Countersink on a Tumbler screw hole.

[mjm46@bellsouth.net]

I just got a Chambers large siler flintlock for a project. The first time I disassembled the lock, when I knocked the tumbler out I noticed the screw hole was countersunk quite deeply (there was no shoulder only countersink showing in the center of the hammer spline). That didn't strike me as too strange, but when I was reassembling the thing and tightening the tumbler (hammer) screw I nearly sheered the threads off the screw. Apparently the deep countersink on the shaft shortens the effective range of the screw threads by half, leaving only about 1/8" of the screw engaged in the tumbler, instead of the usual 1/4". About half or the screw thread was damaged.

I've never had this problem before. I ordered a couple of new screws and a large headed lock bolt to make a longer hammer screw, because the threaded hole is much deeper, I think a longer screw will fix the problem. The female tumbler threads are fine because the tumbler is hardened.

My question is should I be using a clamp to press on the hammer, never had a problem before just tightening the screw to press on the hammer? Or is this a defective lock, never had one with the deep countersink that this one has?

[rich pierce]

I've never used the screw to pull the cock down onto the tumbler.  I use a wooden punch and a mallet to set it then tighten the screw. On many/most locks nowadays the cock is pressed onto the tumbler without either surface being polished at all from the cast state.  That works fine and really seems to bind it on there but takes force to re-seat it.

I had a 1720's Dutch musket lock to study once and the tumbler square was tapered considerably as was the square hole in the cock.  The cock slipped into place against the shoulder on the tumbler nicely.  Just hadn't seen that before.  I bet the "draft" was at least 8 degrees, maybe 10.

[mjm46@bellsouth.net]

Thanks Rich, I'm going to have to start doing something like that.

[D. Taylor Sapergia]

Whether it's right or wrong, I press the cock/hammer onto the tumbler square using brass covered jaws of my bench vise.  I make certain that the cock is absolutely square to the plate, then just squeeze it gently on.  Then I screw in the hammer screw, which I case harden after polishing and engraving.

[Long Ears]

I also use the Vice to press the cock back on the tumbler. That is a tiny screw and short threads to use it to press the cock on I think. The screws on most production locks I have used are very soft and require hardening after they are polished and or engraved. I harden all lock and tang bolts as well. Just my opinion. Good luck, Bob

[LRB]

  I would call Jim Chambers. 1/8" of thread seems a bit on the shy side to me, irregardless of how one acually seats the cock.

[Stophel]

That's definitely not enough thread.   

I taper all mine (which requires me to start from scratch).  Of the few old locks I have been able to take apart, all of them had tapered tumblers and square holes in the cocks.  When it's done this way, tight is tight.  They're often pressed together with a big press now, peeling off a curl of metal in the process!  And, if the square on the tumbler isn't straight, and it's bigger on the outside end than on the inside, when you press it into the cock, it will be wobbly.

I assemble mine by putting the cock on, tapping it on with a mallet, then putting on the screw.

[Pete G.]

I started using the same method as Taylor after I stripped the screw on an L&R Durs Egg. The fit on the tumbler shaft is really what holds everything together, not the thread engagement. I'll usually put a drop of linseed oil on the threads also, but not enough to cause a hydraulic lock in the blind hole.

[mattdog]

The countersink: it is there for a reason.  If you look closely at the threads on your cock screw you'll see that the threads stop .030" or .040" short of the inside face of the screw head.  That's because it is nearly impossible to run the threading tool on a typical threading machine all of the way to the screw head.  Therefore you need to compensate for that short section of shank by countersinking or counterboring the drilled and tapped hole in the tumbler.

The cock should ALWAYS be seated on the square axle by a force other than using the screw to pull it tight.  Clamp it, hammer it, vise it but don't use the screw to squeese it on.  The torque of that large head vs. the relatively small diameter of the threaded shank is asking for it to strip, or worse, to shear off in the hole.  Press the cock on, the screw is only to hold it there once it is seated.

[Stophel]

You definitely need a countersink/counterbore to clear the little unthreaded area of the screw, but I think he's got too much counterbore!

If the hole is deep enough and there's enough thread left in the hole, you might be able to come up with a screw that is longer and will reach the good threads.   

[whitebear]

STEP 1  Before you do anything or make any changes to the lock call Chambers (828-667-8361) and tell Barbie what the problem is.  I will bet that they will make it right.

[Blackpowder Barbie]

My question is should I be using a clamp to press on the hammer, never had a problem before just tightening the screw to press on the hammer? Or is this a defective lock, never had one with the deep countersink that this one has?

Wow!  Turning a mountain into a mole hill here guys. 

Yes, the hammers should always be pressed on, we make sure the fit is very tight so using the screw to draw it into the plate will result in breakage.  Mattdog is correct in his statement above - thank you!

There is no defect in this lock.  Maximum screw strength is accomplished when three threads of a screw are engaged.  You have that with this lock.  You do not have any problems.

[rich pierce]

(making a mountain out of a molehill)  That's what we do, Barbie!  Just a bunch of old guys chewing the fat around the table.   

[Blackpowder Barbie]

(making a mountain out of a molehill)  That's what we do, Barbie!  Just a bunch of old guys chewing the fat around the table.   

   It's quite ok, I live on a mole hill mountain.     If I could only train my moles to dig for gold rather than grubs.....