Cylinder throat adjustment... via lapping

[NikA]

On my list for the past few months has been adjustment of cylinder throats on a new Convertible RH, as well as firelapping of the bore constriction. I managed to get to the range to do some firelapping (just enough to allow a plug gauge to slide all the way through), but I was lacking the tools to adjust the throat size. Fortunately, my Christmas wishlist included a set of brass expandable laps (http://www.acrolaps.com/index_004.htm), which arrived today.

Why lapping over reaming? Firstly, cost: the entire set of laps (11 in 32nds from 3/16" to 1/2") and some diamond lapping compound costs well under 100$, while even an unpiloted .452 reamer would be ~40$ and a reaming kit would be much more (a single lap and compound looks to be around 30$). Replacement barrels and additional lapping compound is likewise inexpensive, whereas a damaged reamer means another large expenditure. Secondly, the laps are infinitely adjustable, meaning I can expand them to exactly the dimension I am looking for (e.g., if I want .4525 vs. .452, etc.). Finally, every description of reaming I have seen has had some discussion of cleaning up surface finish, which is an additional operation, while in lapping the surface finish will only depend on the compound used. For this project, I have selected 30 micron diamond paste (heavy concentration), which should correspond to 600-800 grit.

As a pretest, I took a undersized .358 Lee push through sizer and hand lapped it such that a .358 minus pin gauge would just drop through it. I did this in less time than it took me to write out this explanation (say 20 minutes), using a large tap handle to turn the lap. Much of the time was spent adjusting the lap size to make sure there was enough expansion to achieve the final size. Surface finish was essentially mirrored; I am extremely pleased. Time permitting, I will be taking on an undersized .452 sizer and the RH cylinder using the drill press soon (this weekend), and I'll add a bit more documentation here. I'll note that I intend to lap both these items to the same dimension after the first adjustment for a "perfect" match on bullet size.

The question I've come up with is "Could I determine the lap size prior to lapping so as to minimize adjustment/negate the need for a pin gauge?" After lapping the .358 sizer, I took a measurement of the expanded section without compound on it and came up with between .001 and .002 under the final size. I would say that I would be uncomfortable undertaking this process by measuring without confirming with a pin gauge or appropriately sized bullet. However, since the choice is purchasing a single .452 pin gauge or finding a measured bullet and purchasing appropriately sized pilots for a similar price, I would say the point is moot. Additionally, I have yet to find a major manufacturer that has the appropriate reamers for calibers other than .357, .44, and .45, this seems to me to be a viable (and affordable) option for the DIY guy whose tastes are more esoteric.

 

[contender]

I have the Manson reamer set,,, and can do a few different calibers. I also have the forcing cone cutters,,, and can clean up the throats, or cut a proper angle in the forcing cone. I have done several cylinders in my 45's to where I have a uniform .4525 and haven't had the need to polish them up. I go slow,,, use a lot of good cutting oil, and I have proper pilots & such to keep the reamer centered.
For me,,, reaming is the easiest.
I've only had to cut a couple forcing cones over the years.

PS; If you need a forcing cone depth gauge,,, I have a spare. I bought 2 from Brownells by accident.

 

[NikA]

As luck would have it, I got up to this task faster than expected, and am happy to report positive results. Apologies in advance for the photo quality, it gets dark at 5 here and lighting on my drill press is not ideal. Images appear small on my screen but link to larger shots at that are quite a bit better.

For a start, I set up the lap in my drill press and smeared this tiny amount of diamond paste on it to do the Lee sizer. No pics of the Lee sizer, it was much closer than the one I worked on Wednesday and took basically only one adjustment pass to achieve final size.



Here you see that even that tiny bit of compound spreads to cover a significant portion of the lapping surface. I also wiped a couple gobs out of the sizer itself. Diamond paste is expensive but a little goes a long way.



When I moved on to lapping the actual cylinder, I cut the head off a .45ACP case to use as a guide. For both projects, drill press was set at minimum speed (~260RPM) and work was held by hand.

 

[NikA]

Before lapping: .4520 pin gauge does not fit in throats. I miked this pin before the project to check it's exact size, and also checked my other .45 Ruger throats at the same time. This seems to be the only .45 problem child.



After lapping and some cleanup: pin gauge slides through throats with little resistance. Most people seem to want the throats at .452 or .4525; I'd guess these are .4521-.4522 but the gauge fit is the same for the throats and the bullet sizer I intend to use. You'll see a little streaking in the throats if you look closely, this seems to be remnants of the lapping compound (more on this, suffice to say it is difficult to get out).



