Thanks Mike, I have been going slow, with breaks and cutting oil. The cobalt drill is keeping it's edge, I'm resolved to it being a slow process.

Was thinking of jig too (plenty of thought time when drilling). Was imagining a hardened guide bushing with arm that attaches to other exh stud.

alistair

On 2012-07-04, at 8:36 AM, Mike Riley wrote:

> if your bit has stopped removing material you have work hardened the bolt.
> use a new bit, plenty of oil, SLOW Speed,lots of pressure. only 10seconds or so and stop and clean.
> use a small pilot bit at first.
> Any interest in a drilling jig to hold the drill in position so  they can be drillled while in the car?
> I am trying to judge interest to see if it is worth manufactureing them.
> mike