Drain bolt size?

[Brad520H 17]

Looking for direction on size/thread of drain plug. 1991 520H. Got the tractor last year and previous owner really crossthreaded the trans drain plug. Had to weld on to it just to get it out. So now the threads on the plug are unusable to even get a thread size on, and going to have to retap the hole as well. Any sizing info/tips much appreciated

[Pullstart 59,747]

I think it’s a 1/4” npt pipe plug.  It MAY be 3/8” npt but I am pretty for sure it’s 1/4”.  

[953 nut 51,782]

Like Kevin said, 1/4" NPT

[Brad520H 17]

17 hours ago, Pullstart said:

I think it’s a 1/4” npt pipe plug.  It MAY be 3/8” npt but I am pretty for sure it’s 1/4”.  

Would that be 1/4” -18? Not sure on the tpi

[oldlineman 1,450]

It would be pipe thread which is tapered.

[Pullstart 59,747]

14 minutes ago, Brad520H said:

Would that be 1/4” -18? Not sure on the tpi

Make sure to get 1/4” NPT.  The size of the plug is dictated by the size of pipe inside diameter.  It’s more like 1/2” or so outside diameter.

[lynnmor 6,796]

1 hour ago, Brad520H said:

Would that be 1/4” -18? Not sure on the tpi

Since it is a 1/4 pipe thread, the TPI is always the same, 18.

Edited February 16, 2023 by lynnmor

[Horse Newbie 6,876]

2 hours ago, lynnmor said:

Since it is a 1/4 pipe thread, the TPI is always the same, 18.

I never knew that… I always just went with pipe size and I knew it had to be NPT…

[Brad520H 17]

3 hours ago, lynnmor said:

Since it is a 1/4 pipe thread, the TPI is always the same, 18.

Interesting I learn something every day. Thank you!