Jaguar XJ-S. Manual - part 50

 
 

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operated by a lever that extended up to just below the upper hose fitting.  Evidently, the image of luxury is supposed to 
include being able to drain your coolant while wearing a tuxedo.  For all this effort and expense, Jaguar couldn’t bring 
themselves to provide an outlet out the bottom of the car, so opening the drain valve causes coolant to pour all over the 
structures in the area and dribble out wherever the catch pan isn’t.  The end of the valve is a spherical shape with a 
funny flange, making it rather difficult to attach a hose, but it’s possible with enough determination. 

Before you get too involved with that drain cock, let me provide a description of what you’re getting into.  The drain 
cock itself is a solid brass tapered plug valve that turns 90º from full open to full shut.  There is a spring on the bottom 
that “loads” the tapered plug to keep it sealed; the seal is brass-to-brass, there are no elastomers inside the valve.  There 
is also a little diamond-shaped washer that fits on a shoulder with two flats that limits motion to 90 degrees.  This valve 
is probably repairable from most of its typical failure modes, which is an option you may want to keep open; read on 
before doing anything irreversible. 

Unscrewing the valve from the radiator may be the first irreversible thing you do.  It is not a tapered thread; it is sealed 
by a fiber washer.  But when tightening down, the valve must end up oriented properly to align with the remote handle. 
 To accomplish this interesting feat, Jaguar appears to have used two tactics:  First, the fiber washer may in fact be two 
or more fiber washers, indicating the assembler may have added washers as required to get the proper alignment.  
Second, the fiber washers are thick and compressible, so there is some considerable range of tightness that will ensure a 
seal; the assembler can tighten until it lines up, and leave it.  If you take it out, what do you suppose are the odds that it 
will line up properly and seal reliably when you reinstall it?  Fortunately, the size washer needed is the same as those 
used on many oil drain plugs, so you should be able to obtain a good supply of fibre washers of various thicknesses 
from local auto supply houses. 

If you decide to go ahead and unscrew it, here’s a tip: the hex size is 19/32”, but if you don’t have a wrench that size a 
15mm makes a good fit.  It may be helpful to unbolt the fan shroud and back it away from the radiator a bit to permit 
use of an open-end wrench, since even a crowfoot won’t work well in this space. 

Before you reinstall the drain cock, here’s another tip:  The hole in the end of the valve is 5/16” to a depth of about a 
quarter inch, and then 1/4” the rest of the way through the valve.  With a propane torch, it is a fairly simple matter to 
solder a short length of 5/16” OD brass tubing into the end of the valve to provide something to attach a hose to.  The 
hose can then be routed out the bottom of the car, making draining the coolant a lot neater and easier to collect and 
dispose of properly.  Since there are no nonmetallic parts inside the valve, you don’t even need to take it apart to solder 
on it, but you almost might as well -- it’s only one cotter pin.  Something to keep in mind: the radiator moves around a 
little on its rubber mounts, the oil lines move around a little with the engine moving on its rubber mounts, and the front 
subframe moves a bit on its rubber mounts.  Make sure there is adequate clearance around the valve and attachments so 
they aren’t subject to impacts or rubbing due to these various motions. 

Let’s say your radiator drain valve is toast and you have decided to replace it.  Ed Sowell ordered one from Jaguar:  
“Problem is, it is not the same as the original, even though the part number is correct.  Apparently Jaguar found a new 
source.  The valve is different in several ways.  In fact, the only similarity is that it is a 1/4" BSNB thread, about the 
same length.  The plug is held with a nut rather than a spring and cotter pin.  The important differences are: 

ƒ  The diameter of the bore is 3/16", whereas the original is 5/16".  Slower drain time, but would work.  Also, with 

the original I can jamb a 5/16" plastic hose barb fitting into the outlet to allow coolant recovery (as opposed to 
washing down the front suspension and splashing on the floor).  With the 3/16" bore, it appears impossible to 
attach a hose. 

ƒ  There is a handle instead of the tongue on the stem.  Consequently, the extension for above-car operation of the 

valve cannot be attached.  Very bad.  You have to get under the car to operate it, and there appears to be no way to 
avoid being drenched when you open it!  Could be used, but nobody would want to.  Might as well just slash the 
lower hose! 

ƒ  Due to the height and length of the handle, it can not be installed in the radiator while it is in the car.  No matter 

which way I set the handle, it bumps into something, e.g., the sub frame, or the oil cooler line.  It might work if it 
was screwed in before dropping the radiator into the car. 

