The Briggs normally uses NPT and BSPT will not seal properly. Now of course they might be using BSPT since the engine may have been made in the China and the BSPT is popular China which would explain the thread sealant I see on the plugs.
BSPT (British Standard Pipe Thread) is similar to NPT except for one important difference. The angle across the flanks of threads (if you sliced the fitting in half long-ways and measured the angle from root to crest to root) is 55 degrees instead of 60 degrees as it is for NPT. Thus an NPT male will fit into a BSPT fitting or vice versa but they will not seal. This is a popular fitting in China and Japan but is very rarely used in North America unless the equipment to which it is attached was imported. Thread sealant is needed to seal the male and female fitting together.
