I have a drain plug on my exhaust manifold and it uses a standard 1/4" NPT thread on it and small 1/4" bolt that you can buy in any hardware store.
Problem is the hole is all corroded from rust over the years and I need to clean it out.
For the life of me I can't figure out what size tap to use to clean it out. I have tried 7/16 but its a little too small, and the 1/2" taps I have used are 13 and 20 and the 20 seems to work the best but it wont thread on all the way through on a test pipe I have, it only goes about 3/4" in.
Anyone know the correct thread pitch for this?