Model 10 Cylinder Gap

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ThomasT

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I was given a model 10 heavy barrel gun and was looking at it and noticed the barrel/cylinder looked large. So I measured it with a feeler gauge and the largest gauge I could insert was a .015. Way too big. Is there any way to reduce the gap without having the barrel set back?
 
I believe the shim washer are used to address end-shake not cylinder gap. I believe the only way to address cylinder gap on a S&W is to have the barrel set back one turn and machined to fit. That said although .015 cylinder gap is over spec how does it shoot? My S&W 610 measure .014 and still shoots great and I can still push 200 gr bullets to 1250 fps without issue. Yes your cylinder gap is larger than desired but if the gun shoot well I would not worry about.
 
It's possible that the cylinder has already been shimmed thus increasing the b/c gap. But be that as it may, I concur with mcb that if the revolver performs as it should, I'd leave well enough alone.

Bayou52
 
End shake washers actually hold the cylinder in the widest gap position- good for correcting forcing cone rub, but not excessive clearance.

I had a pretty tired 5" K-frame whose serial number indicated it was a year or two past the cutoff date, but was NOT marked Model 10. It had a .016" gap, which is a total no go for me, but I missed it in the haste of the LGS. I determined the cost to correct would exceed what I wanted to put into the gun, so I consigned it and put the funds toward a better example.
 
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I believe the shim washer are used to address end-shake not cylinder gap. I believe the only way to address cylinder gap on a S&W is to have the barrel set back one turn and machined to fit. That said although .015 cylinder gap is over spec how does it shoot? My S&W 610 measure .014 and still shoots great and I can still push 200 gr bullets to 1250 fps without issue. Yes your cylinder gap is larger than desired but if the gun shoot well I would not worry about.

I just got the gun a few months ago and haven't shot it yet. The gun was bought by my uncle the cop many years ago. When my mother went to work for the sheriffs dept. he gave her the gun to use. She carried it for 18 years until she retired. At 82 she decided she didn't want it anymore so gave it to me. She did have to shoot it in qualification so the gun shoots fine. I'm just anal and don't like for stuff to not work or be out of spec. And it has zero end shake.

Maybe a call to S&W and see if they would even work on it. But that would be after I shot the gun. I wonder if its still under warranty? :rofl:
 
I just got the gun a few months ago and haven't shot it yet. The gun was bought by my uncle the cop many years ago. When my mother went to work for the sheriffs dept. he gave her the gun to use. She carried it for 18 years until she retired. At 82 she decided she didn't want it anymore so gave it to me. She did have to shoot it in qualification so the gun shoots fine. I'm just anal and don't like for stuff to not work or be out of spec. And it has zero end shake.

Maybe a call to S&W and see if they would even work on it. But that would be after I shot the gun. I wonder if its still under warranty? :rofl:

OK. This is a different ball of wax since the gun has sentimental ties. There are should be several gunsmiths that should be able to accomplish this task in the Dallas Metroplex.
 
Endshake washers do not address excess cylinder to barrel gap. In fact, if incorrectly used, they can cause cylinder to barrel binding by removing all clearance. One of their other functions is not often stated. That is to eliminate variable headspace. Since most revolvers headspace off the rim it can affect accuracy if the cylinder floats back and forth too much. I like to check headspace first to ensure that it is at minimum within spec and preferrably close to the minimum end of spec. After that is verified and corrected, then and only then do I set the barrel back. Basically build the correct relationships from the fixed breechface forward. So the cylinder gets any necessary shims at this point, before dealing with the barrel.

Setting a barrel back is a relatively simple process for a real gunsmith or a somewhat skilled amateur with access to a few tools and a lathe. Lacking a lathe, Brownell's did sell a barrel shoulder cutting tool (076-200-319_Barrel_Set-Back_Fixture.pdf (brownells.com) made for the home smith to set back barrels that I believe is available to rent from 4D Reamer Rentals The best chamber reamer rentals and gunsmithing tools from 4D rentals along with the other necessary tools like a receiver wrench, barrel vice blocks, a barrel facing tool, a forcing cone reamer, and a forcing cone gauge. Setting back one turn does not require doing any threading as there is more than enough left to get the job done.

