Breech to cyl clearance tolerance

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bee-kay

Bearcat
Joined
Mar 4, 2017
Messages
37
I have a Super Blackhawk (.41 mag), and I thought that I'd try my hand at closing up the B/C gap by shimming the back of the frame where the cyl makes contact with the frame. This I have done successfully after removing .004" from the front thrust surface of the cyl.
The barrel/cyl gap is now .002" and the end shake is barely noticable when there is a film of oil on the surface.
When it's DRY I can feel a bit of shake, and when the cyl is as far forward as it will go there's about .0005 clearance.
I thought I'd give this a try. I've yet to fire the gun, but if there's any issue I'm expecting it will be due to the additional .004" that was added to the clearance between the breech and the back of the cyl.
At this time the job is clean and sanitary and the cyl spins freely and all the action functions as well as ever.
Any opinions on this?
I'm interested in finding what the tolerances are for breech to rear of cyl face.
I can always undo the job and make it the way it was originally if there end up being ignition issues, due to poor pin/primer indentation.

Thanks in Advance
 

tsubaki

Single-Sixer
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Mar 3, 2013
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Savannah
I'd be concerned it is too close for expansion, say when everything heats up or gets dirty during a reasonable amount of shooting.

Best I remember Dan Wessons are gaped at .006" (with the exception of 357 Maximum gaped at .002"), not that this has any bearing on your firearm.
 

DGW1949

Hunter
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Apr 10, 2005
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3,653
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Texas
If I'm reading your post right, what you've done has increased the headspace by .004 . If that is true, the next step I'd take would be to have the gun's headspace checked by someone whom has the proper gauges.
Plus, as tsubaki has suggested, having a minimum .0005 of a B/C gap probably ain't going to work out all that well once you get around to shooting the gun.

Not trying to rain on your parade, just saying that sometimes shimming works OK, sometimes not. Reason is, when one really thinks about it, shimming a cylinder doesn't really change anything in terms of total clearance numbers, it just shifts them around from one place to another.

DGW
 

jgt

Blackhawk
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Barrel/cylinder gap within .006 and less, does nothing to effect accuracy. It may effect velocity but only enough to read on a chronograph. I once had a .38 that had an atrocious BC gap and was so accurate my father "borrowed" the gun until his death many years later. What could be effected is the ignition. If you start to get misfires, you may want to return it to its original position. Also poor primer strikes can effect accuracy and make a gun shoot patterns rather than groups. An .0005 gap can make life miserable while shooting. It can cause spitting on any cylinder not in alignment, and drag when gun begins to get dirty. All of this is only what could happen, shooting the gun will tell you what is happening.
 

bee-kay

Bearcat
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Mar 4, 2017
Messages
37
Thanks for the info.
I had just been noticing that on Gary Reeder's site, he says that on all his guns where he fabricates a custom cylinder for a revolver that's been through his re-chambering process, he sets the B/C gap at .002.
So I thought I'd give it a try, keeping open the possibility that I can un-do the modification if needed.
My main concern is primer indentation, which will now be .004" less than original.

I'm seeing that the primer indentation will be .016 now where originally it was .020... That's if I remember my measurements correctly - having taken them 12 hr ago and going only by memory
 

NikA

Buckeye
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Keep in mind that when fabricating a cylinder, tolerances can be set on both sides to whatever one desires. When using the current cylinder, as has been said, tolerances may be moved but ultimately the space has to go somewhere.

I think pictures would be helpful here. How did you shim the back of the cylinder? What material did you use to do this? In what way is this reversible, other than by shimming the front area where you removed material?
 

tsubaki

Single-Sixer
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Lets say there will be no consequences in pushing the cylinder farther forward
BUT
what happens to the other rounds when you discharge the one lined up with the barrel?

The recoil will move them forward in the cylinder but when it has dissipated they will migrate to the recoil plate, moving that extra .004" back and forward.
 

tsubaki

Single-Sixer
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I just measure my SBH (44) for conversations sake.

Gap between cylinder and forcing cone is somewhere over .0025" but under .003".
Between upper recoil plate (firing pin area) and cartridge is .003"
Lower recoil plate and lower cartridge is .006".
Those 3 lower cartridges are noisy clanking back and forth at .006".
That extra .004" making it a total of .010" might be extraordinary.
 

bee-kay

Bearcat
Joined
Mar 4, 2017
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413264176.jpg


That didn't quite come thru as I intended
Let me try a few other different things
 

bee-kay

Bearcat
Joined
Mar 4, 2017
Messages
37
413264177.jpg
413264175.jpg
413264178.jpg


Due to my lame graphic capability, those are the best I can do for pics.

