Helicoil size for head bolt

So I managed to snap a head bolt off on my 3.9 and therefore need to drill it out. That's never gone well for me before, so likely will need to helicoil it. Anyone know what size I'm looking at please?

Thanks

Blimey! How did you do that!? Hope the block survives.
Have you thought about using an extractor bit?

I undid all 16 bolts the same, they were tight as expected. This one snapped easily, flush with the block...

I'll try an extractor, but again, never had any success with those.

Just want to be prepared in case it goes pear shaped.

I just worry with helicoil, especially on a head bolt, you need to be very sure you drill perfectly straight.. don't know the bolt size but you should see it listed somewhere i would have thought

make your own mind up, if you think these are right... never done the head on a 3.9 so not sure
https://www.lrdirect.com/err2943-cylinder-head-bolt-4-6v8-short

I think I'll get them measured as needs to be bang on.

Fortunately I have access to a very accurate milling machine and an operator who works in hundreds of a degree.

Lrcat says the head bolts are all 7/16UNC.
Lrcat

Ratae said:

Lrcat says the head bolts are all 7/16UNC.
Lrcat

Click to expand...

Agrees with my link

kermit_rr said:

I just worry with helicoil, especially on a head bolt, you need to be very sure you drill perfectly straight.. don't know the bolt size but you should see it listed somewhere i would have thought

Click to expand...

hi mate

funny enough watched this the other week regarding snapped bolts etc ,plus some guy using a hinge centre drill , wished to share the link in case u found it interesting

j_rov said:

I think I'll get them measured as needs to be bang on.

Fortunately I have access to a very accurate milling machine and an operator who works in hundreds of a degree.

Click to expand...

So ask him to remove the broken stud !

Try welding a washer onto the thread end, give it a tap with a hammer afterwards then weld a nut onto the washer via the inside of the nut. Tap that with a hammer and it may undo.

I feel your pain. It'll need a good drill bit as they'll be hard as hell. Never had to do that yet but good luck!

Left handed drill or mill bit ? That way if it snatches at the bolt, it might unscrew itself ?

pwood999 said:

Left handed drill or mill bit ? That way if it snatches at the bolt, it might unscrew itself ?

Click to expand...

If water has got down that thread enough to snap the head-bolt I really cannot see it shifting.

I'd be tempted to cut a groove up each side of an old head bolt and run it in and out of the old holes once they're all out to clean out the threads. A bit of moly CV grease on the new bolts to help them slide in and out before refitting the head might help too. Still, got to get that last bugger out first!

Once you get the broken bit out, consider investing in the ARP Stud kit. Makes re-torqueing the head much easier.