NPT connectors are not recommended in the Apogee water-block for the following reasons:

Dimensionally the difference between NPT and NPSM is the lack of taper in NPSM; which means that the seal resulting from the insertion of a NPT male into a NPSM female will make contact only at the largest portion of the NPT thread*. Yes, a low pressure seal can be made - but it will have little strength for bending loads from the tubing and will not be reliable.

The threads in the Apogee water-block are ¼”NPSM, the dimensions of which can be seen HERE, and those for NPT threads HERE

A comparison of the features of tapered pipe joints and straight thread adapters, NPSM, [by the SSP Fittings Corp.] can be seen HERE

*“The basic pitch diameter for both the external and internal straight pipe threads is equal to the pitch diameter of the American National Standard Taper Pipe Thread at the gaging notch, which is the same as at the large end of the internal taper pipe thread.“ Robert James Sales, Inc.

2/ NPSM and G ¼ Compared

G ¼, or BSPP, fittings will fit, but may not seal; each must be checked prior to assuming that it will not leak just because they fit together.

Both NPSM and G ¼ (BSPP) are parallel thread and nominally the same size, the principal difference being 18 threads per inch for NPSM and 19 threads per inch for G ¼ (BSPP). Since most male end G ¼ fittings have a short thread length they can generally be engaged in the NPSM threads without much difficulty.

The joint seal is effected with an O-ring which for the NPSM barb is in a groove on the water-block top and compressed by the flange nut barb.
G ¼ fittings have the O-ring captured in a groove under the fitting nut.

G ¼ fittings will seal so long as there is a straight portion under the nut flats sufficient to bring the G ¼ fitting’s O-ring into contact with the bottom of the O-ring groove, a depth of 0.080”.