Caplock Passfire Hole Size?

[Ballistics in Scotland]

I positively do not like the idea of a pinned breechblock. It would be much better if you could find someone to thread the barrel for you on a large lathe... and I don't mean a little better, but much.

In my view if the barrel can't be threaded this gun can't be built. Buf you feel you must go the pinned breechblock route, look for a taper pin reamer and taper pins. Like nearly everything else you will find them on eBay, and dimensions under "Machine tapers" on Wikipedia. I would also epoxy the block to barrel joint - not to add strength, because I don't think it would, after repeated shocks, expansions and contractions. But it would keep fouling out of the joint.

[huntingsgr8]

Well, if it makes any difference I hate the idea too, but I can't thread it. I seriously doubt a 1/2 inch or 5/8 pin would shear, along with a pressed fit. I've even debated drillng in from the rear and putting a couple of bolts into the side walls at the base of the plug.

[Col4570]

You could get someone to make you a Tapping Guide to fit over the Barrel,with a reduced section for the Tap Shanks.It could be Bolted on in a variety of ways or eve pulled on with two long studded rods from the other end of the barrel.You should then be able to Tapp an inline with the bore thread.
The Guide could be Fixed as follows,Two plates drilled to take long studs,One of the plates with a Hole at the centre to take the Guide.This would work and dispense with the need for a Large lathe headstock.

[Ballistics in Scotland]

Well, if it makes any difference I hate the idea too, but I can't thread it. I seriously doubt a 1/2 inch or 5/8 pin would shear, along with a pressed fit. I've even debated drillng in from the rear and putting a couple of bolts into the side walls at the base of the plug.

I don't think it would shear either, but a ½in. or ⅝in. pin requires two ½in. or ⅝in. holes, and that is a lot of metal to remove from the longitudinal strength of your barrel.

What sort of tap are you using? It takes a taper tap to start a hole relatively easily (i.e. just fairly difficult) in this size, but a bottoming tap to cut to the bottom of the hole. I'd consider grinding off the chamfer altogether when the hole was that close to all-threaded. But this is going to be very expensive in such a large size.

Another sort of tapping guide worth considering would be a hole drilled into the front of the tap with carbide drills, to take an epoxied in steel rod which would extend through brass bushes in the bore. You would be as well to have two slightly different sizes of carbide drill for this, since if you try to drill deeply with one, the chances are that the tap and its hole would expand with the heat, and then contract to grip the drill once the source of heat (the actual cutting site) was further along. Carbide drills shatter very easily.

[huntingsgr8]

One of my biggest issue is that it's a plug tap, and it's all I've got. as far as weakening it, I don't think it will, I'll be moving the cap location farther forward, thus allowing me some room for the plug.

[huntingsgr8]

I made the plug .003 thou over sized and pressed it in, but it stopped 1/8 shot of being in all the way. Any suggestions on how to finish putting it in? I do not have a hydraulic press.

[huntingsgr8]

I got it in.