Hello again,
I was wondering if there was an engineering reason as to why the top radiator hose has two different sized ends? Is for thermo-syphoning or something similar? Just curious. 
Cheers,
Tim
I think the hose has two sizes because the radiator outlet is one size and the thermostat housing is another size…
(Sorry Tim, but I had to say it 
)
Yes, you did and I did think it too. However, I’m hoping that there is an engineering solution worthy of the variation in sizes other than two component manufacturers supplied different sizes. 
I’m hoping that it has something to do with syphoning which aids the flow of coolant but that might just be wishful thinking. 
Tim 
Ha, ha. Ok here’s a theory from a mechanical engineer, and if you think it has any value you can insert a coin in the slot_____.
Nothing to do with syphoning, but having to do with thermostat design.
Going back to the SS1, it had no thermostat nor any bypass.
Teddington came up with a thermostat with a bypass. It was all built into a cast iron housing, and the size was governed by the brass disc inside.
This was before somebody else came up with the drop-in thermostat. I think it was Stant.
Initially it was used in the SS1 and SS100 with a single size hose.
Also the 1-1/2 L SS Jaguar.
And the 2-1/2 L.
Then somebody decided the 2-1/2 L and 3-1/2 L needed a better water manifold and a bigger header tank on the radiator. Radiator height and width was dictated by the body stylist (Lyons). The tank became longer, and not so tall, using the empty space above the fan. It needed stiffening braces on the back. The hose spout had to go downwards. But the big hose wouldn’t fit with that brace in there. So they made the spout smaller.
With Mark V, Lyons made the radiator wider, so they could have gone to a larger down spout, or a drop-in thermostat like on the XK engine, but somebody had already purchased 10,000 of those upper hoses, and 10,000 Teddington thermostats, so maybe that’s why they kept the same size. hoo nose
Thank you, Rob, a very well considered proposition. I was hoping for something clever that Jaguar thought up but as you say, “hoo nose?”.
So it seems that Jon might have been right after all! 
Sir Bill was certainly not one to waste anything.
Tim
As good a hypothesis as any!
A little known related fact:
About 100 of the first alloy XK120s had the starting crank handle and special hole in the radiator to accommodate it, but 10,000 XK engines in XK120s and Mark VIIs had the crankshaft damper bolt with 3 jaws to fit that handle.
One time at work I needed a box of 100 screws, and my purchasing clerk ordered 100 boxes of 100 screws.
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Did he also order several Verbal Agreement Forms at the same time? 
Tim
Betcha never needed to order those screws again!
