Re bottom taps. What I've done in a couple of cases was to tap the hole with the standard bottom tap and than grind the end off the tap until it will cit a full thread right to the end of the hole. I might have been fooling myself but it did allow an extra half a turn of threads in the hole :-)
Pretty interesting stuff there.
Here's a tip - doesn't work for the job at hand, but it comes up often enou gh in motorcycle repair - You can fix a stripped m6x1.0 hole by substitutin g a 1/4-20 bolt for the original. Just enough larger, and just coarse enoug h to screw right in with no drilling or tapping.
Back in the day, I did hundreds of those, mostly for the valve covers of Ho nda CB450s. Each cover had 8 screws and it was a pretty rare thing to find a head without at least a few of them stripped. This was an easy, cheap fix .
I'm sure there are other similar combinations of fasteners that can be subs tituted for each other, but this is the one that I know about from actual e xperience.
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