NPT will work but it's usually cheap **** and totally dependent on the sealant. NPTF Dryseal from an industrial hydraulics/pneumatics or tube fitting line should be high quality. McMaster is probably easiest source.
Also NPT for a VW engine? Are these extra holes you're drilling & tapping yourself?
Bill - I will check out the ones at Pioneer.
Bob - yes but they were mostly anodized aluminum.
Bretny - thank you. I have a couple picked out there as potential candidates. Not sure I will use stainless. Coated steel is the recommended plug in an aluminum case like this. Aluminum or stainless work perfectly well in iron blocks.
Matt, the factory plugs are pressed in. They ones in the type IV engines (late North American buses, Porsche 912-E, VW 411-412, Porsche 914-4, and industrial engines for military applications) are notorious for popping out in cold weather, puking the oil, and destroying the engine. Because of the expense these days, and rarity of good parts for these 40 - 50 year old VW buses, many of us pop the plugs out, drill and tap when building a new engine. I'd guess that maybe 5 engines a year of this kind get built in the USA, and it is always a challenge to find things with the changing environment. What was available 6 months ago has changed. There are a few companies left that specialize in these engines but their quality is notoriously bad as they use the cheapest parts, solutions, and labor available.
The T4 engine case is aluminum. Some companies build aircraft engines from them. Coated steel is the recommended plug in aluminum. I've read stainless will work too although it has greater potential of corrosion than coated steel. Brass is not recommended due to corrosion potential. Aluminum on aluminum has the same expansion coefficient and seals exceptionally well, but I am told by those who have been there that once they are in, the next time the engine is rebuilt it is impossible to remove them without stripping them in order to clean the oil galleries because aluminum on aluminum galls. I may make a trip over to Hoses and Fittings etc to see what they have in Parker hydraulic fittings.
Send me your e-mail in a PM and I will send you a picture in the AM.