Is this cylinder gap too large?

[Paper Clip]

Hey all, as per my recent post I just picked up a 629/6 PC Carry comp used in what appeared to be great condition (also one of 500 or less in conjunction with Lew Horton from what i can tell). I took it to the range and just got around to cleaning it. The top strap and cylinder face were both much dirtier than I anticipated with the round count shot. There is some some flame cutting on the top strap, although I read that a little flame cutting is normal and it stops quickly. I then compared the cylinder gap to my other revolvers and noticed that it was larger. My other 629 (-8 i think) is so tight I don't think you can even get a sticky note in there. I saw a couple markings after a good cleaning that made me worry someone may have filed it down to widen the gap. The thin cardboard from a s&b ammo box can be just barely forced in, but i think it gets compacted in the process. My question is: is it too large? Since I bought this gun used, i don't know how much recourse I have and it is a little concerning. Maybe I am just being too worrisome. Thanks for your help.

 

[HughD1]

Do you have a feeler gauge? Only way I know to be sure.


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[Invictus357]

Get some feeler gauges and see what it is.

Doesn't look too bad to me but eyeballing is never reliable.

I think a good gap is .006"

If it's a lot bigger call SW and ask them if it needs looking at.

edit: Just looked into it some more. The famous Iowegan states that this gap should be .004" to .008".

 

[Paper Clip]

Ordering some now. Thanks guys

 

[k22fan]

Maybe it "should be" or ideally would be 0.004" - 0.008" but S&W specified 0.004" - 0.010" until they increased the largest acceptable gap to 0.012".

 

[rockquarry]

I have a 25-2 with a gap of .010", I think. It's been like that for many years. It shoots cast bullets so accurately I hate to disturb it's condition by having it corrected. However, muzzle velocity is about 75-100 fps less than it should be from a 6 1/2" barrel.

 

[Kenny D]

Gap

Maybe it "should be" or ideally would be 0.004" - 0.008" but S&W specified 0.004" - 0.010" until they increased the largest acceptable gap to 0.0014".

I think you got too many zeros in your last acceptable gap measurement. Didn't you mean .014?

 

[Paper Clip]

My 629-3 DX measures .004. My 64-3 that was completely rebuilt at the Factory also measures .004. The 64 had the barrel turned and the cylinder faced.

Something not looking quite right with that forcing cone.

Klyde

Beautiful pieces. If there were something wrong with it, what could I do? That worries me.

 

[cmj8591]

Does it shoot OK? Does it bind or shave lead after a few rounds? Does it have excessive end shake? If not, I wouldn't get too wrapped up about the B/C gap. As one of the replies says: The gap can go out to .012 and still be within specs. The bigger question is "how does it shoot"? If it's shooting well, don't worry about it.

 

[Paper Clip]

Does it shoot OK? Does it bind or shave lead after a few rounds? Does it have excessive end shake? If not, I wouldn't get too wrapped up about the B/C gap. As one of the replies says: The gap can go out to .012 and still be within specs. The bigger question is "how does it shoot"? If it's shooting well, don't worry about it.

I've only taken it out once and it shot pretty good considering it was the first time out. It's a tack driver with specials. The flame cutting did worry me slightly though. I read on FAQ somewhere that some of it is normal. Will just keep an eye on it. You can see it in the second photo.

 

[cmj8591]

I've only taken it out once and it shot pretty good considering it was the first time out. It's a tack driver with specials.

If that's the case, don't worry about the B/C gap.

 

[Paper Clip]

If that's the case, don't worry about the B/C gap.

Thanks for your input! See edit above btw.

 

[stansdds]

The flame cutting is an indication of being fired with full power magnum ammo, perhaps reloads using H110/W-296 powder. If the gap does not exceed 0.012", it is within spec, but that gap does look wide. How's the cylinder play? How much fore and aft movement? This is something you can measure with those feeler gauges.

 

[__steve__]

My 3.5" 500 had a 0.006" - 0.010". It shot very accurately, though the velocity is on the slow side of like 13%. It is currently at S&W for barrel replacement due to forcing cone erosion (with less than 1k of shots fired). Not sure if the worn forcing cone was due to the gap, cylinder/yoke slop, 275-300 gr jacketed rounds, or a combination of all, but it shot fine as such.

Interestingly, my 4" 500 (3" of barrel minus comp) has a gap of 0.004-0.006", yet its velocity stats are only marginally faster than the loose 3.5"

Would be interesting to know if BC gap can accelerate FC erosion or not, if it doesn't I would be perfectly happy with it on the large gap side.

 

[Neal]

Just throwing in my barrel/cylinder gap story: Several decades ago, in my youth as a handgunner, I was all about maximum velocity. Bought a used Model 29 with 8.375 inch barrel at a gun show. I shot the gun for a while and delevoped full power loads for it, but the .009" cylinder gap bothered me. Had my gunsmith remove the barrel and turn down the shoulder so it could be screwed in another turn. Had to shorten the extractor rod, but it was successful and now my gap was around 2 or 3 thousandths. With anticipation I fired it over the chronograph and found it made no significant difference in velocity or accuracy.

 

[Paper Clip]

This is all great information. Thanks everyone! Waiting in anticipation for the feeler gauges to be delivered tomorrow.

 

[gwpercle]

I have a 25-2 with a gap of .010", I think. It's been like that for many years. It shoots cast bullets so accurately I hate to disturb it's condition by having it corrected. However, muzzle velocity is about 75-100 fps less than it should be from a 6 1/2" barrel.

