How do you tighten the water drain tap on the engine block when in the car? It was not installed when I purchased the car. I have one copper and several fiber washers to adjust the alignment. The drain tap has an area where a wrench could fit but, there is no room to maneuver a wrench. Is there a special tool needed for this? Should I put vice grips on the drain tap body to tighten? Use thread sealant on this drain tap? Thanks
I just replaced mine in situ this morning because it was ever so slightly seeping at the top I removed it with a standard Craftsman 15mm combination wrench. You can get the open end on the flats, approaching it from the front of the engine and turn it one flat at a time.
I had the new petcock ready for installation, I used a bit of Permatex #2 on the threads
I waited until the engine was cold, removed the oil dipstick placed a towel under the petcock, removed the old one and installed the new. Less than 1/2 cup of coolant escaped during the process.
Thanks. 15mm was what I did not have, bought one of the short ones at Harbor Ft. I’m going to bend it 30 degrees or find a 15mm wrench with an angle so it clears the obstructions I encountered. Thanks again.
You didn’t say what engine this was, but if it is from the 1950s and a 15mm is close, the tap is probably a 3/8" BSW size, in other words Whitworth.
There are not a lot of Whitworth size hexes on an XK engine, mainly the SU carbs and gearbox, but this drain may be one as well.
You may want to invest in a set of BSW wrenches. Harbor Freight won’t have them, but Moss Motors does.
If it has “eynots” cast into it it’s definitely 3/8"Whit. Can only assume their tooling was so ancient? And they never updated it!
Well, that explains why my SAE set didn’t fit. 15MM is pretty darn close though.
Not to change the subject but they use one on the radiator as well, or did they use the standard radiator drain similar to what us cars of the 1960’s use?
Yes it was the same tap from what I remember!
I tried the one on my XK120; it was 5/16W or 3/8BSF size.
You may find wrenches marked only one size or marked both like this one made by Superslim.
A 15mm Spanner is a good fit , problem is if you want to get a old tap out a block , its more or likely been in there from new !
The edges are not crisp on the taps and will round off easy , trying to get the tap out the block !
I got one out a few weeks back , job was easy as the engine was out , the top of the tap was damaged , so just used a adjustable spanner on the tap body , I did try with a spanner but the edges rounded off .
It would be a good idea , if you are changing a tap to do a back flush , and maybe run some kind of cleaner around the system first , when I removed the tap there was a Orange wall of sludge behind it ,
I removed the core plugs took the block out side and gave the water ways a pressure wash !
Replaced the core plugs and tap , plugged up the water pump hole , and filled the block water ways with Caustic soda and water , left it for a day , back out side and another pressure wash , it took a good 15 mins of washing before the water stopped being a bright Orange !!
If your not in to keeping the car 100% original , you could fit a plug from a XJ6 
A few years ago, when I was going to install a plug, I was told by someone on J-L or a Jag mechanic the drain on my '56 Mk 1, was threaded for 1/4 BSPT Pipe Size, 19 threads per inch. Don’t know how many years Jaguar used that size. McMaster Carr was the only place locally that had it.
As for tightening the drain tap, the half size 13mm wrench was too large, I picked up a metric set of “crowfoot” wrenches. I think that will do it.
BSPP aka British Standard Parallel Pipe threads are also found on the SU carbs and fuel pump fittings and some intake manifolds.
Metric Multistandard Components Corp is another place in the USA to get BSPP plugs and threading taps.
www.metricmcc.com
cam tightening chain is BSW…18mm fits
crank nut is 3/4 bsw, but 1-5/16 or 33mm fit
Metwrinch spanners & sockets cover similar BSW, Imperial and metric with one spanner,
high quality tools
If your block is fully stripped, acid dipping is by far the best method of corrosion eradication.make sure whoever does it,doesn’t leave it in for too long, if not reboring,as bores can get too heavily marked.
If done properly, this leaves the block looking like new and is easy to repaint.