Revolver wrench - Revolver Wrench (Frame) - 4D Reamer Rentals (4drentals.com)
K frame wrench insert - S&W 'K' Frame, MGW Revolver Wrench Insert - 4D Reamer Rentals (4drentals.com)
Barrel set back fixture - Barrel Set-Back Fixture, Revolver (Brownells™) - 4D Reamer Rentals (4drentals.com)
Forcing cone and barrel breech cutters - Revolver Forcing Cone Tools 38/357/9mm (11 degree) - 4D Reamer Rentals (4drentals.com). Note - available in 5* and 22* as well.
Forcing cone gauges - Plug Gauges for Forcing Cones in Revolver Barrels - 4D Reamer Rentals (4drentals.com)
Basic steps are:
1. Clamp the barrel in barrel vice blocks,
2. Remove the barrel pin at the top front of the receiver with the proper cup tip punch (you can make one easily if you cannot find one for sale or rent),
Note: Newer S&Ws do not use a barrel pin and the barrels are installed at a higher torque.
3. Clamp receiver wrench around receiver just behind the barrel, ensure that it is snug and in full contact, but not too tight.
4. Turn the receiver off the barrel,
5. Cut the barrel shoulder in the lathe or with the Brownell's barrel setback tool (K frame thread pitch is 36 TPI so one full turn is .0277", stop a bit shy of that particularly if the barrel was loose or was overtorqued past TDC),
6. Reinstall in the receiver and reinstall the barrel pin (trial fitting and adjusting the shoulder so the barrel torques up correctly will be needed before final barrel install is complete),
7. Attempt to install the cylinder and yoke (probably won't fit),
8. Use barrel facing tool to shorten breech end of barrel about half the estimated amount needed,
9. Repeat steps 7 and 8 until the cylinder will just barely close,
10. Measure barrel to cylinder gap and continue to adjust the barrel face until proper gap (about .005") is reached.
11. Measure and cut the forcing cone with the gauges and cutter.
12. Go shooting!

That's a bit oversimplified and the devil is in the details. Anyone with good basic skill and an understanding of how this stuff works can do it. Patience is key.
 
From a SW armorer.

B/C gap and the rear gauge are measured with the cylinder in battery and fully forward. If you have a .004 B/C gap when the cylinder is fully forward, and a .006" gap when the cyl is held to the rear, then you have .002" of "end shake"

The ideal gauge in a model 10 for instance (according to the spec) would be .004" min at the B/C gap, and .060" at the rear gauge. It these measurements were static, with no allowance for front to back movement at all, then the cylinder would not turn.
To allow for proper rotation of the cylinder, there must be some gauge in there.....ideally .001 - .002".

This allowance is a slight gauge (gap) between the end of the barrel of the yoke, and it's bearing surface inside the cylinder. As wear occurs on the end of the yoke barrel, the cyl moves forward over time and with use. To restore the gauge and move the cylinder back, shims can be used, or the yoke barrel can be stretched or peened.

There are specs in the armorer's manual for these tolerances. Back when I went through the school in 1980 the tolerances were as follows:

The rear gauge beween the breech face and the rear of the cyl:
.060" - .064" for non recessed (like the model 10, 14, etc)
.010" - .014" for the older recessed magnum cyl (model 19, 66, etc)
(k-frames are listed above but the L,N frames are the same )

(rimfire rear gauge was .008 - .010)

B/C gaps were specified as ideal between .004" and .008".

Some guns were shipped with tolerances at both ends of the acceptable range....and the rear face of the barrel was often cut with a file to meet the spec. As you can imagine, as a result of this hand filing, the "squareness" of the rear barrel face was dependent on the skill of the person who fit the gun. The bottom line is that many of the barrel faces are not true and square. This can be fixed if needed using a breech face reamer/cutter. (Brownells)

There is conflicting information because the tolerance specs changed in the manuals published later.....the front gauge (B/C gap) widened out to .004" - .012" I believe.

With respect to end shake, there has to be some gauge in the cylinder assembly for it to rotate properly. I usually try to get as close to the lower end of the specs listed above as I can, and still have approx. .002" of gauge in the assy....or in other words, try to limit the "end shake" to around .002". This gives the cylinder room to rotate and accomodate some lubricant.

Remember on the older guns there is also a potential for end shake on the yoke, as the yoke is held fully back when closed based on it's relationship with the yoke screw. This relationship can be tightened if needed by peening the button on the end of the yoke stem....or by using a .001 - .002 bushing over the stem itself. Armorers are taught to tighten this gauge by peening.
The newer yoke screws have a spring loaded plunger at the end of the screw which theoretically holds the yoke in proper position so this is not an issue with the newer guns unless the plunger or spring is comprimised in some way
 
Thanks Rule3 and BBBBill for the very detailed post. If I had the tools I am certain I could do the work but in the end after I shoot the gun and test it if it bothers me I will just find a gunsmith to make the adjustment. Sometimes you have to know when a job is better left to someone with the tools and experience.

I did watch the video posted above and took the cylinder out and was going to see if someone had installed end shake bushings since this gun seems to have zero end shake but I could not remove the ejector rod. This a 10-8 so should have the left hand threads. But I did not want to force anything so I stopped without getting the rod loosed. If bushings were installed they must have used Loc-Tite on the threads when they put it back together.
 
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