[xk-engine] Thermostat Question

The early XK motors, like the 3.8 in my Mk 2, had a large
thermostat with a cylindrical section that would seem to close off
the bypass slot when fully open. Well, ‘close off’ is a relative
term since there is clearance between the cylindrical portion of
the thermostat and the inner housing that would allow some coolant
to go through the bypass.

So my question is: How important is it to use the original type
thermostat? Is it really necessary to partially block off the
bypass slot when the motor is at operating temperature and the
thermostat is open? Or can I get away with using a more modern
thermostat that wouldn’t close off the bypass slot at all?

Cheers,

Tom–
302Rover
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In reply to a message from 302Rover sent Tue 26 Feb 2008:

You are the designated hitter. Try it and report back. Set up a
reproducible experiment, get the motor up to steady temperature on
a set course, like a long steep up hill when it is 80 outside, then
change the unit and do it again.
If some portion of your coolant is now recirculating through the
engine instead of through the radiator you might see a higher
engine temperature. How much higher is the key question. I would
assume it would make a difference to explain the special
thermostat, but who knows? It could be clever engineering with no
real purpose.
P.–
Peter J. Smith, 1966 3.8S, former 67 MGB
carson city nevada, United States
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In reply to a message from 302Rover sent Tue 26 Feb 2008:

Tom
You can also plug the bypass. Others have had success keeping
temperatures down by restricting the flow using a plug with a 1/4’’
hole in it. If you have full flow through the bypass after the
thermostat opens, about 20% of the coolant bypasses the radiator.
Joel–
ex jag, '66 E-type S1 4.2, '56 XK140dhc, '97 XJ-6
Denison, TX, United States
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3/16" is plenty large - just need enough flow to cool and lubricate the pump
seal and get representative heated water to the wax pellet on the
thermostat.
Bob Grossman-----Original Message-----
From: owner-xk-engine@jag-lovers.org [mailto:owner-xk-engine@jag-lovers.org]
On Behalf Of ex jag
Sent: Tuesday, February 26, 2008 10:17 PM
To: xk-engine@jag-lovers.org
Subject: Re: [xk-engine] Thermostat Question

In reply to a message from 302Rover sent Tue 26 Feb 2008:

Tom
You can also plug the bypass. Others have had success keeping

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In reply to a message from ex jag sent Wed 27 Feb 2008:

Joel,

the bypass is actually a slot in the manifold. So I would have to
weld (or by some other means) the slot closed except for a short
length? And by doing this, a ‘normal’ thermostat could be used?

Am I also correct in reading in to what you suggested, that
improved cooling can be gained by restricting the flow through the
bypass and that furthermore, the original thermostat design with
the ring or cylinder that is supposed to close down the bypass,
really doesn’t close it down sufficiently when the motor is up to
temperature?

Thanks for your help.

Cheers,

Tom–
The original message included these comments:

You can also plug the bypass. Others have had success keeping
temperatures down by restricting the flow using a plug with a 1/4’’
hole in it. If you have full flow through the bypass after the
thermostat opens, about 20% of the coolant bypasses the radiator.
Joel


302Rover
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In reply to a message from 302Rover sent Wed 27 Feb 2008:

Tom
All you need is a disc of some sort, with an OD equal to the OD of
the water pump inlet where the bypass hose attaches. Then, as Bob
pointed out, all you need to do is drill a 3/16’’ hole in the center
of the disc. The disc would cover the inlet of the water pump, and
the bypass hose would go over it. A cap would work too, if you can
find one that fits over the WP inlet. Maybe a 1’’ copper pipe end
cap?
Improved cooling is a result of more coolant being passed through
the radiator, rather than recirculating through the engine.
Joel–
The original message included these comments:

the bypass is actually a slot in the manifold. So I would have to
weld (or by some other means) the slot closed except for a short
length? And by doing this, a ‘normal’ thermostat could be used?
Am I also correct in reading in to what you suggested, that
improved cooling can be gained by restricting the flow through the
bypass and that furthermore, the original thermostat design with
the ring or cylinder that is supposed to close down the bypass,
really doesn’t close it down sufficiently when the motor is up to
temperature?
Tom


ex jag, '66 E-type S1 4.2, '56 XK140dhc, '97 XJ-6
Denison, TX, United States
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In reply to a message from 302Rover sent Tue 26 Feb 2008:

So my question is: How important is it to use the original type
thermostat? Is it really necessary to partially block off the
bypass slot when the motor is at operating temperature and the
thermostat is open? Or can I get away with using a more modern
thermostat that wouldn’t close off the bypass slot at all?
Cheers

Hello Tom,

The factory recommends a brass bung with a 3/16’’ hole inside the
bypass hose when removing the original sleeve thermostat. Take a
look in the Saloons list archives, we have discussed this many
times over the years. You usually have to do this when fitting a
straight port 3.8 with a non bypass 4.2 intake manifold. I use a
brass NPT hose barb fitting that sits on the top of the water pump
bypass inlet inside the hose. Whatever you use, try to prevent it
from falling into the water pump.

Paul–
PS
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In reply to a message from PS sent Wed 27 Feb 2008:

Hi PS this topic has been discussed on the Etype forum some time
ago,a lot of listers felt that these thermostats will not make much
difference because of corrosion in the thermostat housing.I did put
a restiction in my bypass hose about 16yrs ago,running temp on a
hot day dropped 10 degrees.

