September 23, 2010
By Reid Coffield
As a part of converting this Smith & Wesson into a custom target revolver, Coffield timed the three rib-attachment screws. Notice how the screw slots are all perfectly aligned parallel to the axis of the bore.
Okay guys, put away your stopwatches. We're not talking about speed!
Screw timing refers to the orientation of the slot in the screw head and getting that slot positioned in a predetermined and desired direction. I'd heard and read about timing screws long before I went into gunsmithing, but it never really meant anything to me until a customer brought in a very special shotgun.
It was an English-made Westley Richards side-by-side shotgun with detachable locks. I'd never seen one before and couldn't believe anyone in the mountains of western North Carolina had one. It was an absolutely beautiful gun. One of the things I immediately noticed in working on the shotgun was that all the screw slots were aligned parallel to the axis of the barrels. The slot on every cotton-pickin' screw ran north-south, parallel to the bore of the barrels! It was amazing and a darn nice touch. I remember commenting about it to my partner and discussing how it might be done.
Since that time many years ago, I've seen lots of other guns with timed screws. Not all were super-high-dollar guns like that Westley Richards. Some were even factory guns that had been modified by the owner or by a gunsmith. It does add a nice touch, and in many cases, it's something you can do yourself. It doesn't take much in the way of tools or time. In fact, it's a perfect project for the hobby gunsmith.
Let's first take a look at screws that are not usually timed. I have never seen Allen, Phillips, Torx, or any of the other more modern screws timed. I suppose it's possible, but even if you did, it would probably not be noticeable.
With traditional slotted screws, it's a horse of a different color. Those slots, whether narrow European style or the wider standard U.S. type, are readily visible. When you have all the slots lined up in one direction, it's impressive.
Several years ago, I modified a Smith & Wesson revolver for competitive shooting. Part of the modification was building and installing a rib with Wichita sights. When I attached the rib, I used three slotted-head screws, and to make it just a bit more special, I timed all three screws so the slots were oriented with the barrel. It wasn't a big deal, but a lot of folks have looked at this revolver and commented about those three screws.
In the case of that revolver, it was pretty easy to time the screws, which were 6-48 thread with Fillister heads. A Fillister head is distinguished by the bottom of the head being at a right angle--90 degrees--to the shank of the screw. In timing these screws, I first ran my screw down into the countersunk hole in the rib and into the barrel, and I noted the location of the slot.
Now I need to point out that there's an easy way and a hard way to time a screw.
The Hard Way
The hard way is to determine mathematically how much deeper the countersunk hole needs to be to allow the screw to turn just enough to properly align the screw slot. A 6-48 screw will go in .02083 inch each time it makes one full turn.
The top screw has an angled Weaver-type head while the bottom screw has a square Fillister-style head.
If you look closely, you'll see two small marks on the end of this rib screw. The marks identify it to ensure that it goes back in the correct hole in the rib. When working on a screw head, use tape to protect the screw threads.
To determine this, just divide the number of threads into one inch. If your screw slot needs to make just a quarter-turn to properly line up, you take 1/4 of .0208, which is .0052 inch. That's a little more than the thickness of a sheet of paper. Needless to say, drilling out precisely .0052 inch will require some very sophisticated and expensive equipment.
The Easy Way
The cheaper--and more practical--method is to simply take a countersink for a 6-48 Fillister screw, place it in a holder, and by hand carefully remove just a smidgen--to use the official gunsmith measurement--from the bottom of the hole. Once that's done, try the screw and see how the slot lines up. Yep, it's "cut an' try," and it works. It won't take long to develop a feel for this.
By the way, countersinks for Fillister and tapered-head screws can be obtained from almost any gunsmiths' supply house.
Once you have a screw timed to your satisfaction, you need to identify the screw. After all, it won't fit in terms of proper timing in any other hole. On my rib screws, the bottom end of the first--or forward most--screw has a single tiny file mark in the end of the shank. The next screw has two small marks, and the third screw has none.
That's all well and good, but what if you can't deepen the hole? In that case, you work on the screw. All you need to do is take metal off the bottom of the head of the screw. Of course, this is assuming that the shank of the screw has not bottomed out in the hole and can be turned in a bit further. If it can't, you'll need to shorten the shank as well.
Under-cutting, or taking material off the under side of the screw head, can be done in a number of ways. You can put the screw in a lathe and face back the head. If you don't have a lathe, you can do the same job by hand with a needle file. You can also put the screw in a drill chuck and while spinning the screw, use a file to take down the underside of the head.
When using any of these procedures, you have to be very careful that you don't damage the threads of the screw. On a lathe, you can use an adjustable collet to hold the screw. In a drill press, you can generally wrap the shank of the screw with tape to protect the threads.
Another method of timing screws is to use screws that were specifically designed to be timed. You'll often find these screws with some custom accessories such as grip caps, buttplates, and sling-swivel studs. The
screws come with extra-high heads and very shallow screwdriver slots. When you use these screws, you coat the head with machinist layout fluid and just install them as normal. Once the screw is fully seated, you note how the slot should be located and scribe a corresponding line on the top of the head. You also scribe a line on the side of the head to indicate how high it needs to be to be flush. The screw is then removed, and the new slot is cut on the scribed line. The head is also cut down to the appropriate height.
Reid uses a variety of the piloted countersinks to time screws. While you can use a countersink in a drill press or a milling machine, Reid often just uses a hand chuck such as the one below.
A regular hacksaw blade with offset teeth makes cutting easier, but it also makes the slot too large for most screws. The offset needs to be removed for cutting screw-head slots. A screw-slot file only cuts on the edge of the file. The flats (pointer) have no teeth to prevent widening the screw slot as it's being cut to depth.
A variation on this is to just weld up the head of an existing screw and then reshape the head and cut a new slot. When welding a small part like a screw, I've found that it's best to use TIG, though a skilled welder can do it with an oxy-acetylene outfit. I've done it with oxy-acetylene, but believe me, I've also burned up a lot of screws!
When cutting a new slot, I often use a hacksaw with a modified blade. A regular hacksaw blade has teeth that are offset to either side. This helps the cutting process by making the slot of the cut a bit wider than the blade so the blade will not bind. If you use a regular blade that's not been modified, the resulting slot will tend to be a bit too wide.
There are a couple of ways around this. I simply take a standard blade and grind back both sides of the teeth. This results in a narrower slot more suitable for screw heads. Also, since I rarely cut very deeply, the lack of clearance on the sides of the blade is not an issue. At one time, Starrett--the tool company--offered screw-slotting blades in varying widths, but I haven't seen these on the market for many years.
It's also possible to cut the slot with a file, but as you can imagine, it takes a darn narrow file. The only sources I've seen for files such as these are Brownells and MidwayUSA. Both of these gunsmiths' supply houses can provide these files in a variety of widths.
There are a couple of tricks to centering the slot when you cut it. First, secure the screw in a padded vise. You must hold it firmly, yet you don't want to crush the screw or damage the threads. Coat the head with machinists' layout fluid and make a very light initial pass with the file or the hacksaw. This initial pass will show you the location of the slot. If you are too far over to one side, you can still easily make a correction.
That's about it. Timing a screw isn't all that hard, yet it does add a nice custom touch many folks will certainly notice.
Until next time, good luck and good gunsmithing!