Daimler 15: stripped thread

[HenryC]

Hi Dave,

As Stan says and assuming the thread in the block is still ok, you just need to find the correct insert, as per his advice.

I have used helicoil inserts, and other makes, they are indeed diamond shaped in cross section.

Do pm me if you need any more advice. (phone number in DM)

[A.N.Other]

It looks like part of a stud that stripped some time ago and wasn’t removed. It proberbly bound just enough to hold a new stud but then give up the fight when you removed it. The way to find out is measure the hole. If its 3/8ths or slightly more then it hasn’t been previously helicoiled.

[captain bobo]

Curioser and curioser! Finally had time to do a proper investigation today with results that surprised me.

The head stud itself is 3/8 inch diameter with 20 tpi threads at each end which are not 3/8 inch BSF threads, but an oversize thread which requires a different (slightly larger) head nut than the other head studs. The block thread is the same oversize, so I imagine that in the past the block thread stripped and a new thread was cut without enlarging the hole to accommodate a thread insert, and a head stud fabricated to fit, but it's a guess.

Question, is there a 20 tpi thread spec that has a fractionally larger external diameter than a 3/8 BSF? The head nut for the problem stud just slips over a 3/8 BSF threaded stud.

Anyway, I've got hold of a thread insert kit for 3/8 BSF and a BSF tap and die set. I've cut the thread on the stud down to 3/8 BSF at each end, drilled out the head stud hole, threaded the hole, inserted 2 x 1.5D thread inserts and installed the head stud. All I need now is to get hold of some 3/8 BSF nuts, then the head can go back on.

Thank you all for your comments and suggestions, they have been helpful in diagnosing and rectifying the problem

[Brian-H]

Question, is there a 20 tpi thread spec that has a fractionally larger external diameter than a 3/8 BSF? The head nut for the problem stud just slips over a 3/8 BSF threaded stud.

7/16 UNF ?

[Christopher Storey]

The 3/8 BSF has a major diameter of 9.525mm , so the nearest slight oversize is probably an M6 fine , which has a major diameter of 10mm and a pitch of 1.25mm which equates to 20.32 per inch . However, the thread angles are probably different ( I can't recall the exact angles) so you need to be very careful about mix and match

[captain bobo]

The nuts that fitted the correctly threaded head studs didn't properly fit the"rogue" stud - they started to thread ok but then got very tight indeed as the whole nut threaded on. Based on what Chris wrote in that last post I suspect the rogue stud could have had M6 fine thread - my 20 tpi thread gauge best fit showed it having nominally 20 tpi.

I've cut a proper 3/8 BSF thread onto each end of the rogue stud and installed it with a thread insert into the block - it's a good tight fit now.

Just need some new replacement 3/8 BSF nuts and then I can put the head back on.

[captain bobo]

Update: new 3/8 BSF nuts arrived, I decided to use new nuts all round. The head studs have all tightened up to the recommended 41 lb ft (ft lb?), the helicoil did the trick. The head is back on, I'm cleaning and polishing the inlet manifold, carburettor and rocker cover, making up new exhaust and inlet gaskets, degreased the block and repainted while I had good access, replaced the rubber hoses and associated clips, cleaned out the waterways with a pressure washer. Next I'll flush the radiator and then reassemble everything.

Thank you all for your interest and comments.