I have removed my head to change to a new gasket. This means I don't have to have extended length tap(s)
I had all ready gotten an extended bottom tap - however because of being an extended shank model- there were few choices
to pick from.
Taps have very slightly different diameters which cause the tapped hole to have a looser or tighter fit when you install the bolt or stud. The degree of oversize-ness is the "h-limit" of the tap. the numbers go something like 1 to 10- 1 making the tightest thread- and 10 loose fit. When I bought the extended bottom tap I believe it was an H-6. there were no extended ones I could find with a lower limit.
I have tried a couple of holes with this tap. I can feel a slight drag as I am going down the hole. Tapped a couple to bottom OK. Tried stud in these holes and noticed that when the stud is all the way in - or close- I can wiggle the top of the stud
just a little bit (like 1 mm) laterally. This - together with the slight drag going down tells me that this tap is a little bigger
than I would like. Seeing as how we want to maximize the holding power of the studs- I think you would want the maximum
overlapping surface area between the stud threads and the block threads- that is a close fit- as close as I can get to having a
slight drag.
It appeared that there was a much greater variety of taps in the regular shank length- so I am going to try to find more regular
metric taps - but ones with a small h-limit so I can get a snugger hole.
Maybe you think I am being picky- but I think the stud fit could be pretty important to the ultimate strength.
AND- regular taps are pretty cheap and available !
Tim