doublegunshop.com - home Forums General Discussion DoubleGun BBS @ doublegunshop.com Action pin clearances ...

'Just for background, I'm setting myself a project to take a rattly coarse action, and then render it smooth and silky by adopting the best fits, smoothest mating surfaces, spring strengths and correct hardening.'


I didn't think you could afford a Purdey?

Salopian I'm not an engineer , I'm gun maker, We do things by experience and feel .I know that with modern wholey machine made guns, no matter what the makers claim about skilled hand fitting and finishing, it is a different matter. If you had worked in the trade as I did you would under stand this. For example a tumbler would be drilled under size and the reamed out by the fitter to suit .It is difficult to put across in few words to some one from a different disceplin ,but lets face it we have been doing this for a long time and the fact that people are still shooting with 100year old guns, says we can not have been that wrong.

gunman Thanks for the response, but I'm a bit confused by

The pin must sit in the action body and have some relation to it, fixed or moving. How is that achieved, by measurement or trial and error? The same applies to the tumbler hole, it's either got a known dimension relative to the pin size or it's just wiggled and reamed until it feels right. If the tumbler was drilled undersize to start with, without measuring the pin how do you know what the undersize is, and how do you pick the right reamer afterwards?

So just what do you do to get a pin and hole "to suit"? Measure and cut to known dimensions, or wiggle and ream till it feels right?

Regards

Eug

Just a little for those that aren't familiar with machine tools, reamers are available in oversize increments for pin fitting (not necessarily a gun term pin). It's not uncommon to see .0005" oversize increments for a .001-.002" depending on maker. Also, the opposite, smaller, is common.

Rotating parts in a sxs gun are commonly casehardened. I would go along with Miller's suggestion that selecting a .0005" oversize reamer will provide sufficient clearance. However, after casehardening there will likely be some shrinkage of the hole. You don't run a reamer into a casehardened part if you want the reamer to stay sharp. A brass lap with lapping compound would clean up such a hole and leave it free of the relatively loose surface oxides that may be abrasive.

I'm no gunsmith. I spent a few years in a precision machine shop making optical equipment. If a hole was reamed to the same diameter as a hardened dowel pin, the pin pretty much could not be made to go into the hole without some trick, like chilling the dowel pin to shrink it.

If a hole was reamed .0005" over, a hardened dowel pin would press fit in part ways with a lot of force and was considered permanent, as in it could not be removed without damaging the pin or the hole. Getting perfect alignment between the axis of the hole and the axis of the pin was critical to fitting the pin into the hole.

If the hole was reamed .001" over, the hardened dowel pin could be press fit in relatively easily (tapped in with a brass hammer, etc.), though still a tight fit. With effort, such a pin could be tapped out if the hole was not blind.

For the pin to slide smoothly into the hole with little to no force, but with no discernable wiggle, the hole had to be reamed .002" over.

All of this depended on the quality of the dowel pin, i.e. the tolerances of its diameter, roundness, etc., and how carefully the hole was made. Plus, the bigger the pin and/or the deeper the pin was to go, the more room it needed, up to a certain point. This is just hands-on experience with holes 1/16" to 1/2" diameter, and 1/8" to 1" deep.

--shinbone

Use long reamers with very little taper ,the ones I have are specialy made for the job,pentagonal like a watch makers omly bigger . As to how much will depend on experience . You also hve to bear in mind that in time past when hardening wsa done in a hearth or brazier ,scaling can effect the hole size .

PS if you want go to any library and you will find tables of fits and clearences ,there will be an American standard or the British equivolent. In the long run as long as the hole and axle are both round and true, providing the part rotates freelyis all thats needed.

'PS if you want go to any library and you will find tables of fits and clearences ,there will be an American standard or the British equivolent.'

Or look in Machinery Handbook or Kempes.
Obviously you could also refer to the engineering drawings (blueprints) if available.
Eugene,
Once you have received all the knowledge, you can borrow the reamers off me, just let me know what size you require.

Pierre,

See you and The Bride for din dins before long; have I got news for youse!

A clue .. it's as old as I am, done a lot less work, weighs about five cwt. and has MYFORD written on the side.

Eug

I worked in a machine shop for 35 yrs. In all those years I never had to drive in a dowel pin when the hole was reamed with an .0005 oversize reamer. Most dowels run to about a +.0002 for nominal size. Reaming one hole with the nominal size & the other with the +.0005 reamer establishes which side the dowel stays in & which side it "Slips" in & out of.The dowel could be tapped into the one remed with the standard & would slip freely in the one reamed +.0005. If you had to drive a dowel in a hole reamed +.0005" you must have been using a worn out reamer. You can about throw it through a hole reamed to +.001"