How do you measure barrel to cylinder clearance?


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Jim NE
March 31, 2011, 06:07 PM
I've read where you are supposed to measure barrel to cylinder clearance with the revolver COCKED, as this puts the cylinder slightly closer to the barrel. IS THAT TRUE? It seems to be the case on my gun (closer when cocked).

My feeler gauge is automotive, and doesn't have as many thickness as I'd like. Ideas of where to get a better one?

Also, is the gap between barrel and cylinder a function of wear? or is it just where it was set at?

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mmitch
March 31, 2011, 08:37 PM
I measure mine with the gun "in battery" i.e. with the trigger held astern (some folks measure with fired cases in the cylinder).

Go and buy a feeler gauge with more leaves in it. You probably want one that will go as narrow as .006".
For S&W revolvers, for example, you want to be around .008".

If you keep the forcing cone free of powder and lead build-up, your clearance, in a properly functioning gun,should not be a problem.

Mike

The Lone Haranguer
March 31, 2011, 09:04 PM
Surely Brownells would have feeler gauges more suited to firearm usage.

451 Detonics
March 31, 2011, 11:48 PM
Empty or snap cap under the hammer with trigger to the rear same as it would be at the moment of firing. This removes any variation due to end play.

buttrap
April 1, 2011, 05:13 AM
The hammer back deal only works with colts. All the rest you just stick a feeler guage in there you buy at a auto parts store. Most any what used to be a cheep spark plug and ignition guage has brass leaves from .002 to .006 then you go to the steel ones they come with. Hammer back and with cases in them all you get is the specs on the cases in the gun not the wear parts of the gun.

ArchAngelCD
April 1, 2011, 05:24 AM
I think if the gap is .008" it's excessive.

madcratebuilder
April 1, 2011, 06:43 AM
A good set of feeler gauges well go down to .0015. I check the gap with the hammer down and trigger in the pulled position. .004 to .006 is what I like to see. On DW's that I can adjust I have run the gap as tight as .002.

Remllez
April 1, 2011, 10:54 AM
Are you measuring for B/C only or checking for end shake?

SAA
April 1, 2011, 01:45 PM
You do NOT want the cylinder pushed to the front to properly measure the actual gap. Do whatever you have to to make sure the cylinder is as far BACK as it can go while taking the measurement.

Smaug
April 1, 2011, 02:19 PM
I have an automotive set too, and it works fine. Get an automotive set with more feelers, and you'll be OK.

Remember, you don't need to measure it down to the 1/10,000". Just knowing within a thousandth or two is enough to know if it is "good." Think of it as a go/no-go test.

Drail
April 1, 2011, 09:22 PM
You need to measure with fired cases in the chambers and the cylinder held back against the recoil sheild. Having it cocked or not should make no difference. (If it does you have other problems.) The fired cases need to be in place.

oldfool
April 1, 2011, 09:54 PM
a little surprised only one poster mentioned endshake, measuring both front and rear
if checking for problems using feeler gauges, why would anyone not ??
to set a benchmarks , if nothing else
as noted, a good automotive set will do fine

if you have shot a variety of revolvers over time, you can check the gun just by eye and feel (with surprising "precision") actually, but it's nice to have a set of feeler gauges to verify in any case
(then again, I never seem to have a bore light in my pocket, either, when it would be of best use, and I am pretty much guaranteed to misplace the few feelers most needed out of a feeler set, happens every time; some sort of Murphy's Law corollary in all that)

hotchihuahua
April 1, 2011, 10:03 PM
Just for the heck of it, after doublechecking that you have six empty chambers, hold the weapon sideways up to a good light and slowly dry fire it double action while watching the gap between forcing cone and cylinder. My 625 when brand new appeared to be perfect, but when fired double action the cylinder would touch the forcing cone hard enough to stop the cylinder from rotating. This was never evident in single action fireing. Sent it back to S&W and they replaced the crane.

Ratshooter
April 2, 2011, 12:14 AM
The funny thing about the gap between the cylinder and the barrel is we all want a good, tight gap. I agree. I bought an F.I.E. revolver off gunbroker that looked like a J-frame S&W chambered in 32 S&W long that had a .022 cylinder gap.:eek:

I thought about it before I shot it and just knew it would pepper me with powder and lead but you know what? It didn't do that at all. Matter of fact it was one of the faster shooting 32s I owned and was damn accurate to boot. I sold it at a loss because it just seemed like such a cheap gun but it sure changed my ideas on cylinder gap numbers.

The sad thing is I sold a blued 4" model 29 several years ago that had a .009 gap and I thought it was defective. I bought it for $160 and it was the most accurate 29 I have ever owned. I wish I would have known better.

Shimitup
April 2, 2011, 01:40 AM
One more more thing, obvious but just in case you haven't though about it, clean the gun first. Any leading on the cylinder or gunk or powder under the extractor will give you an adversely small reading.

Drail, I don't follow the reasoning on the fired cases in place, thought about it a bit and all I can come up with is that the cases could cause an errant reading if there was a headspace problem or the cases were stretched. Please explain.

Shim

gamestalker
April 2, 2011, 01:44 AM
I can understand why you would measure the gap in full battery mode, but I don't understand why it needs empty cases in it? Do the cases drag against the firing pin cone in fully cocked mode?

