Tap size for 1/4" NPT bolt?

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?

It's a pipe tap. The threads are tapered.

Where would I get that type of tap?

snapon .com or a snap-on dealer would have one also maybe home depot or lowes in plumbing.

You need a 1/4 NPT tap. Do you have a NAPA auto parts store around ya! They should be able to get you one.

orchard supply will have them too.

Also Sears or www.mcmaster.com

got one at ace hardware last week

gonna check around thanks

1/4" NPT tap needs a 7/16 drill

sounds like you need just the tap...