I'd say I'm happy with the concentricity to the chambers given they all look like this:

 

[NikA]

Finally, some notes on the process:

1) I expected to be able to adjust the lap for the die and have it be close to dimension for the throats. This was not the case, likely due to the harder steel in the cylinder. Fortunately, the lap barrel has five cuts in it so it is very easy to mark progress and gradually expand to achieve the final desired dimension. I adjusted 1/2 cut (36 degrees) for the most part and found this rate of progress satisfactory.

2) I suspected and was pleasantly surprised to confirm that the cylinder on Redhawks can be removed from the crane without removing the crane from the revolver first. I think this is also true for GP-100s. This made the necessary disassembly/assembly process much easier than a complete teardown to get at the cylinder.

3) I started off with conservative use of the lapping compound in order to pre-empt a mess but found that it was easier to recharge the lap between holes than to have the lap heat up and stick in the throat I was working on. I was able to recharge the lap accurately while it was spinning and still have 95+% of the 5 grams of diamond paste I started with.

4) CLEANLINESS IS INCREDIBLY IMPORTANT WHEN MEASURING DURING THIS PROCESS. It was not easy to clean the throats between lapping sessions: I used cotton swabs and isopropyl alcohol but found that the solvent mostly removed the oil on my parts and not the compound. When I was able to push the pin gauge into the throat with difficulty, I stopped and used a cleaning jag with thick cotton pads (cosmetic type) to really clean the cylinder, and ended up right at the dimension I wanted. 30 microns is a little over a thousandth of an inch, so any residue of compound will increase resistance when attempting to insert the pin gauge and the dimensions are non-trivial.

 

[NikA]

contender, I'm not suggesting that reaming is a bad option, just that lapping is also an option if the amount of material to be removed is small. I've seen used Manson reamer sets for sale and considered purchasing them, but the cost is still higher than lapping. I wouldn't want to lap a throat from .450 to .452 as it would require too much time and effort, but for these small dimensional changes, this seems to be a viable approach.

I don't have enough experience at this point to know whether recutting forcing cones will help me: I've yet to see one that is truly ugly. Mostly I've encountered mild thread chokes, which I've been approaching by firelapping and using PC lead bullets as opposed to traditional lube. I'm not entirely sure what a forcing cone depth gauge will do for me? The ones I see look like go/no go gauges for cartridge cases but I'm not certain what that measurement would tell me or whether it would be something that would be worthwhile to correct.

 

[contender]

I'm not saying lapping is a bad way to go. But,, as you've said,,, if you have a larger dimension such as .450 to .452 to remove,,, lapping will be a chore.
And once you buy the reamer set,,, you don't have to keep supplying lapping compound. I also bought my stuff when I could get a good deal on it by sales, gun shows, or gift cards etc. Plus,, having over a dozen 45 cal handguns & always willing to add more,,, it's a cost savings to me to do it myself.

I too have had VERY few forcing cones that needed "adjustment" as most are done properly. And as for my spare gauge,,, it is a go/no go type for the F/C only. You have to be very careful to not cut the F/C too deeply,, as you do not want any bullet "jump" from the cylinder to the rifling. The ideal set-up is to have the bullet still centered in the throat by the rear band,,, and the front band firmly in contact with the rifling. The 2 F/C's I've had to do had very rough machining marks,,, and caused leading. All I had to do was clean up the entry. You have to be REAL careful to not over cut the F/C. It's easy to do. I also think too many folks over think the forcing cone issues,,, and often think it's a problem area. It's purpose is to assist in aligning the bullet from the cylinder to the barrel. A proper bullet that has the 2 contact points I mentioned will rarely suffer from F/C issues. Barrel thread constriction can be an issue that fire lapping can cure.
I too use cast bullets that are PCed. I truly enjoy the PC method of things now.

 

[29aholic]

What's PC'ed bullets?

 

[RoninPA]

Powder Coated, many manufacturers and bullet casters are using this method on lead bullets to cut way down on leading. Powder coating is not hard to do, but just like anything else, take your time and follow the directions and you'll get a good product.

 

[NikA]

I bought my commercial coated bullets from Monmouth/Shooters Cast bullets. However, I shot them before I adjusted the throats in some guns and still experienced some minor leading. I've since started casting my own and coating them with HF paint, but I don't have enough data to endorse that method yet.

 

[contender]

Powder coating is an excellent method over traditional lubing in my book. I started with Harbor Freight red like many folks,, but graduated to different colors, (HF quality doesn't allow this,) from a better quality supplier.
No where nearly as messy, easier to apply, and no leading.

I had just bought 10 sticks of Vearl's Blue when I tried PCing. I STILL have all 10 sticks, after about 3 years now.