DWFan
April 2, 2011, 03:00 AM
I believe current S&W quality control allows for a .010" gap. Dan Wessons came from the factory at .006" and, as mentioned, can go as tight as .002" without problems as long as you keep the cylinder face clean.

Jim NE
April 2, 2011, 01:53 PM
Thanks for the input everyone. Sure a lot of different perspectives on this subject.

One of the things I read on the S&W site forum is that a suprising number of guns (I don't mean a LARGE number...just more than you'd think) from S&W have somewhat uneven gaps. Some claim that some of these are from the factory. My gun has variation, though it's quite possible it was maybe a non-professional with a file trying to deal with lead/powder deposits or something. I'm going to see how my gun shoots, then worry. On average the gap seems to be within specs, as far as I can tell.

Drail
April 2, 2011, 11:11 PM
Gamestalker, you need the case rims in place because that is what sets the limit on how far to the rear the cylinder can move. (without the case rims the cylinder will set back on the extractor) Then you measure the cyl./bbl. gap to get the correct measurement that indicates the actual gap when the gun is fired.

SAA
April 3, 2011, 12:28 AM
Gamestalker, you need the case rims in place because that is what sets the limit on how far to the rear the cylinder can move.
:confused:

A gun that depends on the rims of the cartridge for headspace should be sent back to the factory to have a new cylinder properly fitted.

Shimitup
April 3, 2011, 01:05 AM
Drail for your own edification, you should take any revolver you own put a case in the cylinder close it with case lined up under the hammer the and grip it by the barrel muzzle down with your thumb pushing the cylinder forcibly to the rear, now bring it up to the light. You WILL see daylight between the case head and the recoil shield. If you don't your gun has a serious headspace problem. The extractor star is indeed what controls the rearward limits of the cylinder and the headspace dimension This design is common to all revolvers that I know of.

Drail
April 3, 2011, 04:39 PM
Then why does the Kuhnhausen S&W manual instruct you to place empty cases in the chambers when measuring the cyl. gap?

Remllez
April 3, 2011, 05:49 PM
Don't have the Kuhnhaesen manual but have been checking B/C gaps for 40 years,never have I used empty cases to check B/C. I've been setting the gap on the only pistol that allows the owner to do it......Dan Wesson. B/C gap is set hammer down on Empty Cylinder with a slip fit between barrel and cylinder. I usually set for .003 on the longest charge hole. There is a difference of almost .002 between the longest and shortest charge holes on my cylinder.

If you put empties in the cylinder they should not touch the recoil shield on rotation or you have a problem. Think about it....they would create such a drag as to be compared to the cylinder dragging on the forcing cone up front. The rear gap at the recoil shield is not set with shell casings....Endshake is measured by pushing cylinder to rear measuring gap at front then pushing cylinder forward check gap again then subtract one from the other. B/C gap is done in machining at the factory. Endshake is usually caused from wear at the bearing surface on the ejector star. Headspace problems are caused by excessive endshake.

SAA
April 3, 2011, 06:34 PM
Then why does the Kuhnhausen S&W manual instruct you to place empty cases in the chambers when measuring the cyl. gap?

Are you positive that wasn't refering to checking headspace?

rcmodel
April 3, 2011, 06:46 PM
I agree empty cases would have no function in measuring B/C gap.
They could be used in measuring headspace, but a real headspace guage is preferable.
Too much variation in rim thickness between brands of brass to make cases reliable.

I use two small wood wedges to push all the end-shake out of the cylinder to measure B/C gap.

Then the same wedges to push it fully to the rear to measure headspace clearance with a max headspace guage.

rc

oldfool
April 3, 2011, 07:26 PM
frankly never heard of using snap caps or brass to check gap, prior to this thread
"rims in place...sets the limit on how far to the rear the cylinder can move."

Remllez
"Think about it....they would create such a drag as to be compared to the cylinder dragging on the forcing cone up front."
precisely !

DWFan
"can go as tight as .002" without problems as long as you keep the cylinder face clean."
yes... powder residue on face will show faint scuff marks if you look, but you will feel the cylinder drag first, if you do not keep it clean, so just keep it clean
some love it that tight, some hate it
me, I love it on one of my k-17s, but don't need it on all of 'em
depends on what you most like doing with the gun, of course
(another good excuse to own more than one anyway)

Notoast
April 3, 2011, 08:48 PM
If I use spent brass or snap caps, I get a different measurement (0.001 smaller) than if I use decapped brass or no brass (all with hammer forward, trigger back). I assume it's the firing pin pushing on the case?

Shimitup
April 3, 2011, 08:56 PM
Drail, that kind of baffles me that it would say that, as SAA suggested double check the manual to make sure he's not talking about a headspace measurement. If you've done what I suggested about holding it up to the light with muzzle down you can see what I mean about seeing a gap behind the case head. The revolvers I've worked on over the years S&W Ruger or Colt have been pretty liberal with their allowance for headspace. As Rcmodel alluded to, the rims can vary a bit in thickness.

Drail
April 4, 2011, 12:02 AM
I stand humbly corrected. Kuhnhausen was referring to a headspace check. Thank you for straightening me out. Sorry for the misinfo.

Shimitup
April 4, 2011, 12:54 AM
No sweat, easy thing to confuse. I find myself digging around in reference books way more than I used to to find out what used to be in my head, it's kind of annoying. :confused:

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