
and some shim stock, or maybe from a pair of needlenose pliers and some shim stock. 

If that idea works, the next idea should work too:  Replacing the piston rings.  If you have the liner out and the piston is 
flopping around above the opening in the block, it shouldn’t be too difficult to carefully remove the old rings and slip 
on some new ones. 

I’d like to report that you could even replace the pistons themselves, but reportedly the piston does not come up high 
enough for the pin to clear the ledge that the liner sits on. 

 

CLEANING HEAD STUD HOLES:  Before reinstalling the head, it is of utmost importance that the holes for the studs 
be thoroughly cleaned.  Any crud remaining in these holes may be kicked loose when the head is slid down over the 
studs and it might fall out the bottom and sit on top of the head gasket while you’re closing it up, and you’ll have a bad 
seal. 

For cleaning the stud holes, a suitable item would be a wire brush shaped like a “bottle brush”.  If you’re real lucky, you 
might find one in an auto parts store, and if you’re even luckier it might be somewhere near the correct size.  Forget 
about luck and visit any sporting goods store or department and look at the tools available for cleaning rifle and shotgun 
barrels.  Outers and Hoppe’s make cleaning tools involving 3-piece shafts and little copper wire brushes that screw onto 
the end for very reasonable prices.  Buy one shotgun-cleaning brush assembly and additional interchangeable brass 
wire brush attachments for 10, 12, 16, 20, 28, and .410 gauge shotguns, and you’ll be able to clean just about any size 
hole you encounter.  You can opt for the rifle and pistol cleaning tools as well for cleaning smaller holes yet, but note 
that the threads on the brush itself might be different and therefore require either a different rod or an adapter.  Also 
note that Outers and Hoppe’s shotgun brushes are not interchangeable, so you’ll need to choose one brand for all the 
brushes you buy. 

As opposed to the items found in an auto parts store with handles that are merely a continuation of the twisted wires 
that form the brush itself, the shotgun brush assemblies have a finely machined 3-piece aluminum shaft.  Hence, it 
becomes quite reasonable to chuck one section of the shaft up in the variable-speed drill and clean those stud holes up 
in a hurry! 

 
 

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REPLACING HEAD STUDS:  Some of the head studs on the V12 are immersed in coolant.  If the coolant has not 
been maintained properly (changed on schedule), some of these studs may get corroded.  A small amount of surface 
corrosion is no big deal, but large-scale erosion can signficantly reduce the cross-sectional area of the stud -- which is a 
formula for blown head gaskets. 

Head studs can be thought of as springs.  When you tighten the nuts to the specified torque, you are stretching these 
springs to a particular load value.  Even though parts may expand a little with changes in temperature, the studs are 
designed to be long enough that this growth is minor compared to the stretch of the studs so the compressive force on 
the head gasket is maintained.  But if the shanks of the studs start corroding away, this preload is reduced.  And if the 
engine is reassembled without replacing such corroded studs, it may prove impossible to apply the specified torque; the 
bolt may yield at the narrowed section first. 

Peter Hyslop shares experience with these studs:  “This applies mainly to old V12 engines (ie >20 years old) with a few 
miles on them and which have never been apart: 

1) The studs can get stretched if over-torqued by some fool trying to take a short-cut on a head gasket leak ... this is an 
unusual degree of incompetence, so never use that mechanic again (and if you did it yourself, give up, there’s no hope 
for you). 

2) The studs immersed in the coolant can corrode and pit.  The others are usually fine. 

3) The studs immersed in the coolant can become adherent to the block and become brittle.  Some of these studs stand a 
reasonable chance of snapping off when you try to remove them ... yes, you read it correctly, the stud will snap before 
the aluminium block gives way. 

The solution: 

- Make sure that you really want to go this far before you start, and are prepared to stick it out when the going gets 
tough. 

- Leave the studs not in the coolant alone. 

- Pull the studs in coolant only if corroded. 

- Expect some of the corroded bolts to snap and land you with a big machinists’ bill. 

How do I know?...I’m up to my arse in the alligators right now with items two and three above (yup, snap, snap snap)!” 

Note that the AJ6 6-cylinder engine used in some XJ-S’s has an entirely different theory in head studs.  They are 
designed to be tightened until they yield, which provides a very closely controlled amount of compression on the head 
gasket -- but requires that the studs be replaced whenever the head is pulled. 