“I find it had to believe Jaguar would say this was a replacement for the original.  They must be blind.” 

 
 

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So, now you’re in the market for something non-Jaguar to fit that hole.  Go ahead and measure the threads; they are 
13.16 mm OD and 19 threads/inch.  As mentioned above, it is not a tapered pipe thread; it’s a washer-sealed installation 
like an oil pan drain plug.  Tony Bryant in NZ says this drain fitting is “1/4" BSP (British Standard Pipe).  Very 
common in this part of the world.  Cost me less than $1 for a brass plug.  Any competent hydraulic fitting supplier 
should be able to find one, or at least a thread adaptor.  BSP is extremely common here, but NPT is a lot more difficult, 
but still possible.”  Well, here in the Bubba Belt in the good ol’ USA, my local hydraulic fitting shop calls it BSPP 
(British Standard Pipe - Parallel, as opposed to a tapered version) and charged over $5 for a fitting for connection to a 
1/4” hose -- and only had that one type of fitting on hand to choose from.  The hole through the middle is only about 
1/8”, so it would drain very slowly indeed; this fitting was clearly designed for hydraulics, not radiator drains. 

The closest thing you’re likely to find in auto parts stores in the US is a 1/4” NPT (tapered), but it will not fit properly -- 
it’s 18 TPI. 

Bryant also suggests that the coolant drain plug on the block (note: not the one on the radiator) of many Japanese cars is 
BSP, although the tapered version.  Still, the tapered plug may be usable to plug the parallel fitting on the XJ-S radiator. 

Another option: find a way to use the original valve as a plug.  It’s solid brass and very meaty (weighs about five 
pounds, I think) so it should be easy to work with.  One possibility is to cut the valve portion off, just leaving the hex 
and threads, so it looks like a plug except it has a 1/4” hole through the middle.  So, you can put a stainless steel or 
brass bolt through the middle and tighten a nut down on the other end, and use the assembly as a conventional plug -- 
just remove the whole thing when you want to drain the coolant.  If you wanna get fancier, you can tap the hole in the 
center for a threaded plug instead of using a bolt. 

Of course, using a simple threaded plug as a radiator drain is messy.  For neater work, you can solder a piece of brass 
tubing into the hole in the plug, connect a piece of hose to it, route the hose out the bottom of the car, and plug the end 
of the hose with something.  Then when you want to drain the coolant, you don’t even need to open the hood -- just 
reach underneath and remove the plug from the end of the hose.  You can stick with the 1/4” hole in the plug, but it 
might be a better idea to enlarge the hole to 5/16” and use larger tubing and hose; it’ll drain quicker and larger chunks 
of crud can pass through. 

A similar idea might be to drill and tap the hex portion and screw in a common fitting.  It’s too small to fit a 1/4” NPT 
fitting, but a 1/8” NPT will fit nicely.  One might think that draining through a 1/8” NPT fitting would take forever, but 
believe it or not you can find fittings with 1/8” NPT on the outside and a clear hole through the middle that’s larger than 
1/4”!  One such fitting is made by Brass-Tite!, part number 43275, and has a 1/8” male NPT on one end and a 3/8” 
hose fitting on the other; it is perfect for this task. 

Yet another option would be to make the piece of the original valve into an adapter to fit a standard drain cock.  This 
would involve drilling and tapping for the threads of whatever drain cock you buy.  Most of the universal ones seem to 
fit a 1/4” NPT, but you’ll need to choose a smaller drain cock with a 1/8” NPT.  You also need to choose your drain 
cock carefully, since many of them have a moving plug at the inner end that would require more space inside the hole 
than you’ll be able to provide within that chunk of the original valve. 

If you have the radiator out, you can get more radical: toss the original drain cock in the trash and drill out the threads in 
the boss on the radiator and retap it for something readily available.  The fitting on the radiator appears to be pretty 
meaty, so it could be drilled and tapped for something considerably larger than the stock drain cock. 

 

WHAT RADIATOR DRAIN?  Somewhere around 1988 -- possibly coinciding with the introduction of long-life 
phosphate-free coolant -- Jaguar went from the overdesigned drain cock to no drain cock at all.  On later cars, it is 
necessary to disconnect the lower radiator hose to drain the coolant.  Wearing a tux is not recommended.  In fact, this 
author once suggested to an owner that he do this task au naturel, and just jump in the shower afterwards. 

