Ramrod tools come in two different thread sizes, both of them being standard American sizes.
They are the #8-32 and the #10-32.
The #10-32, because of its larger size (.190 diameter threads) is more popular for muzzleloaders.
Although a #8-32 will screw into a #10-32 threaded hole, do not be tempted to use one this way. There is so little thread engagement that the smaller #8-32 tool can easily pull out of the ramrods hole.
While I'm talking about ramrods, make sure the tip on yours has a cross drilled pin going thru the wood.
Many factories supply their ramrods with the tip just glued on and if this glue joint fails (as they usually do) it can leave the whole tip down somewhere in the bore of your gun.