Bladerunner919
Active Member
I need to re-tap a head bolt-hole in the block. I think the head bolts are M12 x 1.5 but can anyone confirm that?
I take it some of the holes are tight. You don't need a tap just cut a few slots in an old head bolt at the end for an inch or so along the length of the thread with an hacksaw. Then lightly oil and work in and out. That will clean the threads. Then pour a little paraffin into each hole and blow out with an airline to clear any residue. Make sure all the new bolts screw in fully by hand before re assembly of head. Lightly oil threads and under bolt head on assembly. Or better still a light coat of moly grease on threads and under head onto washer.
Bladerunner919
Active Member
Unfortunately that's not the problem. Someone has had the engine apart before and, for reasons that I am unable to fathom, every single bolt-hole for the head bolts has a helicoil-type insert. One of them is not able to hold the torque of the head bolt, so I need to re-drill it and put another insert in. Hence the need to know the exact thread spec.
benthomas
Active Member
According to Euro parts they are M12 as you thought, they will be M12 x 1.5 as this is standard metric pitch, they are definitely not MF "metric fine".
To be sure measure the pitch from peak to peak with a vernier if you can get hold of one, or take a head bolt with you to the engineers supplies your getting the helicoil kit from
To be sure measure the pitch from peak to peak with a vernier if you can get hold of one, or take a head bolt with you to the engineers supplies your getting the helicoil kit from
benthomas
Active Member
Yeah sticking with my M12 X 1.5 as per the attached page for M51 head bolts.
Cylinder Head Bolt BMW 5 Touring (E34) 525 td - de 1993 115 HP buy cheap online - Cylinder Head Bolt - Engine
Cylinder Head Bolt BMW 5 Touring (E34) 525 td - de 1993 115 HP buy cheap online - Cylinder Head Bolt - Engine
In that case you have a big problem. In what way does it not take the angle tightening. Has it pulled out or has it stripped the bolt? You can't drill out an helicoil.
benthomas
Active Member
That's right you cant drill them out, they come out the same way they go in, screwed with a tool like this.
M3 to M10 (3/16 - 3/8) Helicoil Wire Insert Removal Tool | Engineering-Supplies.com
I've teased them out with a flat head screwdriver before now.
Just so, they must be wound out. That is why i asked the question what has failed.
Bladerunner919
Active Member
I think the hole for the helicoil was either slightly oversize or not perfectly round. I think that allowed the bolt to slip during the tightening and damage the threads. The helicoil just wound out, so that's not a problem. I just need to redrill, tap and use an insert to get it back to the original thread size. I say, 'just'!
So you are going to use a solid insert rather than a Helicoil. Depends how much meat you have available around the hole.
