What is the nipple thread on a nipple for a Lymans Plains rifle and a Thompson center Hawken rifle.  Both .50 cal.

FUUUUUU

OP....you had one job!

I wasn't expecting a GD response.

6-.75 thread for the Lyman. I have had a couple of the GPR's over the years. My absolute favorite muzzleloader.
I'll let you you look up the TC spec.

Lets see if I can do this right.nipple

My first muzzle loader I built was a CVA mountain rifle kit.   I shot it for 25 years.   The lock spring broke for the third time.  There were 3 different spring for that gun. I happened to have teh rarest one.   After I could not find a new spring I decided to get a GPR.   It has been a love hate relationship..  The percussion cap does not always go off on the first try.  I have tried 3 different nipples on the gun. and it always the same thing.  One of the guys at the Muzzleloader club has had the same problem and he took a lathe to the nipple and turn his down just a little bit.   I don't have a lathe so I will take a file to the nipple.  

I picked up a used TC hawken and would like a new nipple for it to have as a spare.

As a general rule I always found what drill bit size would fit as tight as possible and still go through the hole on the end that threads into the drum. Then I drilled it out with the next bigger size. I use a little bit of never seize on the clean out screw so I can remove it after every range/hunting session. This allows me to be able to be sure the entire breech area is thoroughly clean as well as the barrel.
I have several thousand rounds through my GPRs with fewer than a handful of misfires or hang fires. I think it is due the level of care and cleaning I apply to my muzzleloaders. Also I try to use the hottest caps I can find, ie the CCI #11m. Although recommended for the replica powders, I find it works just fine with the Holy Black. Prior to the #11m, I always used the CCI #11 caps as they gave me the most reliable ignition over the years. ymmv