It appears water is leaking from my cylinder head from the top of the small head bolt, radiator end. Does that mean the water jacket near the water pump is in the block has a hole and if so can JB Weld fix this or is more required?

Hi, I do not believe from factory that holes goes through into the water pump area, but someone with better knowledge may correct me. From original it is a 5/16 bolt, and is tensioned less than the rest.

Maybe out two gurus with drawings can agree on this one. lol

However, when people drill the two extra holes for cooper S studs, they often break through into the water pump area for the bolt at that end, and often make it 3/8 which is not needed. If it breaks into the coolant the thread of the bolt has to be sealed. Once upon a time we used Stag paste, GM sealer , or some sealer meant to seal threads. And I guess if you retension head, it should come out and reseal. cheers Lindsay Siebler

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1967 EX works rally mini restoration virtually complete OBL45F
1965 Mk1 Cooper S
1970 Colt 1100F SS
2014 Mitsubishi Imiev fully electric daily driver
1948 Allard M coupe Olds 215 v8 project
1971 Twin cam Escort ex Uk championship winning rally car project

It appears the smaller bolt hole has been enlarged to the same size as the other 10 stud bolts and therefore may have broken into the water pump area.

That 5/16inch hole goes through into the water jacket, just behind the thermostat. The thread has a helicoil in it from factory.
Watch this from Bill Sollis at 1 hour 59minutes 30 seconds for explanation:
https://www.youtube.com/watch?v=BaaPXIKwDXY

QUESTION

Is the 11th head stud hole (radiator end) the same diameter as the 10 other holes?

No. It also has a different torque setting if I remember correctly

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1964 smooth roof panel van
1967 Mk 1 X-Police cooper s
1970 mini k
1975 leyland

I think I've found the head stud problem but how can it be fixed?

That hole through the head casting is the same diameter as the other holes through the head casting for 3/8" studs.
The special bolt AEG248 for that 11th hole is 5/16 18 UNC with a larger diameter shank just below the hex head.

If it were mine I would seal a stud in there (stag, threadlock, silicone et al).
Or if you consider the bolt not necesary then seal a grub screw in there.

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Cheers Dave.

Thread sealant as mentioned previously

It’s pretty common for engine studs to end up in the cooling jacket on many different engines

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I use to use QH stud sealant on 308 v8 motors and never had a problem
Also used it on a mini at the engine steady bar end when someone put a to long a bolt in and broke through into the water jacket

I am sorry to be late in giving a reply to this question. This hole is threaded through into the water jacket 3/8 x 18 TPI to take a 5/16 helicoil for the 5/16 18 TPI special bolt as pictured above. If you insert a normal 3/8 bolt or stud here, you will almost certainly ruin the thread since normally, 3/8 is 16 TPI for UNC holes. There should be a helicoil in this position as fitted originally. The extra hole on the other side of the head is a normal 3/8 x 16 tpi threaded stud. The special 5/16 bolt appears in the service parts list for YDO6 and YDO23. Confusingly, a Service Liaison Summary dated 1966 refers to "two extra studs" fitted to Morris Cooper S engines implying that the thread in the water pump position is for a normal 3/8 x 16 stud. Head stud kits advertised online also say 11 studs for Cooper S models. If you have a bolt, then almost certainly it is (or was) a helicoil fitment as described above. If there is a stud there, I don't know. I would have concerns about screwing a 3/8 x 16 stud into 3/8 18 hole and torquing it up to 42 ft lb. For the standard bolt, the torque setting is reduced as mentioned above. Having had some recent experience with helicoils into the water jacket, I recommend putting plenty of thread sealer on the stud thread, and on the helicoil as well if renewing it.

So is it possible to replace stud thread with a helicoil for the 5/16 bolt?

The thread size in the block is 3/8 threaded 18 TPI. This is for screwing in a helicoil to give a final thread of 5/16 (sorry, not 5/15, my editorial error) x 18 TPI thread for the special bolt. That is the standard condition.

If the helicoil has been taken out by some person, and then a 3/8 head stud or bolt at 16 TPI put in there, the stud might screw in a few turns, but if forced in, will ruin the original 18 TPI thread. You could try re-tapping the thread back to the original 3/8 18 TPI in the hope it will clean things up, but of course somewhere along the way there will be a cut or two in the threads. The steel helicoil might save the day (at least as far as the special bolt is concerned), and it may hold at the reduced torque setting. Either way, with the 3/8 x16 stud or the 3/8 x 18 (5/16 helicoil), the thread is compromised and there's probably not much you can do about it. I am sure there are probably many cars with the same problem. I am told by someone who is knowledgable that the reduced torque and helicoil was put there because of a weakness in the block casting around that area. Just why it needs to be there in the first place I don't know, a Longbrdige 1275 engine doesn't have such a bolt in this position but that casting is different of course to a Morrris Motors block. Regards Tony

If I understand you correctly 'eightfifty' this bolt could be be dispensed with and the hole could be plugged so it never leaks.