Warning / Advice ...If it shoots accurately , LIA -
Leave It Alone ! I fixed one and the accuracy went out the window ... I regret ever "fixing" an accurate revolver...
Trust me ... don't fix an accurate revolver !
Gary

 

[gwpercle]

This is all great information. Thanks everyone! Waiting in anticipation for the feeler gauges to be delivered tomorrow.

You can buy them at any auto parts store ... even Wally Mart has them . I hate having to wait on the mail person / postal system and try to get stuff locally when I can ... sometimes my orders get " misdirected" and end up in some distribution center ...that kind of stuff is aggravating ... I would rather drive to the store and get it myself. Sorry about the rant , ordered stuff not arriving is a pet peeve ...
Gary

 

[stansdds]

Just throwing in my barrel/cylinder gap story: Several decades ago, in my youth as a handgunner, I was all about maximum velocity. Bought a used Model 29 with 8.375 inch barrel at a gun show. I shot the gun for a while and delevoped full power loads for it, but the .009" cylinder gap bothered me. Had my gunsmith remove the barrel and turn down the shoulder so it could be screwed in another turn. Had to shorten the extractor rod, but it was successful and now my gap was around 2 or 3 thousandths. With anticipation I fired it over the chronograph and found it made no significant difference in velocity or accuracy.

Yes, barrel-cylinder gap has a minimal impact on velocity. Ballistics by the Inch did a test on barrel-cylinder gap and velocity.



BBTI - Ballistics by the Inch :: Cylinder Gap

 

[Hapworth]

Just to add to the S&W specs confusion...

I know it's been posted many times that the current factory acceptable gap goes out to .012", but I have dealt with S&W a couple times on this and they state that .010" is still the upper allowable limit, and they have corrected (under warranty) a couple of mine that came from the factory over .010".

 

[Hapworth]

This is all great information. Thanks everyone! Waiting in anticipation for the feeler gauges to be delivered tomorrow.

Remember, b-c gap is measured with the cylinder held to the rear; check on each charge hole and from both sides of the gap...

...after you have checked, double-checked and triple-checked, then checked again, that the revolver is unloaded and safe to work on.

Depending on your measurements, if you're in spec and the revolver functions correctly under fire without spitting or shaving, I'm squarely in the camp of leaving it alone.

If you have an extraordinary gap or develop issues shooting, there are plenty of options and we can walk you through them, but basically it's about getting to a qualified revolversmith.

Long as you have the feeler gauges, check your endshake and headspace, too.

 

[rockquarry]

Yes, barrel-cylinder gap has a minimal impact on velocity. Ballistics by the Inch did a test on barrel-cylinder gap and velocity.



BBTI - Ballistics by the Inch :: Cylinder Gap

I'm not familiar with Ballistics By The Inch, but I don't believe it's possible to make a valid one-size-fits-all blanket statement about cylinder gap having minimal effect on muzzle velocity. Consider several revolvers from the same manufacturer that have average cylinder gaps and varying barrel lengths (3", 4", and 5") that give approximately "book velocity" (handloading manual references). Compare these figures with a 6 1/2" barreled revolver that has a wide cylinder gap of .010" - .011". The muzzle velocity on the latter gun is less than the velocity on the 3"-barreled gun.

No extensive testing here, just reasoning based on experience. That's also considering many changeable factors in play here. While it's true there are "fast" barrels and slow "barrels", that observable fact generally has much more signifcance with regard to rifle barrels and rifle ballistics than with revolver barrels.

 

[Paper Clip]

Remember, b-c gap is measured with the cylinder held to the rear; check on each charge hole and from both sides of the gap...

...after you have checked, double-checked and triple-checked, then checked again, that the revolver is unloaded and safe to work on.

Depending on your measurements, if you're in spec and the revolver functions correctly under fire without spitting or shaving, I'm squarely in the camp of leaving it alone.

If you have an extraordinary gap or develop issues shooting, there are plenty of options and we can walk you through them, but basically it's about getting to a qualified revolversmith.

Long as you have the feeler gauges, check your endshake and headspace, too.

I'm not a revolver newbie, but I am one when it comes to diagnosing this stuff. What does that mean and how do I check it?

 

[series guy]

Before you can accurately check the barrel cylinder gap you must insert feeler gauges of the correct thickness to set the rear gauge. Otherwise you may be pushing the cylinder back too close to the recoil shield.

 

[terry_tr6]

Feeler gauges! wow has it been a while since I've pulled mine out. Cars without points or plugs to gap every couple thousand miles, and selling the old triumph cycle so no valves to set every thousand or so miles.... I just recently got rid of my timing light and dwell meter....

 

[rockquarry]

I use a feeler gauge regularly in handloading, but rarely for checking cylinder gap. A feeler gauge is the easiest and fastest way of setting up a Forster case trimmer for a run of brass.

 

[Hapworth]

I'm not a revolver newbie, but I am one when it comes to diagnosing this stuff. What does that mean and how do I check it?

Endshake (known also as end play) is fore and aft play in the cylinder. A little (about .001") is necessary for proper function. Opinions vary on how much beyond that needs correction. Too much causes excessive batter under fire, possible binding and failed ignition.

You measure endshake by holding the cylinder forward and measuring the gap between the barrel and cylinder faces. Then hold the cylinder back and measure again. The difference is your endshake.

Headspace is the distance between rear of the cylinder and breech face (blast shield area by firing pin hole). Someone check me on this, but current spec I believe is .060"-.064".

 

[Benchrest1]

I have a old 686 no dash that has been shoot a lot. It has a gap of .010" and is still one of my best shooting guns.