Coolant drain tap XK120/140

Me again! Is the thread different between the engine and radiator drain taps? They look similar but the radiator tap is C980 and the cylinder block one is C2354.
I have what I think is an XK140 rad drain tap with a soldered-on piece of downward curved copper tube. I like the design - would it fit the XK120?

Thought this would be an easy one for someone to answer!

I can give you a definite maybe. I don’t know anything about the XK140 part. I believe that the threads on the two taps I have on hand (EDIT: not sure if they came from the radiator or block Correction, these are block drain cocks) are both 1/2-20. Both have a fiber and copper washer, so they are definitely straight thread, not tapered. A BSPP 1/4 pipe -19 tpi thread gauge rocks slightly, 1/2"-20 is tight. Way back when, I used a 1/2"-20 bolt (same bolt as the oil galley bolts) and copper washer to eliminate the block tap, and I am not the kind of guy who intentionally cross threads a bolt. PS: the block in the photo is from a 1964 Mk2, not a XK120.

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The hard part is finding the parts in my stash of stuff to measure.

The other hard part is telling a 1/2-20 UNF thread from a 1/4-19 BSPP thread. They are very close, and I don’t have a 19 pitch thread gauge.
The tap with the curved tube is most likely from a pushrod engine. My Mark V and SS both have it on the blocks. It is part C.2000 in the Mark V book.

As nothing else on these pushrod engines is UNF, I would say it is most likely a 1/4-19 BSPP thread.
The handle is threaded and breaks off rather easily BTW.

As everything else on the XK engine is UNF or UNC, I think it is very likely that the XK right angle drain tap would use a 1/2-20 UNF thread.

As the C.980 straight through drain tap is also used on pushrod car radiators, I would say it is 1/4-19 BSPP.

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Amen to that. I went back and forth several times between the 19 and 20 tpi thread gauges. 19 tpi rocked a tiny bit but the 20 did not.

Thanks Mike & Rob. The top picture in your last post, Rob, shows two drain taps. I bought a new one which looks the same as the one on the left as being correct for an XK120 radiator drain tap. I don’t like the look of it as much, and apart from anything else, it would squirt coolant straight out making it difficult to collect if you wanted to save it. But I will compare the threads with the one I described which I thought was XK140 and see if they are the same. If so, I guess it should fit the rad OK. You could also attach a piece of rubber or plastic tubing to make collecting the coolant even more accurately. I’m still confused by the different part numbers between block and rad drain taps…

Here is a better answer. The brass plug on the left is 1/2"-20 tpi and fits the threaded ports along the main oil galley and the tap for the block drain valve. The stainless steel plug with the integral o-ring on the right I just removed from the bottom of my XK120 radiator and it is BSPP 1/4" pipe, 19 tpi. So the radiator drain plug and the block drain plug are not interchangeable


The drain taps ,be they block or rad, are 1/4 BSPP, but the threads in the brass
bottom tank of the rad do not corrode, while the threads in the cast block do, hence
the looser fit.
Drain taps with curved outlets were /are used on SS engines, should fit a 120 Block
or rad as long as you can turn it.

Peter B

I’ve just had a thought… I have an aluminium radiator and I’m wondering whether a brass drain tap might cause some adverse electrolytic reaction between the two metals? Hmmm…

Metallurgists step up !! I hope it`s not a problem with bronze valve guides :sob:

Galvanic potential tables are written based on exposure to flowing seawater. The effects are also proportional to the relative mass and surface area of the two parts. So long as you use a garden variety antifreeze solution that includes the usual corrosion inhibitors, you will be fine.

Bronze valve guides are safe because they are generally not exposed to a liquid, except the mineral oil at the valve spring end, and mineral oil is not electrically conductive.

The biggest risk is to the 40 lb block of aluminum that is the cylinder head bolted to an iron block. If you use plain water, you can expect a galvanic reaction to occur over time.

For the two cocks I have in hand, and plug I have used in the bottom of the 120 radiator, I am dead certain of the thread form. The cocks are 1/2’-20 and the stainless plug is 1/4-19 BSPP. What I do not know is the origin of the cocks. They could easily be XK150, Mk9, 420 or any of the other engines that have passed through my hands. In addition to the Starrett and Parker thread gauges I examined the fittings with, I tried each with a 1/2"-20 nut. The nut spins readily onto the two cocks and the brass galley plug without resistance, but not more than one thread on the radiator plug. Cocks and plugs with 1/2-20 nuts shown in photo.

Yes, this is the reason we find corroded oval ports on the bottom of our XK heads.

The C980 thread OD is .514" and a 38 pitch thread gauge fits on it so it is 1/4-19 BSPP.
The C2354 thread OD is .494" so this is 1/2-20 UNF.

It looks like we are coming to the conclusion that pushrod and XK120 radiators have 1/4-19 BSPP threads, as do pushrod engine blocks.

XK blocks have 1/2-20 UNF threads.

Chris, you can use your C2000 curved tube tap in your radiator but not your engine block, or if you want to you can solder a short piece of copper tube inside a C980 drain tap so you can put a hose on it any time you want to drain the radiator.

OK Miike Im half wrong (note half wrong not half right) Ive checked the quantity I have and the type shown in your post, the type fitted in the block, are indeed 1/2 UNF, but the other type with the sraight
outlet , which when modified to accept the control rod, MK1 /2 etc, are 1/4
Perhaps there is a part number change for the block drain for the early
120 / MK7 blocks say mid 51, anything pre 50 BSPP methinks.

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The evidence of the XK120 SPC suggests that there was only one block drain tap C2354 from the beginning of XK blocks.
This same C2354 is listed for all XK blocks up into the 1960s, but changed with the 3.4 MkII to C31616.
Early Mark V radiators had a different drain tap C.3736 with a long control rod, but this was changed back to C.980 before mine was made.
We can learn from this discussion that it is possible to cram a wrong thread drain tap into a block or a rad, or to mistakenly run a wrong threading tap into a block or rad to clean up a tapped hole.

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It’s too bad these little drain cocks lack any sort of identifying marks. They all look very much alike.

Rob does C2354 change to C31616 re 3.8 XK150 / MK9 to
3.8 MK2 same as MK2 3.4., thread change at this point perhaps.

The C.2354 drain tap continues on all XK150, Mk IX, 2.4 MkII, 3.8 Mk X, 4.2 Mk X, 420 saloon, 3.8 E-Type, and 4.2 E-Type, so apparently there was no change in the thread.

The C.31616 drain tap is listed for 3.4 Mk II and 3.4 S-Type. Nothing is listed for 3.8 S-Type, but there is a footnote referring all other items not listed back to the 3.4 block. I do not have a SPC for 3.8 Mk II or 340.

There is no explanation as to why these 3 small saloon models would have a different drain from the other models, but on the XKs Unlimited website this part number is a brass hex bolt plug, not a petcock.

But I think it unlikely to be a change in the thread, especially to a BSPP thread, which would be a step backwards in the move to UNF standardization.

However, we have identified the possibility that a BSPP threaded drain COULD have been mistakenly installed in an XK block by some careless or misinformed owner or independent shop mechanic, either by just cramming it in by force, or by re-threading the hole in the block. In either case, better investigate thoroughly before deciding what to do about it.

Ding! The engine block shown in my photos above came from a 1964 Mk2, so that 1/2-20 hex bolt I used in place of a petcock may not have been original thought.

Thanks Rob , the change would be from BSPP to UNF not the reverse of course.
What number MK V block and rad?
Peter B.