 

VALVE REMOVAL:  Jan Wikström says, “The normal generic valve compressor from K-mart will do fine; there's 
nothing special about the V12 valves.”  Apparently, K-mart is a quality tool source in Australia where Wikström lives; 
here in the US, the vast majority of valve spring compressors on the market are either cheap junk, designed specifically 
to fit a Chevy and nothing else, or both.  Trying to use cheap junk on valve springs can be seriously dangerous; if that 
thing snaps loose or breaks while holding the springs fully compressed, it can fire keepers and collars around the room 
like bullets. 

Some of these pieces of junk are intended to be operated from above only, which may be handy if you are trying to 
replace seals without pulling the heads, but these tools try to hook onto the coils directly; they compress part of the 
spring rather than the whole length of the spring, and they don’t compress the inner spring at all -- you have to push 
down on the tool to get the keepers in and out.  And you must hold the valve itself in place by other means, possibly 
applying compressed air to a spark plug hole or even inserting some rope and turning the engine around until the piston 
pushes it against the valve head. 

 
 

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If you have the head off, Gerald Foster recommends a valve spring compressor sold by Sears.  It costs less than $20, is 
a substantial tool that wraps around the head (like a big C-clamp) to push on the spring and the valve head at the same 
time, and is a Craftsman tool with a lifetime warranty. 

When using the C-clamp type compressor, you’ll find it helpful to tighten down on the spring a little bit and then give 
the top of the spring a little rap with a plastic hammer to pop the collar loose from the keepers.  Then you can proceed 
to compress the spring further. 

 

VALVE SEAT WORK:  It’s why you’re reading this section, right?  Well, when replacing the seat, the first thing you 
need to get right is the fit.  In the General Specifications section of the ROM under “Valve Guides and Seats”, there is a 
spec for “Interference fit in cylinder head” of 0.002”-0.006”.  This is not for the valve seats -- it’s the spec for the valve 
guides.  The correct interference fit, 0.003”, is in the ROM, but not here; it’s in section 12.29.18, step 14.  It’s followed 
by instructions to heat the head before installing. 

The ROM also talks about two oversize seats being available, but here in the US good luck finding a machine shop that 
cares.  The boring tool used to ream the hole in the aluminum and the sintered iron valve seats are both standards of 
some sort and they will use their own, not Jaguar’s.  If they’re any good at their jobs, they will machine the ID and the 
seat area of their insert to the same configuration as the original so you really wouldn’t know the difference unless you 
interrogated them about it.  It’s not like you have any good reason to insist on genuine Jaguar seats here; those were 
genuine Jaguar seats that fell out! 

Craig Sawyers talks about the hot/cold fit:  “Assuming that the temp difference between the seat and the head is 130°C, 
the differential contraction is 10ppm/°C and the seat OD is 1.5 inches, there will be a 1 thou interference under these 
conditions.  If the seat was chilled in a freezer to -40°C, it would be a precise fit.  Cooled to liquid nitrogen, it would 
just drop in. 

“Machining to give a greater interference than 3 thou may be counterproductive, as the seat would have to be driven 
home, and shave the aluminium on the way in, reducing the interference again (to about 3 thou?).” 

You need to make sure they cut the seat properly in order to ensure that the valve itself sits the same “depth” into the 
head, for two reasons.  First, the position of the closed valve greatly affects the combustion chamber volume and 
therefore the compression ratio in that cylinder; the farther off it is, the more fiddling somebody’s gonna have to do to 
cc the heads to make sure that each cylinder has the same volume.  When you see what these guys do to get these 
volumes the same, you’ll wish they had seated the valve right.  Of course, the correct valve position may be even more 
important on the H.E. engine since they are part of the swirl configuration of the head. 

The second reason has to do with adjusting the valve clearances.  The shims are only available in a limited range of 
sizes, and grinding away on the valve stem is a no-no. 

 

VALVE TRIMMING:  After the valves and seats have been machined to renew the contact surfaces, the valve will 
obviously sit lower in the head.  This closes up the valve clearances at the tappets.  Jim Cantrell points out:  “Often, 
people at this point will then cut the valve stem to get the additional clearance.  This will then cause the valve stem’s 
life to be reduced since the stems are hardened.  This hardening only penetrates a few thousandths of an inch and 
cutting it off exposes the softer valve material.”  The proper solution to inadequate clearance is to replace the valve, the 
seat, or both. 

 

SO YOU’RE DOING A VALVE JOB:  To many of us, doing a valve job means pulling the heads off the car, taking 
them to a machine shop, picking them up later, and bolting them back onto the engine.  However, the valves are a prime 
place for easy improvements when the heads are off.  I will mention a few ideas. 

