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Reaming Cylinder Throats
MikeG on 2013-11-08

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Cylinder Throat Reaming on the Cheap....

 by

MikeG


It is a well-known problem with many modern .45 Colt revolvers that the cylinder throat dimensions are all over the map.  Not only confined to any particular brand, I've seen a Colt SAA with .458" cylinder throats!  My own guns have run from 0.456" (a "Liberty" model .45 Colt / .45 ACP) to an early 1990s production "Bisley" which ran only 0.449" or so.  It would literally not chamber some ammunition!  Although my "Liberty" Blackhawk shoots surprisingly well with 0.451" jacketed and 0.452" hard cast, the undersized throats are a recipe for problems, especially with cast bullets.  Lately the trend seems to be toward undersized throats.  Ideally, cylinder throats will run 0.001" or so above groove diameter, and most modern .45 Colt guns will have barrel groove diameters of 0.451" - 0.452."  So we'd like the cylinder throats to be at least 0.452," possibly 0.453" depending on the gun and bullet used.

Rounding out the sample, in the 'middle,' dimension-wise, was an Accusport Bisley (stainless) and a Vaquero,both with about 0.450" throats.  The Vaquero in particular seem to have some variance between the individual chamber throats.

With every problem comes opportunity, and there is now a market for specialty tools such as Brownell's kit for reaming .45 Colt cylinder throats.  The reamer is offered in a useful size of 0.4525" and comes with onepilot (important!) and extra pilots are available.  What could be wrong with this?  Well - it's expensive!  At $70 for the reamer, and an additional $44 for extra pilots, the costs add up fast.  Unless you have several guns to do, or can share the cost with a friend, it may make more sense to pay a gunsmith to do this simple task, right?



Or maybe not!  With a little searching, some basic tools, and small amount of work, this task can be accomplished MUCH more cheaply.  Note that simple, 'non-gunsmith' reamers can be had MUCH cheaper.  In the top center of the picture are two plain reamers, one is a 'decimal' reamer, sized 0.452," which should certainly cost less than $20 from any industrial supply house.  On-line sources include Enco, Travers (my source), Wholesale Tool, McMaster-Carr, and no doubt plenty of others.  The other reamer is a 'fractional' reamer, by happy accident 29/64" equals 0.453125" or 0.453" for our purposes, and it's even cheaper than a decimal reamer!  Occasionally, one might find a really good deal on an adjustable reamer, pictured just above the micrometer.  This one isn't even marked in inches, it is metric, from 10.75mm to 11.75mm (0.423" to 0.463"), and this can be set to any size desired in that range.

The only problem with a plain reamer is the lack of a pilot; there needs to be some sort of guide to ensure that the reamer stays concentric with the chamber.  The Brownells reamer uses a pilot that leads the reamer into the hole; this actually can still let the reamer cut off-center if not used with care.  To solve this problem, I used a tubing cutter, some old cases, and a bit of aluminum muffler tape.  The top reamer appears to have both a brass and silver band around it.  This is about half of a .454 Casull case (range scrap!) that's been cut in two with the tubing cutter.  The tubing cutter will leave a bit of a burr in the case, but that's OK; just use another case to carefully push the piece onto the reamer.  It needs to go up the reamer far enough that the reamer comes completely through the front of the cylinder, as shown it was about right.  The burr on the case prevents it from sliding around on the reamer.  Don't use a hacksaw to cut the case; it will end up out of round, and with burrs on the outside, where you don't want them.



Since there was still a bit of clearance between the case and the chamber, a strip of muffler tape was wrappedaround the case.  Muffler tape is very thin and this batch measured about 0.002" which was just right.  If there was less clearance it is probably not needed.

Clean out each chamber and leave a light film of any kind of gun oil.  The reamer should get a drop oftapping oil on each flute.  Put the cylinder in a vise; I've deliberately chosen a very small vise to show how little effort this takes! We are only removing a few thousands of an inch at most, no ham-handed efforts needed!  All that protects the cylinder in this case are two shims made out of thick paper, so you can see that very little clamping pressure was needed.  Since the reamer shank is round, some tool is needed to turn it; ideally a couple of flats should have been ground on it so that a tap wrench could be used.  I'll admit to just being lazy and not walking to the grinder in the garage..... there was a small pair of vise-grips handy which I attached for a handle!  



Again, for emphasis, the picture shows me turning the vise-grips with literally one finger; no more pressure is wanted or needed!  Any more could cause the reamer to cut off-center.  The other hand is used to steady the reamer just above the cylinder, not show because I'd need a third hand to run the camera.  The weight of the reamer will let it bite and start cutting, just keep turning gently, and soon it'll poke through.  In this case, the reamer would extend through the cylinder by about a quarter inch before the brass guide sleeve kept it from going any farther.

Remove the reamer by continuing to turn it in the same direction you did while cutting; do not ever turn a reamer backwards as it will trap chips under the edge of the blades and can damage the blade or scratch the item you are working on.  Most reamers are 'right hand' and turn clockwise, when viewed from the back.

That's it.... clean the chips off with every hole, and put another drop of cutting fluid on the reamer, and go to the next one.  The only thing that happened to slow this down at all was the muffler tape coming off about halfway through the second cylinder.  Another strip was cut and soon it was back in action.  It took less time to ream three cylinders than to type up this tech note, by the way.

My plan is to shoot all three guns (a blued Bisley, a Vaquero, and a stainless Accusport Bisley) with the throats at 0.452," then ream them to 0.453" and see if there is any change.  By the way, if these guns are to be fire-lapped (which Rugers often need to shoot plain-based cast bullets without severe leading), the reaming MUST be done before fire-lapping.

To verify the dimensions, a couple of soft 0.457" roundballs were pushed through (shown to the right of the cylinders in the first picture) and mic'd right at 0.452," as desired.  Also shown is a simple expanding hole gage; not needed for this but I had one so also check the hole size with it.

The main cost for most hobbyists would be the reamers, and they are the most expensive part by far.  The tapping fluid was around $6 (you'd need that anyway with the Brownells kit), the tubing cutter is less than $10, scrap brass is free, muffler tape was a couple of dollars..... if you don't have a pair of vise-grips, you'll surely need them for something else!  Even the micrometer is not expensive; from the same industrial supply houses, a micrometer that reads to 0.0001" should be well under $20.  The hole gage and adjustable reamer are not needed, but could be used.

For quite a bit less than the cost of the Brownells kit, and a few minutes of time, a vexing problem with these fine
guns has been solved!

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