I’m puzzled as to why air doesn’t leak out through the drone switch rod from the inside when the drones are on. Isn’t it just a straight brass rod running through a narrow tunnel which needs to be sufficiently wide enough to let the switch operate freely, but why doesn’t it leak air?
DavidG
The inside of the “narrow tunnel” is lined with cork sheet for about an inch at the bottom to prevent leaks.
djm
I’d always suspescted something like that, and going by how difficult it can be to push and pull the switch on some sets I assumed that was all there was to it, but when performing a gig once sitting during a song waiting for my turn to come it, I was idly fiddling with the switch flicking it in and out and it struck me just how easy mine is to manouvre in and out and wondered if there isn’t any resistance to its movement, surely there must be air leaking out too. Mine’s the type that has the switch connected to a hinge at the bottom and the ‘in-out rod’ connected to the switch with a brass pin.
DavidG
Depending on the age of your set and the amount of use it gets, the cork may very well get worn. It shouldn’t be any harder to detect a leak here as anywhere else on the set. It might possibly be time to look into getting the cork replaced. I would suggest you get input from an experienced pipemaker before trying to do this yourself.
djm
No, I don’t have a leak. And it’s always been this loose since the day it got it. I’m just wondering how it can be that there be no leak. I’ll just have to take your word for it that there is a very efficient piece of cork inside there. 
DavidG
Another possibility is the presence of some other blocking mechanism. A leather or rubber pad may be provided to block air egress when the drone switch is “fully engaged”.
A very slight leak is likely to go undetected when the drones are on, of course.
In the Kenna hollow mainstock reported on in the SRS Journal volume 2, the switch bar was tapered so as to form a tight seal against the inner surface when the switch was engaged, which is a neat trick. When the switch is open, a small spring on the side of the bar keeps it from rattling around, and the valve at the far end prevents air from entering the tapered hole bored for the switch bar.
Bill
Michael Dow has a new article on his website that explain’s how he made his drone stock switch airtight… http://www.archcarving.com/Bagpipes%20&%20Bellows/bag_pipes.htm#Projects Scroll down to Common Stock.
Michael’s brass-in-brass idea (the brass rod slides in a brass tube) makes sense, but it carries some danger if the pipes go unplayed for a significant time (heaven forbid). For that reason, when I use a similar rod-inside-tube arrangement, I use aluminum for the outside tube. This is because even when corroded, brass and aluminum don’t bond very well to each other, thus the arrangement is unlikely to ever ‘stick’. There is also a natural “lubrication” effect that happens whenever aluminum and brass are in contact with each other (I recall from engineering school)… Thus they make a nice bearing surface whose only downside is possible wear. For our purposes I don’t think that’s a big problem, though for an axle or slide that was in continuous use for thousands of hours would probably give trouble.
[Edited to add]
John Mulhern has rightly noted that aluminum is quite anodic to brass, thus galvanic corrosion is to be expected in this setup. That’s true, although in most applications aluminum’s corrosion is somewhat self-limiting, because the aluminum oxide coating is electrically insulating and also protective. In a friction bearing situation like the drone slide, time will tell how well this really works out; it could be that decades in a moist climate like Ireland will cause the aluminum sleeve to rot away and require a re-sleeving, or at least introduce leaks, The trouble with just using metai in a wooden sleeve is of course that you get either leaks at the outset, a sticking value when the wood shrinks, or both depending on the seasons. All the more reason to put some sort of
semi-airtight “stop” on the drone switch so that air can’t readily leak around the switch shaft when the drones are fully ‘on’. The Kenna tapered shaft solution is really the most elegant, but as is so often the case, the “best solution” is a lot more work than the easier ones.
regards
Bill