Where to drill the touch hole?

[skoda]

Hello,

I am working on getting a new (Rice) barrel into my Thompson Center rifle. It was a percussion but I'm a flintlock guy and so am converting it to flintlock. I have the barrel fitted now and need to drill the touch hole. I was wondering where is the best place to center the hole. Should I put it so that the bottom of the hole is tangent to the pan bottom? Center the hole somewhere in the middle of the pan? Or center the hole with the top of the pan? Or some other place? I know that we are talking about an 1/8th of an inch difference but if I am going to do it I might as well do it in the best place.

Also, I plan to use a T/C vent liner. Does anyone know the thread size and the chamfer angle and depth?

I appreciate any words of wisdom and experience.

 

[Pete G]

Center @ top of pan (sunset position).

 

[tenngun]

Too low means slower ignition. You don’t want a fuse but the heat of the flash setting off the charge.
You don’t want it too high or not enough heat gets in.
Draw a line across the top of the pan, half the hole above and below that line. When you prime keep powder just to the bottom of the hole.

 

[SmokepoleSam]

The threads are 1/4-28 but I don't know the chamfer angle here is a picture:

 

[skoda]

So like this?

 

[Larry (Omaha)]

Hello,

I am working on getting a new (Rice) barrel into my Thompson Center rifle. It was a percussion but I'm a flintlock guy and so am converting it to flintlock. I have the barrel fitted now and need to drill the touch hole. I was wondering where is the best place to center the hole. Should I put it so that the bottom of the hole is tangent to the pan bottom? Center the hole somewhere in the middle of the pan? Or center the hole with the top of the pan? Or some other place? I know that we are talking about an 1/8th of an inch difference but if I am going to do it I might as well do it in the best place.

Also, I plan to use a T/C vent liner. Does anyone know the thread size and the chamfer angle and depth?

I appreciate any words of wisdom and experience.

Make sure you know exactly where the end of the breech plug is. You do not want to intersect the breech plug!!! Others will have their opinions, but I like my TH liner threads to start about .015 away from the breech plug. Center of pan for left right is perfect, provided your breech plug cooperates.
Larry

 

[Sidney Smith]

Inlet barrel, then mark where breech plug is on outside of barrel. That is easily done by inserting a wooden dowel until it touches the breech plug, then Make a mark on the dowel at the muzzle. Then hold the dowel on the top flat of the barrel and align the mark on the barrel top flat at the muzzle and mark where the breech plug is on the outside of the barrel with a thin sharpie at the end of the dowel. Then inlet the lock so the back curve of the pan is either on the breech line or just behind it. Remember when you drill the hole for your breech plug, that about 1/10 of an inch of the liner is going to be near the breech plug. Thats not an issue as long as the touch hole is in the middle of the pan as in the illustration.

If you drill a little too far back and your touchhole is touching the edge of the breech plug, this isnt a problem. You can remove the breech plug and file a slight V notch in it to allow powder to access the flash channel. You dont want to be too far back where the flash hole is well behind the face of the plug. And you dont want to be too far forward either or light loads may not cover up your flash hole and your patching will interfere with and block the flash hole.

When I did my flash hole for my .32 caliber , I thought I messed up as the flash hole wound up measring almost .200 from the breech plug face. But since in a .32 the powder charge is long and thin it works fine. Its when you have a large bore diameter that a light load could result with the above issue if the hole is too far forward.

 

[Sidney Smith]

The threads are 1/4-28 but I don't know the chamfer angle here is a picture:View attachment 79534

I dont think chamfer angle matters much.. Just tap 1/4 -28 threads, tighten plug and file flat with barrel. Thats all I ever do.

Just get the right tap drill size so you dont drill too large a hole and have threads that are too shallow. Drill number for flash hole liner escapes me at the moment. Im going to say its a no.10 but dont quote me.

 

[Woody Morgan]

Use a #3 bit (.2130") for 1/4-28 threads.

wm

 

[skoda]

I have a quarter inch from the front of the breech plug to the middle of the pan. It's a 50 caliber barrel and doing a little bit of math a 70 grain FFg powder charge will fill about 1.5 inches of bore so I am plenty good there.

Thanks for all of your help. This forum is great, really, truly great, and it's because of the awesome people that come here and contribute.

 

[mustanger]

I would advise checking the threads supplied on the touch hole liner. Not all of them are 1/4 28.. The Jim Chambers white lightnin is 32 tpi and the diameter can be 1/4,5/16, or 3/8.

 

[tenngun]

Use a #3 bit (.2130") for 1/4-28 threads.

wm

Track sells it as a set, tap and pilot hole drill bit

 

[mustanger]

I like to use a reamer in mine. Drills don't always make round holes.

 

[Musketeer]

So like this?

View attachment 79543

That looks perfect to me.

 

[skoda]

My liner is a Thompson Center that I kept from a rifle that I parted out. I'll check it any way just to be sure.

I have enough tooling to get the job done. I have a small milling machine to hold it. It's a nice barrel going into a really nice rifle so I don't want to mess things up.

 

[M. De Land]

I like to use a reamer in mine. Drills don't always make round holes.

The tap itself will ream the correct sized, drilled hole, round.

 

[mustanger]

You may not notice it in a hole that is mildly oval, but the threads will not be the same depth and the hole will not be rounded by the tap. If you have a hole that has a lot of oval in it's dimension, then it may not even be possible to have threads of sufficient strength in parts of the hole. I must say that the concept of a tap reaming a hole round is mistaken. I have years of machine experience and I can assure it this does not happen. Ream a hole before you tap it if you want the best threads.

 

[M. De Land]

You may not notice it in a hole that is mildly oval, but the threads will not be the same depth and the hole will not be rounded by the tap. If you have a hole that has a lot of oval in it's dimension, then it may not even be possible to have threads of sufficient strength in parts of the hole. I must say that the concept of a tap reaming a hole round is mistaken. I have years of machine experience and I can assure it this does not happen. Ream a hole before you tap it if you want the best threads.

The major thread diameter will be cut round in a drilled hole and the tap will find center of a drilled hole when used in a jig or tap guide. The minor thread diameter will be out the same small amount as was the drilled hole but is inconsequential for any practical use. Drilled holes made through jigs and drill guides are only very minor-ly oval and perfectly adequate to tap without reaming in my opinion. One needs reamed holes when chambers, bores and dies are made and the reamer should be in a floating holder for best accuracy.
You will find that even reamers will cut oval holes if not floating or coaxial with the hole they are working in. We see this in revolver chambers all the time.

 

[Sidney Smith]

I think some are making a mountain out of a mole hill. If youve got relatively decent drill bits and a drill press, youre not going to have issues. Ive always used dewalt non wandering drill bits in my Craftsman drill press and have not had any problems with out of round holes. And even if they were it would be to the order of maybe a couple thousandths of an inch. Not enough to be an issue unless you drill the liner hole too big in the first place.

 

[M. De Land]

I think some are making a mountain out of a mole hill. If youve got relatively decent drill bits and a drill press, youre not going to have issues. Ive always used dewalt non wandering drill bits in my Craftsman drill press and have not had any problems with out of round holes. And even if they were it would be to the order of maybe a couple thousandths of an inch. Not enough to be an issue unless you drill the liner hole too big in the first place.

Amen