Joe Montgomery suggests another way to get most of the coolant out of later cars:  “Having seen at the local shop the 
way coolant goes everywhere when the bottom radiator hose is pulled and since the storm drain that would collect it is a 
straight shoot to the bay, I found a better way.  I got about 2 feet of 1/4" copper tube and 4 feet of plastic tube that 
would fit over the end from Home Depot.  By inserting the copper tube through the bleed hole on the right hand side of 
the radiator I was able to syphon better than 3 1/2 gallons into a bucket for recycling.” 

 
 

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Another suggestion: if you can locate a suitable tee, you can cut the bottom radiator hose and install the tee with two 
hose clamps and use the tee to drain the coolant.  There is a similar tee used for adding a coolant heater in subfreezing 
climates; perhaps that tee could be used. 

Something to note: the lower radiator hose does not connect to the bottom of the radiator in an XJ-S.  It’s perhaps four 
inches from the bottom at the left side.  So, if the only way to drain and flush the coolant is by disconnecting this hose, 
you will never get the junk outta the bottom four inches of the radiator!  And the connection is on the downstream side; 
anything larger than the tiny passages in the radiator core will pile up on the upstream side indefinitely.  Hence, if you 
have the radiator out for service, you might consider having a drain fitting added at the bottom right corner. 

 

RADIATOR REMOVAL:  Both the official Jaguar manual and the Haynes manual state that removing the radiator 
requires discharging the air conditioner freon circuit.  They lie.  In fact, as Jim Isbell reports, “In the Haynes manual 
there are 21 steps under section #21 that describe the removal.  Steps 1, 3, 5, 9, 10, and 11 are all unnecessary.”  Most 
are merely extra work, but discharging the freon is a waste of serious money. 

Both manuals also provide the same illustration which shows the radiator sitting on top of the oil cooler.  This may be 
the case for cars that weren’t equipped with air conditioning, but for the rest of us the oil cooler is in front of the 
radiator, and the A/C condenser is on top of it. 

Forget the manuals and just dive in.  The radiator comes out vertically upward, leaving the A/C condenser and oil 
cooler in place.  Drain the coolant and disconnect the hoses, remove the air purge system tubing from the top of the rail 
over the radiator, unbolt the A/C dryer from the rail and leave it hanging, unbolt the fan shroud, and remove the rail.  
Either remove the hood, or simply remove the grille, disconnect the struts, and tilt it forward until it rests on the 
bumper.  Disconnect the hoses from the transmission cooler and the wire from the coolant level sensor (early models 
only) and whatever other little things are hanging on, and pull the radiator out straight up. 

Now, if you have an early model such as Bernard Embden’s ’78, things are a bit more difficult.  “The radiator has two 
brackets welded to the bottom. The oil cooler is bolted to these brackets.  To remove the radiator the oil cooler lines 
must be disconnected and the radiator and oil cooler removed as one unit.  Once removed, the oil cooler can be 
unbolted from the radiator brackets.” 

Embden recommends changing things while you’re in there.  “Why does the damn oil cooler have to be bolted to the 
radiator?  After removing the radiator I decided I was not going thru this again.  I went over to the radiator and sawed 
off the two tits (sorry, "oil mounting brackets") that the oil cooler bolts to.  Now properly motivated, I fabbed two 
brackets out of 1/2 inch wide, 1/8 thick flat iron available at any hardware store.  I offset the brackets (wider).  Made 
two 1-1/4 inch straps.  Bolted one end to the oil cooler mounting bracket, the other to the brackets that I fabbed up.  
This was so the brackets would fit below the still existing part of the bracket that remains on the bottom of the radiator. 
(I was concerned about clearance).  Two bolts per side, drilled the holes thru the boxed lower radiator support.  
Brackets had to be angled slightly (hammer took care of that).  Now the oil cooler and A/C condenser sit where they 
should be with the radiator out.” 

 

RADIATOR WORK CHECKLIST:  If you have to pull the radiator out for any reason, you should take advantage of 
the opportunity to do all of the following: 

1) 

Rod, recore, or replace the radiator. 

1a) 

Consider converting to a single-pass system; see below. 

1b) 

If the coolant level sensor is located in the radiator, have the radiator shop plug the hole and relocate 
the sensor to the header tank -- see page 207. 

2) 

Clean fins of A/C condensor and oil cooler. 

