Hi, I am at the point of mounting the regulator to my 3 1/2 inch Rob Roy boiler.
Martin Evans specifies 8BA stainless steel bolts, which suggests to me that he means drilling and tapping right through the bush into the boiler steam cavity.
I was planning to drill and tap only as deep as the bush itself, but that is trickier and means a few less threads to grip. Am I being too cautious, is drilling right through reasonable ?

…. well I went ahead and tapped blind holes. I used the S/S screws anyway. All held up at 120 psi test so all good.

I'd still like to know people's opinions on this though.

Thanks for the remarks Jeff. If I had thought ahead I would have arranged for another 1/16 or even 1/8 inch of depth on the bush which would have eliminated the whole concern ! There are several aspects of this design where a little more metal would have helped. Having said that, I’ve never built an engine before and having the whole build in one book was helpful so I’m not complaining. Like many things you learn your own way by doing it wrong on occasion.