Regards Gerry Ireland 878269 Ontario Canada–
The original message included these comments:

So my question is: How important is it to use the original type
thermostat? Is it really necessary to partially block off the
bypass slot when the motor is at operating temperature and the
thermostat is open? Or can I get away with using a more modern
thermostat that wouldn’t close off the bypass slot at all?
Cheers
Hello Tom,


gez
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In reply to a message from gez sent Thu 28 Feb 2008:

Thanks to all of you for the advice about restricting the bypass
flow. I’ll do that; now is the time to make these mods since the
motor is out of the car, on an engine stand and I am in the process
of reassembling it.

Tom–
The original message included these comments:

Hi PS this topic has been discussed on the Etype forum some time
ago,a lot of listers felt that these thermostats will not make much
difference because of corrosion in the thermostat housing.I did put
a restiction in my bypass hose about 16yrs ago,running temp on a
hot day dropped 10 degrees.
Regards Gerry Ireland 878269 Ontario Canada


302Rover
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The XK cooling system is of a type called “constant flow”. The idea
is that whether the engine is hot or cold, coolant will circulate
around the block at approximately the same rate. This is done to
prevent hot spots during the warm-up process, as well as to allow the
engine to warm more quickly. It’s essential to the design that the
bypass work normally.

The idea of putting a restriction into the bypass hose comes from the
Jaguar race prep guide (you can find it reproduced in the back of a
Bentley’s E-Type manual). But the restriction in the bypass hose came
along with eliminating the thermostat and installing a restrictor
plate in it’s place. I’m convinced that this was not a great idea,
and probably arose from some confusion as to the purpose of the
bypass. (Sorry, I’m not a believer in the infallibility of Jagur
engineering when it comes to cooling systems). If you are replacing
the thermostat with a restrictor, then it’s fine to plug the bypass
entirely, since you will have constant flow in all conditions. Warm
ups will take longer, but who cares for a racing engine.

Unfortunately, several things work against you if you are working
with a 3.8 or older motor. First, the thermostats today are not the
original bellows type. Instead, vendors will weld a shutter onto a
modern thermostat. These seal with varying efficiency. Many of the
ones on the market are 160 degree thermostats, in the mistaken belief
that cooler is better. But the biggest problem is corrosion. The
bypass slot is EXTREMELY prone to cavitation erosion, due to the fact
that it’s located at the point where temperature is highest and
pressure is lowest. Spot boiling will occur in this area as the stat
opens to reveal the slot, and that will blast away metal over time.
Forty or fifty years of that has probably ruined many if not most of
the examples out there. It’s very important to run a 50/50 mix of
antifreeze, which will minimize this damage, and not succumb to the
water-is-better nonsense that is often offered as the solution to
cooling problems. If your thermostat housing is already damaged,
there’s not a lot of choice other than to install some sort of restriction.

With a street engine, a restriction placed in the bypass hose should
be matched by an opening drilled in the thermostat flange or poppet.
This way, there will be at least some flow around the block when the
engine is cold. You need to maintain flow in a cold engine, first to
ensure that the thermostat opens, and second to prevent stagnation
and hot spots in the engine passages. This arrangement will lengthen
the warm up period, but will limit the amount of hot water
circulating back around the block due to bypass passage corrosion.
It’s not an ideal solution, but probably a necessity in most early engines

Beginning with the 4.2, a dual acting thermostat in a revised housing
was specified. Dual acting thermostats are in current production, and
should be used on all 4.2 XK’s and V12’s. These are more certain in
their action, and will seal adequately, as long as the bypass opening
isn’t too badly corroded. If the opening is corroded, it’s relatively
easy to machine smooth, and there are good replacement 4.2 thermostat
housings available. A properly operating bypass is preferable to
plugging the bypass and drilling the thermostat.

Mike Frank

Mike Frank

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In reply to a message from Mike Frank sent Sat 1 Mar 2008:

Mike,

thanks for your detailed and information-packed reply. I replaced
my thermostat housing with one that had been sitting on a shelf for
many years so it doesn’t have any corrosion around the bypass
slot. I also have a replacement thermostat (not yet installed)
with a restricting ring fixed to it but it is not the bellows
type.

Based on responses on the Forum it appears that if I add a
restrictor in the bypass flow I could realize an operating
temperature drop but, based on what you are telling me, I run the
risk of creating hot spots in the motor, especially during the warm
up period.

One improvement I am thinking about for the cooling system is to
install an aluminum radiator. Perhaps I should leave the bypass
system alone and go with the aluminum radiator.

Thanks again,

Tom–
The original message included these comments:

The XK cooling system is of a type called ‘‘constant flow’’. The idea
is that whether the engine is hot or cold, coolant will circulate
around the block at approximately the same rate. This is done to
prevent hot spots during the warm-up process, as well as to allow the
engine to warm more quickly. It’s essential to the design that the
bypass work normally.
The idea of putting a restriction into the bypass hose comes from the
Jaguar race prep guide (you can find it reproduced in the back of a
Bentley’s E-Type manual). But the restriction in the bypass hose came
along with eliminating the thermostat and installing a restrictor
plate in it’s place. I’m convinced that this was not a great idea,


302Rover
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