3) 

Consider revision of radiator drain scheme -- see page 194. 

4) 

Revise air bleed banjo fitting -- see page 187. 

 
 

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5) 

Install foam in all undesirable air paths -- see page 226. 

6) 

Install coolant filters -- see page 190. 

You might also consider the opportunity to replace the front oil seal on the engine or to check that the bolt holding the 
crank damper on is properly torqued (page 89). 

 

IMPROVING THE ONE-AND-A-HALF-PASS RADIATOR SCHEME:  As mentioned in the cooling system 
description beginning on page 172, the OEM radiator in the XJ-S will result in the left bank running warmer than the 
right.  If you’d like both banks running at the same temperature, it will be necessary to revise the system so that both 
thermostat outlets see the same backpressure.  Perhaps the simplest and most straightforward method to accomplish this 
would be to increase the backpressure on the right side thermostat outlet.  Performance outfits like Summit Racing 
(page 717) offer flow restrictors for cooling systems, but their intentions are different and the parts may be difficult to 
adapt to the Jaguar V12.  Perhaps the better tactic would be to simply adopt the idea, and provide a simple orifice in the 
upper right radiator hose or radiator inlet.  We have no idea what size the orifice should be, unfortunately; if someone 
wants to take the effort to install pressure taps and experiment, the orifice should be just the right size that the pressure 
at both thermostat outlets is exactly the same -- but even with that criteria, the desired orifice size may vary with 
radiator condition and possibly even engine RPM. 

This author installed a 1/2” stainless steel washer in the right side radiator hose as an orifice.  The OD of a standard 
1/2” flat washer proved to be exactly the same as the OD of a 1-1/4” Gano filter (see page 190), so a couple of tiny 
holes were drilled in the edge of the washer and it was screwed to the inlet end of the Gano filter with the existing 
screws that hold the Gano filter together.  The ID of the 1/2” washer is 14mm.  The car ran fine and stayed cool, as 
evidenced by the gauge sender in the right side thermostat housing; thus we can conclude that a 14mm orifice is large 
enough, although we really cannot decide if it is small enough without measuring the effect on the left bank 
temperature. 

Note that the installation of such an orifice doesn’t merely make the cooling of the right bank as bad as the left.  It 
actually improves the cooling of the left bank.  It accomplishes this by increasing the pressure loss in the right bank, 
which reduces the pressure in the right side end tank on the radiator.  Since the flow through the upper 1/3 of the 
radiator is a function of the pressure difference between the left side inlet and the right side end tank, lowering the 
pressure in the right side end tank will increase the flow in the upper 1/3 of the radiator -- and hence in the left bank. 

 

ELIMINATING THE ONE-AND-A-HALF-PASS RADIATOR SCHEME -- HOME VERSION:  The optimum fix 
for the XJ-S cooling system would be to eliminate the one-and-a-half-pass scheme altogether and plumb this cooling 
system the way Jaguar should have in the first place.  If you’re handy with a propane torch or have a radiator shop 
willing to do some fiddling, you may be able to modify the OEM radiator to make it a single-pass.  You will need to 
remove and cap the upper left inlet, and install a second inlet on the right side header tank just under the existing inlet.  
Remove and throw out that hokey radiator drain lever if your car has it (see page 194 for ideas on the radiator drain).  
Also, you need to make the left end of the radiator into a single tank the full height of the core.  The radiator on this 
author’s car has a single left end tank with a partition hidden inside, so you can remove the tank, remove the partition 
dividing the upper 1/3 from the lower 2/3, and reinstall the tank.  Other owners have reported their radiators have two 
distinct tanks, an upper 1/3 and a lower 2/3.  This latter type would obviously require more effort to convert into a 
single left side tank, such as perhaps adding a pipe between the two.  Once you’ve got the radiator converted to a 
single-pass, see the plumbing issues below. 

A testimonial from Joe Bialy:  “With two electric fans and the original two pass scheme, the fans would run for a long 
time after shutdown.  Sometimes cycling on and off a few times too.  With my one pass scheme, only one fan runs for a 
minute or two after shutdown, never cycling on and off afterwards.  This car absolutely runs cool as can be now, far 
and away the best it's ever been.  

“Yank the radiator, slice out the baffle, solder it back up, buy a couple of 1'' copper fittings and two radiator hoses, put 
it all back together and fergettaboutit.  This will be your best weekend investment under the hood ever and it'll only set 
you back $50 or so.”