Mark V thermostat

Hi,

I have 2 one sort of works at about boiling point the other does not, I have removed the centre so I can run the car, I have noticed some commentary regarding the by pass hose being blocked off
can any one shed some light on this and what can I do to make it work correctly?

Thanks

Grant

Hello Grant, a lot of the Mark V thermostats have worn out in various ways over the years. Doing a search on “thermostat” in the pre-XK section will help you learn some of the problems and solutions.

If the centre is removed, then water flows need to be considered. Full flow to the radiator will allow engine to operate at normal temperatures or lower. Full flow to the radiator means the bypass flow is blocked. Flow into the bypass line is not cooled by the radiator before it goes back into the engine. Water boiling in the engine while the radiator water remains much cooler can occur if substantial flow in the bypass is permitted when the engine is warm. The bypass line can be plugged by a wood dowel easily sized so as to prevent unwanted plug movement inside the bypass line. Depending on the climate in which your car is operated, centre out with bypass plugged could work pretty well.

Before condemning an old thermostat it is worth checking that its failure to open or close is not simply because the central rod is seized in the casting. If this is not the case and the bellows have failed and you don’t have a replacement then Ed Nantes has a good modification that allows you to use a modern stat within the original casting and still retain the correct operation of the bypass.

Blocking the bypass is a bodge and you really should only do this as a temporary fix.

Peter

Does any one know how to retrieve old posts ,[ about this ] from the old site.

Basically you need to put an XJ thermostat in from the front of the housing , Which means reversing the operation of the by pass take off
DRill out the front of the bypass casting to 3/4"mill the surface flat , and solder a penny in the back to block off the original outlet to the bypass. The screw the modern thermostat on the front . YOu will probably need a1/4"spacer und r it. Try it out in a saucepan with a thermometer before putting in the car .

Ed’s article…

AN EASY CONVERSION FOR Mk4/5 THERMOSTATS.

Since late side-valve cars, Jaguar have used a thermostat that is a 2 way switch. When cold the water runs through the bypass hose and as the temperature rises it is diverted through the radiator core.
When the originals got old and gave up the ghost, a variety of fixes were tried.
Removing the internals altogether… not a good thing as the water will prefer to take the easy route through the bypass rather than fight through the radiator core.
Blocking the bypass and putting in an on/off thermostat… better , but takes longer to warm up which increases wear.
Making a new housing to incorporate an XJ6 thermostat… good but not cheap.

This conversion is suitable for 6 cylinder Mk4 /5 thermostats which have the bypass entering the housing in the direction of the centerline. i.e. not SS and 1 ½ litre cars where it enters at a tangent.
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The original mechanism blocked the bypass by blocking the feed which faces the rear of the car.
In the conversion, the bypass is turned 180 degrees so that the XJ6 thermostat can push from the front and close it that way.

The first step is to remove the old brass mechanism… unsolder the nut and remove the screw at the front to take out the closing plate and the rest is pretty much force. Cut the 4 tabs which hold it in at the back but don’t damage the housing.
It’s good to clean the housing by sandblasting as you need it clean to braze it.
Mount it in a vice with the bypass vertical, and use a 1 1/8” hole saw [ a good one not the hardware shop cheapies ] in a pillar drill. Remove the pilot drill BEFORE starting.
The saw will cut around the bypass, being guided by it, to separate it from the housing.
Thermostat%20conversion
The saw will bottom out before it cuts completely through but a light tap with a hammer on the end of the bypass will sheer the last 1/8” tags of metal remaining.
The bypass is now free and can be rotated 180 degrees.
It has a 3/8” hole through the centre and a with a piece of allthread and washers and nuts it can be held in place with the large opening squarely facing forward while it is brazed. It will sit slightly forward up against the edge of the hole but not to worry, the bypass hose has enough give to compensate.
Start brazing at the front edge and finish at the back.
Tap the 3/8” hole remaining in the centre to 7/16” [ UNF ] and shorten a set screw so the it is just long enough to block the hole when held in with a spring washer.
The XJ 6 thermostat mounts in the front opening. To get the correct spacing between the flange and the bypass opening, I insert a ¼” [6mm] spacer. The local laser cutters make them for me as a ‘washer program’, but if you haven’t access to one they can be cut from brass or steel with a hole saw, turned off the end of solid bar or if your local steel supplier has 2 ½” thick-wall tube he might cut a slice off for you.
Braze , silver solder or soft solder the spacer in place and mount the thermostat with 2 1/8” [preferably] stainless steel screws tapped in to the spacer.
Total cost , here thermostat about AU$20, spacer lasered AU$10 , screws and a bit of gas and time.
Here’s what it looks like finished with one of the spacers shown for illustration.
Thermostat%20Conv2
Oh, and while not mandatory, drilling a 1/8” bleed hole in the thermostat will make it easier to fill the radiator.

1 Like

Hi,

I did this according to Ed’s instructions and it’s been working really well in all conditions. I have replaced the (4.2L XK) thermostat once and tested again in a kettle that it opens up and at the same time closes the bypass (which has one hole plugged but the other one open to allow the engine to warm up faster as designed).

Cheers!

Thanks to all

I will now have a crack at modification as per Ed’s article and re post once completed for interest sake, picking up the mark V up next week from the paint shop…very excited!!!

Cheers

Grant

Did Jaguar provide a bypass plug for the Mark V thermostat?

Spare Parts Catalogue page 19 reads “Part No. C.1952 Thermostat Body Assembly (Complete with By-Pass Plug No. C.1956) No. per Unit 1 Remarks. Supplied to Special Order Only”

Offering a guess, it may be a body without a thermostat, and blocking the bypass port, for someone who wanted to run without a thermostat. Possibly in hot desert countries? Then all the water would circulate through the radiator all the time.
Here’s what’s inside one without thermostat.


Here’s my guess as to what a plug might be like.



There would probably be a couple of rubber washers under the flat washers.

Perhaps something to keep in your traveling parts box for if you ever have a thermostat failure out on the road somewhere.