Replacement Thermostat from Moss

Hi All
I have been looking to replace my thermostat and came across Moss motors offering a reproduction bellows type one which I believe is what is fitted to the MKIV can anybody confirm this is suitable, I don’t see any part drawings of the thermostat in the spares manual, Moss motors link below

Not for the Mark IV, unless your thermostat housing has been modified.
Here is what you should have.

If yours is dead, you can remove the brass parts, block off the bypass port, and run without a thermostat, so all the water circulates all the time.

The Mark IV and Mark V use the C.527/2 assembly as shown in Rob’s photos and mine below. The later XK and Mark VII engines use different thermostats. People have modified the C.527/2 to accept other thermostats (with varying success), perhaps see Ed Nantes entries on such topic for more successful route.

In the photo below, the housing shows how the internals just to the left of the housing are oriented. The ether-filled bellows expand when heated, opening flow to radiator and closing bypass to water pump. This ether fill often has leaked out over time. Assembly of the internals includes unscrewing the top and screwing it back on after insertion of the internals.

The leftmost part of the photo below shows top unscrewed from bellows and top hole with ball bearing inside the hole for one-way backflow when bellows have shrunk to close path to radiator.

I’m not so sure. I think this would work correctly provided the sleeved section attached to the top of the bellows blocks the side bypass opening when fully open.

The original items were locked in place by the bayonet twist system shown in the second photo. This replacement item would require the outside diameter of the top to be an interference fit in the top hose. The way to install it is to fit it into the start of the hose, then push the hose on, which slides the thermostat further into the hose, but still in contact with the housing. This also means you don’t need to do a dress up of the top of the housing to create a seal for the valve to close, as required for the original.

At that price some of the brass would have to be gold, surely.

No, the XK120 thermostat slides past a side port to cover it.

It would not work with the pushrod housing.

Here is what’s inside the pushrod housing.

The thing in the middle is a small chamber that connects to the bypass hose port.

When my '38 SS came to me, I found some PO had removed the insides but still had the bypass hose in operation. Meaning some of the water was not circulating through the radiator. I used a large flat 1/4" fender washer and a 1/4" bolt and nut through to block off the center port. So it never overheats, runs about 80 C most of the time.


Ok here is what I have to work with, pretty crusted up and seen better days, this came with a common thermostat mounted in the housing as shown with a couple of stainless screws, opens at 70F

So is the consensus of opinion that I block up the centre hole as Rob says and let all the water circulate through the radiator from cold start ? I am hoping to obtain another thermostat housing which may be in better condition

The bypass should be blocked if no thermostat is used. A thermostat blocking the radiator flow while also having the bypass blocked is not a good idea.

The bypass hose can have a wood dowel about 2 inches long put into the main straight section. The dowel can block all bypass flow and also cannot move into the hose curves, so it is stable in that location.

I have used the thermostat housing without thermostat along with bypass blocked quite satisfactorily in Southern California. I’ve also equipped other Mark V cars with this bodge when their thermostats failed.

As Roger said, block off the bypass at the side by inserting a plug in the hose. The plug should be of a diameter and length that prevents it slipping further back in the hose. It is usually recommended to drill a small hole through to prevent any differential pressures each side.

The stainless screws prevent the thermostat slipping further into the return hose, but I cannot see any small bleed hole in the thermostat top plate. It is normal to have one there as an air escape point and pressure relief. These small bleed holes are about 3mm.

The housing has suffered a bit from the ravages of time but is still serviceable. You don’t need to do anything to it or modify anything, just plug the bypass hose. The bottom flange is very thin on one side and a good gasket and cement will seal this as it is a non-pressurised system.

The reason for the bypass in the original system is to enable the engine to warm up fast because the thermostat remains closed and the water just circulates vertically through the block and not the radiator. When the thermostat opens, the movement of the valve also closes off the bypass so all water passes through the radiator. If you use the generic thermostat, it cannot close off the bypass and the the system could more readily boil. You can run with or without a thermostat, but plug the bypass hose.

This, and the cylinder head, both being cast iron, are the culprits for causing the extreme corrosion of the aluminium water manifold by electrolysis.

Rob’s bypass block method appears functionally similar to the dowel plug. Take your pick.

If you can’t find an original thermostat then Ed Nantes suggests the following modification that permits the bypass to still operate with a modern thermostat.


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.


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.


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.

Here’s what it looks like finished with one of the spacers shown for illustration.


Total cost , here thermostat about AU$20, spacer lasered AU$10 , screws and a bit of gas and time.

Oh, and while not mandatory, drilling a 1/8” bleed hole in the thermostat will make it easier to fill the radiator.