The oil galleries in all Dart blocks are threaded with NPSF (National Pipe Straight Fuel) threads. Most OEM blocks use this same thread for pipe plugs. It’s important to know that the NPSF standard is a straight thread, not tapered.
NPSF threads (defined as American Fuel Internal Straight Pipe Thread) form a seal by the interference of the root and crest of the mating threads. These threads are intended for use in soft or ductile materials with NPTF external threads.
The pipe plugs used in Dart and factory blocks have tapered NPT threads (American Standard Pipe Taper Thread). When installing an NPT plug in a threaded hole, the diameter of the plug determines how deeply it goes into the hole. This dimension can vary signi fi cantly among different plug manufacturers. If the plug is too tall (sticks out of the hole), try using a plug from a different manufacturer. If that does not solve the problem, the block can be tapped carefully using a conventional NPT tap to allow the plug to screw in farther. It is very important to use caution when doing this. If the plug screws into the block too far, it could block or restrict internal passages. Never overtighten or force a tapered plug into a threaded hole – you could strip the plug’s socket hole or cause a crack in the block.
If an NPT plug goes into a threaded hole too deep, do not use an NPT tap on the threads. This will simply make the hole larger and the plug will go in deeper.
The following are common tap sizes for threaded holes:
1/8-27 NPSF = 1/8-27 NPT Std.
1/4-18 NPSF = 1/4-18 NPT Std.
3/8-18 NPSF. = 3/8-18 NPT Std.
1/2-14 NPSF = 1/2-14 NPT Std.