XK120 Bypass hose connection to water pump

Hi all,

First time really getting my hands dirty with 1953 FHC #680000. Digging into my overheating problem a little more, I pulled off the top hose and thermostat housing, and sure enough - she was running with no thermostat, and full open bypass.

When I first looked at the bypass hose that was fitted, it looked like a franken-hose, made of five clamps and three individual hose pieces. I pulled it apart (unfortunately before looking at my new bypass hose (#C3676 from Moss). Thinking I had the new part in hand, I didn’t hesitate to destroy bits of this franken-hose when removing

I go to fit my new bypass hose - only to find that the water pump end is completely the wrong size. The new hose actually fits inside the nipple on the water pump. And the reason for the franken-hose from the previous owner was actually to adapt to just that - have the bypass hose go on the inside of the water pump nipple, and then two extra pieces of hose wrapped and clamped around the outside to stop it leaking.

So it sounds like the obvious conclusion is that I have a different water pump fitted. Can anyone help identify what I have, and advise where I might get a bypass hose to fit (without rebuilding a franken-hose)?


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OK, so a little research shows that I likely have an XK140 water pump. And with that, it looks like neither the XK120 bypass hose nor the XK140 bypass hose would work…

Any suggestions? How challenging is it to go back to an XK120 water pump once it has been upgrade to an XK140 version?

2 suggestions.

  • use the Parts Manual to establish the correct Part number for your water pump, then purchase it by Ebay, JL classifieds, or new (but they can be expensive)

  • have a metal tube formed with the correct diameter on each end, cut 2 hoses to make it fit together

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I guess it wouldn’t be polite to laugh at that frankenhose.
Yes, bypassing and no thermostat would make it overheat.
One or the other, not both.

You are best off going to a 120 pump, but there were several, and reproductions exist.
I see a narrow fan belt there too.
The 120 belt was much wider, about 5/8".
Different fan pulley too.
So we need to figure out what else has been bodged up.
Crankshaft pulley and generator pulley perhaps.

Here is mine, but it is earlier and there were several changes between mine and yours. For instance I don’t have my heater installed.

Chris, my XK120 spent many years in Arizona and was fitted with several innovative adaptations to aid its cooling. One of them is an XK140 water pump, which allows the use of the larger diameter, eight-blade cooling fan from the XK140.

To adapt the bypass on the XK140 water pump to the XK120 thermostat housing, a brass plumbing fitting was used. It’s a 5/8” barb X 3/4” MIP. The water pump bypass was internally threaded using a 3/4” MIP tapered tap. This allowed the brass plumbing fitting to screw into the water pump’s bypass. Then the standard XK120 bypass hose can be used, although it will need to be shortened by an inch or so on the water pump end.

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Thanks all. I love how my 20 minute “change a couple of hoses” has turned into hours of research and potentially a “rip a bunch of stuff out” project! That’s why we love this stuff, right? :wink:

So I am a big believer of “do it quickly, or do it right”. I do like @Mike_Balch 's suggestion of threading the existing pump with a brass fitting so as to use the original hose. I think I am going to try that

In parallel, I’m going to start collecting the (long) list of parts to get this thing back into original shape. I think I have decoded what I need. Interestingly is a 6-blade fan (not the 8-blade Mike mentions), so at least that is one less thing. I think I am going to be collecting:

  • New (remanufactured) Water Pump - steep, and I don’t know if they’ll give me a credit for the XK140 pump core…
  • water pump pulley
  • Generator Pulley
  • Crank Pulley
  • Two banjos, long banjo bolt, and copper washers
  • bypass hose and water-pump-to-heater-pipe hose
  • fan belt (which I already have)
  • maybe a fan?

I also noticed my damper looks a bit dodgy, so while I’m digging into all that, I may as well replace the damper too…

Fun stuff!


Why stop there? With a bit of effort you should be able to keep working your way down the car until you reach the reverse lights… :crazy_face::crazy_face::crazy_face:


What damper? - - - - - - - - -

Kind of like “If you don’t have time to do it right, where will you find time to do it over?”
Anyway, Damper Doctor in Redding CA rebuilt my balancer must be 25 years ago and it’s still fine.

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Hi all,

Quick report. I followed @Mike_Balch’s suggestion, tapped the pump outlet, fitted the brass fitting, and trimmed the new bypass hose to fit. Everything went together smoothly, no leaks I can see, and my new (sleeve-type) thermostat is now in place.

Out for a test run, it worried me at first how quickly the temperature gauge moved - but then I realized that was the whole point! :wink: Warmed up nice and quickly, and stayed around 80 degrees. Stopped at a long traffic light, it wandered up to a bit past 90, but came back down to 85 as soon as I got moving again. Longer test drives coming soon!