The valve guides are sleeves that are press-fit into the heads, and protrude a little bit into the inlet and exhaust ports.  
Jim Isbell suggests that, before you install the valves, you take a hand grinder and grind off the protruding parts of the 
guides flush with the surface of the port. 

 
 

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The valves and seats on the Jaguar V12 are supposed to be machined with 44½º angles.  Most machine shops will 
suggest that the seats be “triple cut”,  in which additional cuts are made at angles greater and less than 44½º in order to 
control the width and location of the contact area.  Typical angles for these cuts would be 32º and 60º and the contact 
area width should be about 1½ mm wide.  Chad Bolles suggests that a similar triple cut on the valves themselves would 
be beneficial. 

John Milne suggests that, after the valve contact surfaces have been machined, some machinist’s bluing be applied and 
the valve trial fit to determine the location of the actual contact on the valve surface.  Then, carefully avoiding the 
contact area, the inner edge of the machined surface should be blended to form a smooth continuous surface with the 
“tulip” shape of the valve.  This helps the flow through the valve, since it makes a smooth passage instead of that 
corner.  It also slightly enlarges the opening, since the smallest flow area when the valve is open is between this inner 
corner and the seat.  “It’s kinda like getting a little extra valve lift for free.”  Blending the inner edges of the machined 
surfaces of the seat may have similar benefits. 

Of course, every high-performance enthusiast will suggest that you do a little “porting” while you’re in there.  This 
means carefully enlarging the intake and exhaust passages.  In the case of the intake passages on the Jaguar V12, 
however, this may produce undesirable results.  These intake passages are designed to provide a certain amount of 
resonance-induced flow enhancement, and this requires that the flow rates be fairly high.  Enlarging the ports makes for 
slower flow, which means better flow at high RPM but less boost at low RPM.  In other words, enlarging the intake 
ports may increase high-speed horsepower at the expense of low-speed torque. 

The next automatic suggestion is “polishing”.  Quite literally, the passages may be polished using successively finer 
abrasive compounds until a mirror finish is achieved.  This supposedly will reduce surface friction of the flow. 

Before you reinstall the head, Jim Isbell suggests you “cc” it.  This means that you measure the volume of each 
combustion chamber, and grind a little metal away here and there to make sure they are all the same.  This makes for a 
smooth running engine.  To measure the volume, you can set the heads upside down on a level surface and fill each 
chamber with a carefully measured amount of light oil. 

Cc’ing must be done after the valves are installed for the final assembly.  Clearly, if valves are ground or relocated after 
cc’ing, they will sit at a different level than they did before and this would significantly change the volume of the 
chamber.  And this is the reason that cc’ing is recommended whenever the valves have been redone. 

If you happen to be working on a pre-H.E., cc’ing doesn’t apply.  The head is flat, so there are no chambers to cc. 

 

TEFLON VALVE STEM SEALS:  Michel Carpentier says, “Teflon valve seals are the way to go.  They last forever, 
plus you can fit them on the exhaust guides as well.  Less blow-by, reduced crankcase pressure, no more oil leaks, and 
cleaner oil to boot. 

“Our engines came from the factory with 24 valves but only 12 valve stem seals.  Jaguar probably realized that the 
British rubber intake rendition would soon fry on the exhaust side.  

“Why would any (effective) exhaust valve seal affect crankcase pressure?  When the exhaust valves open, gases gush 
out of the combustion chamber with tremendous pressure.  Even though the valve itself acts as some sort of an 
umbrella, there is still a significant pressure differential between both sides of the exhaust guide.  Any clearance 
between valve and guide will let exhaust gases into the crankcase area.  Jaguar old trade secret to keep engines leaking 
oil!” 

Of course, the decision to switch to Teflon seals, and to install 24 instead of 12, may not be quite that simple.  This 
author had some discussion on the phone with the rep at Silver Seals about the differences between Teflon valve stem 
seals and normal nitrile seals.  There is apparently a functional difference: the nitrile seals always leak, and the leakage 
allows a small amount of oil down the valve guides to lubricate them.  The Teflon items will not leak, so the valve 
guides run dry.  According to the rep, this is a problem on some cars and not a problem on others.  I’m betting it’s not a 
problem on the Jag, since it uses good materials on both the valves and guides.  Also, the action of the cam on the 
tappet applies no sideways load to the valve stem, as there is in any valve train with rockers.  Still, recognize that you 
may be trading oil burning for faster valve guide wear when you install Teflon seals.