The area just to the center of the loading gate has been shimmed .004"
The cyl itself has had .004 removed from the front - so as to close the B/C gap - this has widened the headspace area at the rear of the cyl between the back cyl face and the breech,
 

bee-kay

Bearcat
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Mar 4, 2017
Messages
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I might phone Gary Reeder. He's always been friendly.
I'll see what he has to tell me, cause it's because of his statements in his mods that he does that got me curious to try this
 

NikA

Buckeye
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I think your photos are fine but they don't tell me what I want to know. What material is your shim made out of and how is it secured to the rear area of the cylinder window? People have reported ratchet peening on the rear area of the cylinder window, I'm wondering (1) how your shim will hold up to repeated impacts from the ratchet teeth and (2) whether its position may shift under recoil and tie up the gun? You've definitely increased headspace by 0.004", which may or not be within spec depending on what the headspace was to begin with.
 

bee-kay

Bearcat
Joined
Mar 4, 2017
Messages
37
The shim is made from a piece of feeler gauge JB welded to the frame where the star turns.
Deburred and lapped to remove any high spots.
It's much flatter there on that surface now than it was from the factory. New, that thrust surface was covered with tool marks and barely deburred... Pretty much like every Ruger I've owned.
From what I'm reading here, this isn't something that is commonly done.
I wouldn't do it to someone elses revolver. But I'm willing to find out how the experiment works out.
If I keep my eye on it, it's the sort of thing that will give me some warning signs before something goes critical.
I'll be sure to be cautious of hangfires.

Thanks for the input
 

Yetiman

Bearcat
Joined
Jan 5, 2015
Messages
53
The proper way to do this job without effecting headspace (assuming it was good to begin with when the cylinder was all the way back) is to first address the endshake. This is done by installing a new endfloat bushing ( http://parts.bowenclassicarms.com/index.php?main_page=product_info&cPath=8&products_id=52 ) in the front of the cylinder and machining it to the right thickness to very nearly eliminate endshake.

Then, to close the gap you would pull the barrel and cut the shoulder back the length of one turn of the threads.

Then the barrel is installed and measured, and if it now protrudes too far (it will) the barrel face is turned down to achieve the desired gap.

Now you will have a nice tight cylinder with whatever gap you desire (which varies with your intended use of the gun).
 

bee-kay

Bearcat
Joined
Mar 4, 2017
Messages
37
That's true - Yetiman
Turning the barrel on a SBH Hunter can be sort of a bee-yatch
This operation I've done is pretty mickey mouse.
I just wanted to try out this revolver with .002" B/C gap to see if I even notice a difference before I attempt to do it the right way, cause it really is not a simple process.
 

Yetiman

Bearcat
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Jan 5, 2015
Messages
53
I have to admit, I cheated on mine and had David Clements perform the job along with quite a bit more on my Bisley Hunter.

I understand wanting to experiment and to do things yourself though too.
 

Carry_Up

Single-Sixer
Joined
Dec 22, 2007
Messages
376
Location
Dallas, TX
bee-kay said:
I have a Super Blackhawk (.41 mag), and I thought that I'd try my hand at closing up the B/C gap by shimming the back of the frame where the cyl makes contact with the frame. This I have done successfully after removing .004" from the front thrust surface of the cyl.
The barrel/cyl gap is now .002" and the end shake is barely noticable when there is a film of oil on the surface.
Maybe a little research and study on revolver repair might have saved you a lot of unnecessary trouble. First, what was your b/c gap before you did anything? Looking at your numbers it works out that your original b/c gap was .006". Guess what? .006" is perfect for that type of revolver. Fact: Any closer than about .004" and you will not be able to fire 6 shots in a row without the cylinder heating up and binding.

CU
 

Carry_Up

Single-Sixer
Joined
Dec 22, 2007
Messages
376
Location
Dallas, TX
bee-kay said:
The shim is made from a piece of feeler gauge JB welded to the frame where the star turns.
That's basically how its done, except that the shim has to be silver brazed to the frame. JB weld is a great glue, but doesn't cut it for gunsmithing.
 

Carry_Up

Single-Sixer
Joined
Dec 22, 2007
Messages
376
Location
Dallas, TX
bee-kay said:
This operation I've done is pretty mickey mouse.
I just wanted to try out this revolver with .002" B/C gap to see if I even notice a difference before I attempt to do it the right way, cause it really is not a simple process.
The goal is to define the problem correctly in the first place. What was the original problem, again? If your recoil face was being slap peened by the cylinder, digging a trench into the recoil face...that would be a problem worth fixing with a shim on the recoil face. But, in such a case, your goal would have nothing to do with improving the b/c gap.

B/C gap, by itself, isn't enough information. You also need to know the endshake, and the headspace to get the full picture. As mentioned previously, b/c gap by itself will have a small effect on velocity. Much more important for revolver accuracy is the angle and depth of the forcing cone, the accuracy of the chamber to bore alignment, and the size of the cylinder throats in relation to the bullet type you use.

Please do not make the mistake of thinking that a little rotational play in the cylinder is a sign of poor gunsmithing or a worn out revolver. Only the most carefully hand made revolvers can get away with no rotational play because those cylinders are line-bored to guarantee perfect registration on each chamber. Installing an oversized cylinder stop (bolt) on a production gun will just lock the cylinder at some unknown position vs the bore - there again a fix waiting for a problem. Or rather a fix that could easily cause some problems that weren't there.

CU
 

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