3.5 Litre Mk1V Head Stud Block Threads

I need to repair a number of head stud block threads with recoils. It has been suggested that I should go to 7/16" UNC 14 TPI rather than the original BSF 18 TPI & have some new head studs made. Opinions please whether a UNC thread in the block would be better than the BSF?

Regards Peter

I think you mean helicoils.
I have never used them, but I have many times used Keenserts.

I think I would go with 7/16-20 UNF, if 7/16-18 BSF is not available anywhere.
You can make your studs with 7/16-18 BSF on the other end so you can use the same nuts on all studs.

If you have cylinder liners then beware of stressing the top deck with excessive torques.



Hi Peter,

I, for myself, would use a Helicoil of the same thread for a couple of reasons. One is that the same studs can be used, but the other is mechanical. Though the threads are stripped, the remnants and interference effect of the original t.p.i. will remain. Changing to a coarser t.p.i. will cut across some of those grooves, removing more metal. Would that matter? I don’t know. The Helicoil BSF replacement uses the same pitch and will cut the existing threads deeper. This will give more bite. In addition, the coil insert is of hardened steel (stainless?), so it won’t strip and the only way it can strip is for it to pull out of the cast which will be harder to do.

Just a hint if you go the insert way. Run the insert down 3 or 4 turns and try the stud to make sure it is all seating correctly. It often helps to run a die or die nut down the stud thread to ensure it is precise as the coil profile is very accurate. The trap is that if something goes wrong, you can’t unwind the coil - you can only wind it through to the other side. If the hole goes through to the water cavity, no problem. If it is a blind hole, make sure you test the stud frequently.


1 Like

Thank you Gentlemen, Rob, recoil is a Oz brand of Helicoil, Peter, no liners, the block I am using is one that I acquired to replace the original one which had an issue between 5 & 6 bores.
It had been laying outside for a number of years with head studs missing & others bent. I used it as it was std. bore & gave me more options for pistons when reboring it. Admittedly I had to drive the seized original pistons out even after soaking in diesel for a week. I retapped the stud holes, had the block tanked & bored. I should have checked the threads in hindsight but just coated the studs threads with Holymar & reinserted them. The first time it torqued up OK but when I filled the engine with coolant there were leaks just below the carbies. I had the head checked & they just gave it a clean up. Thicker hardened head nut washers were used when reassembling the head but when I got to 55 ftlbs two studs started to go soft, a wonderful feeling. I pulled the studs thinking they may have started stretching but no such luck. In all four studs went soft so I assume to be on the safe side I should repair all 14 stud holes.
Why I mentioned UNC is that someone recommend that a coarse thread is better in cast iron? As far as I know that is the go with aluminium.
As the engine is assembled & in the car what are my options re drilling & tapping the holes, a jig, a magnetic base drill press or?

Regards Peter