Head stud repair options

My combat head has stripped out a helicoil on the front r/h stud leaving an almost smooth 5/16 hole, my instinct is to tap this out to 3/8 unc but I keep reading stories about threads breaking through into the pushrod tunnel, also if I tap it 3/8, is there any problem drilling the hole in the head out to 3/8 and using a straight stud or should I step it back down to 5/16.
What depth is this hole usually, I can only measure 14.2mm from the top of the barrel but I'm not sure if I can see a piece of sheared stud at the bottom, it's definitely not from the stud that's come out, it may have been left in by the person that bodged the helicoil in there.
The answers are probably simple but I don't want to find out the hard way when many of you will have dealt with this problem before, any advice greatly appreciated, many thanks in advance, Paul.

Hi,
Find out if you have a piece of broken stud in the bottom of the hole by using a slender piece of magnet. Hopefully you don't as that would complicate any repair that you plan to do.

GB

Talk to Matt @ Colorado Norton Works. He has a head bolt set up that is top notch. ARP quality bolts.
Cheers,
Thomas
CNN
Edited: Re reading your thread. The three studs out of the head should be almost .375” or 3/8 in diameter. Head torque sequence # 2, #7 and #8. Just to be sure. Was it a helicoil or just the remainder of the stripped out aluminum from the head? If that is the case then you need a thread repair insert to fix it. If you do not have a machinist close by then I would recommend sending the head to have head studs all repaired. The studs are common to pull out. You must insure that you do not go too deep with the helicoil drill. Comnoz would know what to do or the other option is to order from Walridge Motors on Page 161 in his catalogue for a thread repair kit if you want to keep it stock and do it yourself. The price of the kit is probably not worth it if it’s just for one repair. You must be bang on with the hole placements or you will have difficulty dropping your head back down onto your barrels. Where are you located?

A number of ways to skin this one so will let experts detail the various thread replacement beef ups, but do mention that Al is not very fast reactive to strong inorganic acids [way more so in alkali like detergents so beware] so old tractor restorers dam around the stud hole with clay or wax/grease then put Nitric or other strong acid in via syringe and periodically suck-flush out the reddish goo of dissolved iron/steel till not much left but to just flush-brush and thread chase out, no lost of Al surfaces. Might be a piece of steel heli coil jammed in bottom if not also some stud but studs generally don't break very far down from the surface, There is a very expensive electro ablative method but tricky in holes with Al threads. Likely also a reverse plating electro galvanic method that'd dissolve to point any pieces left just fall out but I ain't up on that one.

brxpb said:

My combat head has stripped out a helicoil on the front r/h stud leaving an almost smooth 5/16 hole, my instinct is to tap this out to 3/8 unc but I keep reading stories about threads breaking through into the pushrod tunnel, also if I tap it 3/8, is there any problem drilling the hole in the head out to 3/8 and using a straight stud or should I step it back down to 5/16.
What depth is this hole usually, I can only measure 14.2mm from the top of the barrel but I'm not sure if I can see a piece of sheared stud at the bottom, it's definitely not from the stud that's come out, it may have been left in by the person that bodged the helicoil in there.
The answers are probably simple but I don't want to find out the hard way when many of you will have dealt with this problem before, any advice greatly appreciated, many thanks in advance, Paul.

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I would suggest using a 5/16th-26 helicoil insert to repair it. Jim

Sorry for misleading you CanukNortonNut but the stud I mean is no.10 that comes out of the barrel, I had to phone A.N. this morning for parts and whilst I was on the phone they looked up the drawings and informed me that the hole in question should have been drilled to a depth of.500", so as I can already measure it at over 14mm I presume it mustn't be a sheared stud, just the bottom of the hole.
They also told me regarding breaking through into the pushrod tunnel, that most barrels were hand fettled in the works, so it depends how zealous the bloke was that did your particular bike.
Thanks Paul.

I agree with Jim, I performed the repair last summer and did all 3 while the head was off.
at the same time I converted to a 12 pt nut on the rear stud, now I can access the rear stud with 7/16 dogbone and use a torque wrench. I converted the front nuts to unf with a helicoil, I think Jim suggested this as well.
The stock inserts were smaller od than modern helicoil.