I’m still thinking about collecting all the parts to go back to original style. I do think I need to get the radiator rodded or re-cored, but as everything seems good for the moment, I’ll put that all down for some winter-time maintenance…



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Hi Chris, I’m glad you were able to eliminate the “franken-hose” with the five clamps between the water pump and thermostat housing. Your engine may still be running a little hotter than typical, so I’m wondering about the type of sleeved thermostat you’ve installed. Below is a link to a Moss Motors description showing the two types of sleeved thermostats that are available today.

The first thermostat shown is the type originally supplied by Jaguar. When the engine is cold, the sleeve is in the down position giving the coolant ready access to the bypass circuit of the water pump. As the engine warms, the sleeve moves up and does a decent (but not perfect) job of blocking-off the bypass circuit as coolant is directed into the radiator.

The second thermostat (in my opinion) is poorly suited to an XK engine. When the engine is cold, the sleeve is in the up position blocking access to the bypass circuit of the water pump. As the engine warms, the sleeve moves down leaving the bypass circuit completely open as coolant is also directed into the radiator. If you’re using the second type of sleeved thermostat, the engine will likely run hotter than if the first type were used.

Thanks Mike. Indeed I have the second type (wax capsule). I guess I misunderstood when looking into the type, I had thought this type still did the same thing (blocked the bypass when hot). I’ll look into swapping it out for the bellows type…



Don’t know about the situation in the US but I don’t think these bellows versions are still being made and sold. So you’ll have to look for an original version on the internet. There are several brands but all were made by only two companies. These NOS or used versions are still available in the UK for acceptable prices. Survey of the brands and types: I assume you will need the 72 Celsius opening temperature. If you drive in wintertime you might opt for 80 C. Always ask for the temperature stamped in the thermostat.

  1. AC-Sphinx and AC (DELCO) type TF-1 for 72° C (numbered 2235 or 2663)
  2. Lucas (Motostock) (made by AC Delco) type LF1
  3. Quinton Brivec or Quinton Hazell (made by Quinton Brivec) type QT100/72
  4. Remax (made by Quinton Brivec) type NT100
  5. GH thermostats (made by Quinton Brivec?) type A70 or possibly A72.

Bob K.

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Hi @Bob_K1 ,

Looks like Classic Gold is making them. Available in the US from Moss:

@Mike_Balch , here is a quote from the Wax type notes: “As this unit approaches the preset operating temperature and begins to open, the sleeve moves downward (along with the cylindrical body) to block the bypass port.” That is why I thought the wax type would be correct (and it has the benefit of being wayyyy cheaper… :slight_smile: ).

Anyway, new bellows type on order, and I will report back here on any differences in operation. Certainly the radiator was still showing signs of bypassing (IR thermometer showed ~=100C at the top, and 60’s C at the bottom…) So hopefully the new bellows type will block more bypass, and give me a little breather until I get the chance to “do it right”…


Moss Motors began making the original type thermostats a couple of years ago under part number 434-156. Looking at their website there’s just one available. Otherwise, NOS units do turn up occasionally on eBay UK in sub 72 degree temps.

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I noticed that statement by Moss as well. I believe it refers to some other make and model of car that the Moss product line serves. They should clarify their publication.

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Good initiative to re-manufacture these bellows type of thermostats. Like Mike mentioned, you’ll find them (in particular) on eBay UK for prices between GBP 25 and 40.
I meanwhile have some experience with these “old” thermostats and I purchased a few “used” ones. I was surprised that even after decades, these thermostats were still functioning fine: I checked them in hot water with a reliable thermometer. The temperature stamped is the opening value and after a temperature increase of about 5 to 7 degrees Celsius, the valve is fully open.
As these bellow types functioning on basis of expanding gasses (alcohol as far as I remember), the pressure inside the bellows has to work against the pressure of the cooling system itself. Hence the maximum value of 4 psi for the Radiator Cap.

Bob K.

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The new old stock sleeve bellows of the models name prior by Bob K can be found on internet $45 to $100 USD. Check to be sure it is what is listed…don’t go by photo…and that it has both bellows and sleeve, and some are winter stats…at 86D C. Look for the 72C. The MOSS Classic Gold…was made for the Healey…but fits the XK120 with a tiny bit of “dressing” with a file of the upper lip. It fits…with that done…you will see when you try to fit it into the housing what needs to be done. Don’t just force it or not notice. Retails about $100. Test any new to you stat in a pot of water…to be sure it opens/closes. A note on the bypass hose…C3676, pictured,

it is a different inside diameter size at each end…thus someitmes the frankenstein work around…which I did once while awaiting a new proper hose. Some of the metal nipples that accept the hose have a taper in them…wider at the base…this makes the hose want to slide off. Ridiculous that they are made that way…but they are. The more you tighten the clamp the more it wants to push the hose off. There are various fixes you may think of…filing the metal flatter, a bead of JB weld, a gluey sealant (which will later be regretted) : but a good tight fit hose–between the two ends helps keep it in place. Nick