what depth to drill for helicoil?

[Davenott]

I'm drilling out a fractured inlet manifold bolt, and intend to helicoil it. [can't remove it any other way with this one, all others out]
The thread on the fractured bolt goes about 26mm into the alloy head, but the helicoil is less than 1cm. Do I need to drill down the full depth of the fractured bolt and fit the helicoil and use a full length bolt, or can I drill down less and fit a helicoil and put a shortened bolt into it. At present I have drilled the bolt out to a depth of 12mm, Just asking as the bolt thread is surely just going to engage in the length of the heliocoil and I want to minimise chance of damaging cylinder head.
Thanks,
Dave.

[stagmuffin]

I would normally drill 1 and a 1/2 times the heliciol length ,if you look at the taps they are tapered depending on the set you have and the first part probably about 4/5 mm is not effective . but if the holes are as deep as you say then 18mm would be fine too, I always use 2xd here which is 2x diameter of length of helicoil just means the bolt has plenty of purchase there.

[Davenott]

I would normally drill 1 and a 1/2 times the heliciol length ,if you look at the taps they are tapered depending on the set you have and the first part probably about 4/5 mm is not effective . but if the holes are as deep as you say then 18mm would be fine too, I always use 2xd here which is 2x diameter of length of helicoil just means the bolt has plenty of purchase there.

Thanks Steve,
I've checked my helicoil and they are only 1.5xd, so will pick up a 2xd. I had overlooked the tap only cutting properly further along its length, but 18mm will be fine.
Regards,
Dave.