I suppose that is an option, and probably Graham Russell can advise you better than I. All I am saying is that the Longbridge 1275 as fitted to a Mini LS or Moke Californian doesn't have this bolt, but the block is a different casting to a Cooper S block and may not need it. On the other hand, the Special Bolt is supposed to be only done up to 25 ft lbs compared to the stud nuts at about 42 ft lbs so you have to ask is it actually doing anything? If it were my car, I think I would (depending on the condition of the thread), tap through with the 3/8x18 helicoil tap, put the helicoil in (with plenty of sealer applied) and install the bolt and tighten it to about 10 ft lbs and see how it feels. You can probably do this with an old head gasket for the sake of trial. However, this is my opinion only and others like Graham would have had first hand experience with this problem and can give better advice.

The extra stud and nut and bolt are supposedly to cope with the extra compression ratio of the high compression engine. Whether it is needed or not and whether you can get away with out is probably a personal choice. However I have been running a high compression engine with only the standard nine studs and nuts for a number of years with no problems at all. I sealed a grub screw into the water pump end hole but I just left the clutch end as was.
I cannot see that an extra stud and nut and an extra bolt at virtually no torque makes any significant difference.

_________________
Cheers Dave.

ditto, even with 13+ PSI of boost from a supercharger and 100bhp at the wheels...

for a while I had my S head on an 1100S block, the spare holes in the head were really handy for holding screwdrivers

_________________
did I tell you that I won a trophy?

Hey Steam, What did you use to seal the grub screw?

I actually can't remember but it would hve been something I already had in the shed . Being a tight a** i wouldn't have bought anything new. I will have a look and a think and get back to you.

_________________
Cheers Dave.

Any experience in this sealer?

https://www.seallock.com.au/products/fluid-weld

No experience but reading the blurb it sounds like it would work.
I probably used black high grade acetic cure RTV silicone that I have here.

_________________
Cheers Dave.

Here is the hole I am hoping to repair. It has been butchered and is now 9.36mm in diameter and has broken into the water jacket as seen in the 2nd picture.

Iam hoping to recoil it with the correct thread but don't know which kit to buy. There appears to be a drill bit required, then a tap and then the 5/16" UNC helicoil. What size drill bit do I need?
Once the helicoil is inserted I am going to block the water pump end with some PC Fahrenheit expoxy.

Hope that does the trick.

I think you will be having trouble here. A 5/16 x 18 helicoil kit comes with an 8.3 dia drill and a special tap for the inserts to screw into. Since your hole is already 9.36 mm dia, the special tap has no chance of creating any thread for the 5/16 inserts. Out of interest, the drawing shows that this hole does go through to the water jacket, and the factory 5/16 helicoil does have a part number AEA684. regards, Tony

As I see it from the attachment, the hole does go all the way into the water jacket and then the special helicoil is put in which must be blanked off so water does not leak into the helicoil.

I am not sure I understand your description, but the only thing that stops the water leaking up the thread of the helicoil, whether the inside or the outside of the coil, is the pressure arising from tightening the special bolt, plus the thread sealer that would no doubt have been applied. The bolt does not extend down into the space as shown in your photo, but the hole does go right through. The helicoil is usually screwed into the threaded block so it is positioned just beneath the top surface and then the special bolt screwed in. I suppose you could go to 7/16" UNC thread in the existing 9.3mm hole and make a plug. A 3/8 helicoil would be 25/64 drill (9.92mm) and then you'd have a 3/8x16 threaded hole in which you could put a head stud in, or, try (if there's enough metal to work with), make a 7/16 x 20 UNF OD with a 5/16 x 18 ID "coil insert" to bring things back to normal appearance.

Edit: Something like this shown below which I made after dinner. 7/16 x 20 TPI outisde, 5/16" x 18 TPI inside. The outside diameter of a 7/16" external thread is about 11 mm so you are sacrificing a little under a mm around the circumference of your existing hole (assuming there is no thread there at all) but at least you get a thread back, and then you can use the special bolt 5/16, but I wouldn't do it up to 25.

Nice skills , eightfifty

Wow eightfifty. I would like to know if you would like to sell that beautiful work so I can a least have my engine looking good.

-

Thank you eightfifty PM sent.