I know of no acme thread w/o plugs that were tapered. Regardless, as far as my knowledge goes, during the later No. American
steam era, the fine threads were all cut from the center line. Boiler studs, pipe, w/o plugs, taper crown stays, boiler fittings, etc.
There were some applications of boiler studs using interference fit thread and use angle of spindle taps but that is not part of this discussion.
My preference safety wise, would be to convert all w/o fine threads to square or acme sleeve/plugs. However, opening up holes for threaded
or welded sleeve application in the shell requires review due to maximum unsupported opening diameter.
One problem to consider is the manufacture of square threads taps to produce new or for clean up of sleeves in use.
True square thread taps are quite expensive and most square threads applications are made with a 10 degree included angle profile.
Additionally, sleeve reseating capability is tied into thread clean up in tap design. This keeps everything in line.
It generally is wise to go with existing designs & materials as engineering for use and liability concerns will have been
addressed. It never hurts to check with proper authorities for any changes that may have surfaced. Keep it simple.
Chris A.