Having just had some engine parts bead-blasted, I see that one of my Pantah heads has a poor spark plug thread. There's some thread left, but it's not good enough to put into a rebuild engine. All the threads in my other heads are OK. So today I took the problem head to BLR Engineering of Wallington. They do a recovery service which consists of boring out the hole, tapping a thread, tightly screwing in a bronze insert, and boring and tapping the insert back to standard 14mm spark plug thread. The result is considerably stronger than a Helicoil, and stronger than the original head casting ever was. I hope to get that head back in a couple of weeks, ready for assembly.
This head had been sitting in the back of a damp garage for 20 years with the sparking plug rusting in place. There was no way the old plug was coming out without bringing some of the alloy threads with it. Rebuilding old bikes involves all kinds of jobs to recondition components, and £60 or so to make a Ducati cylinder head perfect again is not a lot in the scheme of things.
Collected that rear cylinder head today, with the sparking plug thread fixed. BLR managed to get the bronze insert in without fouling the valve seats. Now I have left with BLR my collection of footrests and brake/gear levers (all steel on a Pantah) some of which need straightening and/or welding to get them right before I have them chromed.
Incidentally the standard plug thread is 14mm diameter and 12.7mm long (i.e. 1/2 inch short thread). I have arranged for the new thread to be 19mm long (i.e. 3/4 inch long thread). So with NGK plugs, it will now take a BP7ES instead of a BP7HS.
I have been dealing with BLR for over 30 years now and can thoroughly recommend them. They have taken on jobs that others would not touch. So if you do have a tricky thread repair these people are definitely worth a phone call.
I would guess the insert is located off centre to miss the valve seats, then the thread put in the original position .
one of the ford heads is prone to seized and breaking plugs in the thread, using an impact gun to remove the plugs sems to reduce the risk, go easy with plugs with tapered seats
No, the insert is not off centre. What BLR have done is bore the hole centrally but carefully, stopping short of full depth so the bore-hole does not quite go as far down as the valve seats - it is in effect a "blind" hole. The alloy of the combustion chamber is left undisturbed on that side. The bronze insert is screwed into the full depth of the hole, and also pinned in place with a small axial pin. Then the plug thread is tapped centrally in the bronze insert and in exactly the same position as originally, but now 3/4 inch long instead of 1/2 inch long. The new thread must be at least twice as strong as the original was when new.