 

[NikA]

After measuring throats on the new .44 GP and other .44 Redhawks, I thought I might give a try at lapping a .430" Lee sizer to .432". I cheated on this one and used 400 grit Clover valve lapping compound (SiC) since it's much cheaper than the diamond paste and should remove material faster. I was pleasantly surprised here at how fast I went from an (undersized, as typical with these sizers) .430" to .432": about 30 minutes of hand lapping with a tap handle. As I mentioned before, I wouldn't want to do this six times for cylinder throats that are .002" and think a reamer would be more appropriate here, but I would just highlight the versatility of this method: so far, I've done three different bullet sizers and both .41 and .45 Redhawk cylinder throats.

 

[Carry_Up]

I'm certainly glad you got the results you were looking for. It seems that most of the decisions were based on cost, but no mention of whether you plan to do this job ever again. If not, why purchase a whole set of adjustable lapping tools? You mention that you got a very nice surface finish and that's always fun to look at but I don't think the lead bullet is going to be impressed with a mirror surface any more than the "grey" matte finish that is usually produced by hand lapping on a flat lapping table.

Throats have to let the bullet through without squashing it. But lead bullets cannot be made to tolerances in the tenths, nor would that be of any real value to your accuracy. How small is too small? What are the tolerances of the .452 bullets from batch to batch?

Personally I think of lapping as a very fine adjustment of dimension, usually a few tenths. In other words, it is just one step above choosing a surface finish. Plus this type of lapping will not maintain roundness - not that it matters in this case. Depends upon the runout and wear in the drill press bearings! Except for the manufacturing of precision replacement parts, lapping is really best suited to adjusting a fit between 2 precision parts. But...whatever works !

CU

 

[NikA]

A single lap and compound were 30$; the whole set was 90$. By the time I had the three sizes for .358, .410, and .452, I was pretty close to the cost of the whole set. As I am a habitual accumulator of tools, I figured it was easier to get the whole set for future project rather than save the cost of a single lap now. I am certain this set will be used in the future (I'm already thinking about future project involving "custom" Lee sizer dies).

There is a diminishing return to better surface finish, as you stated. I just wanted to avoid the possibility of having a gouge from a reamer in my throats. At some point I stated that I had previously measured the .45 RH throats at around .4518, so I was thinking of this as a method to remove a few tenths. I was actually surprised that I could remove 2.5 thou from the Lee sizer as fast as I did (guess they probably use soft steel here than the cylinders).

While it's true that casting bullets is not a precision process, there's no reason sizing them shouldn't be. Most lead alloys have very little or no spring back. Thus, since the .452 sizer and throats were lapped to the same size, I've now set up a precision relationship between my sizer and my revolver throats. Will this make more consistent loads? There are probably larger errors elsewhere in my loading process, so perhaps not.

I would be surprised to get an out-of-round result from this type of lap. I think it's about as likely as getting an out-of-round hole from a reamer. There's probably a larger risk of ending up with something that is out of square in the depth of the hole. In any case, press runout was checked and a spacer was used to prevent canting/misalignment.

 

[DougGuy]

I used brass ACRO laps to clean up after the reamer and they worked pretty dang good. Since those days I have invested in a Sunnen hone, along with all the mandrels needed for pistol calibers from .22 to .500 Linebaugh. The precision in the Sunnen method of honing is SO far beyond what you can do with a lap, and I do enough cylinders per year to need one since ACRO barrels are rather expensive unless you get them NOS off ebay. For doing one or two cylinders on your workbench they work but for production work, they wear quickly and as mentioned earlier, are nowhere NEAR as precise as the Sunnen hone.

Now.. The thing that the reamer has that the ACRO laps do not have, is a PILOT. You stick an ACRO lap in a cylinder throat and there is absolutely ZERO centering or piloting to keep it concentric with the chamber. The lap is free to wander as it pleases and if there is an oval or belled throat, it will follow that shape where the reamer stays strictly on course as the very tightly fitted pilot holds it from wandering. THIS ALONE, is worth the cost of the reamers and pilots. I don't ream ANYTHING that isn't very snugly piloted. If a cylinder or barrel comes in that I don't have a pilot for, I order one.

The reamer cuts a ROUND, PARALLEL, CONCENTRIC throat which the ACRO lap, simply by it's design, cannot do. The ACRO lap will center itself based upon equal resistance, and will not round an egged throat like a reamer and the Sunnen hone will. It will not correct a belled throat like a reamer and Sunnen hone will.

Customers send me work with confidence that it will be done professionally, and accurately and they really like the quick turnaround. For what I charge to do a cylinder, you could not buy or rent the tooling to do one cylinder. In this day and time finding fast, affordable, precise top notch craftsmanship is not an easy thing to do. My pricing and turnaround make it pointless to buy the tools unless you plan on doing several cylinders all the same caliber. It's cheaper in the long run to send them this way, and the quality of the work speaks for itself..