Good progress cobber!
For the broken studs on the cylinder head I usually do the following:
1. Sand or file the studs flat, being extremely careful not to sand the cylinder head itself. If the stud stands proud of the head that is a good thing.
2. With a very sharp prick-punch lightly mark the centre of the stud with the gentlest tap of a hammer.
3. Check that the punch mark is centred. If it is off-centre, put the punch in the mark and lean it over until is pointing towards the centre and give it another tap.
4. When the mark is centred, give it a firm whack to increase the depth of the mark, whilst the punch is plumb to the stud axis.
5. Select a HSS drill bit as a pilot - for a 1/4" stud I would use about a 3mm drill bit; generally a bit 1/2 the size of the tapping drill will work well on small studs - and then commence drilling. You will need at least one assistant with a good eye to help line the drill bit up to plumb. This is absolutely essential if the operation is to succeed.
6. Using frequent brushings of soluble oil on the bit, gauge how deep you have drilled by measuring the depth of the hole on the one you have the bolt in. You may even feel the drill bit break through into the cavity below the stud.
7. When the pilot hole is complete, commence drilling with the tapping-drill, which can be found from a tapping-chart if you know the diameter of the stud and preferably the thread. I bought a left hand drill bit some time ago, which can unwind a lightly stuck thread; in my case of the six fasteners I had to drill, it unwound one of them.
8. When you finished drilling two things are likely to have happened: if all went well the thread is still full of old stud, which can be carefully cleaned out with a taper-tap; or if you have drilled off-line, more work will be needed. In the second case you will need to purchase a re-thread kit such as Helicoil and follow the directions in the kit to restore the thread.
Finally before fitting the new studs you will need to consider the following: the originals rusted in place because water leaked from the cooling system and into the thread, so this needs to be remedied. So either use an anti-seize paste on the thread to protect it, or use a thread-locking fluid such as Loctite, which has the additional benefit of glueing the probably not-perfect-thread solidly in place onto the stud.
Let me know if you want me to make you a new housing,
Cheers Charlie
