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Old 01-20-2009, 06:05 PM
falcon195 falcon195 is offline
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I just checked the air gap on my 642 and the .070" shim is slightly tight and the .060" shim is slightly loose. So I guess my air gap is .065". From what i have read this is within toleraces. But how good is .065"?
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Old 01-20-2009, 06:05 PM
falcon195 falcon195 is offline
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I just checked the air gap on my 642 and the .070" shim is slightly tight and the .060" shim is slightly loose. So I guess my air gap is .065". From what i have read this is within toleraces. But how good is .065"?
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  #3  
Old 01-20-2009, 06:18 PM
JohnK JohnK is offline
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I've been told that Smith passes anything up to 0.012" barrel-cylinder gap.

I'd personally accept what you have.
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Old 01-20-2009, 06:39 PM
Texas Roots Texas Roots is offline
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Perfect.
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Old 01-20-2009, 08:23 PM
bountyhunter bountyhunter is offline
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Quote:
Originally posted by falcon195:
I just checked the air gap on my 642 and the .070" shim is slightly tight and the .060" shim is slightly loose. So I guess my air gap is .065". From what i have read this is within toleraces. But how good is .065"?
Do you mean headspace, the gap between the cylinder and the breechface? I recall .060" - .064" was the nominal value or around there.


If you have a set of auto feeler gauges you can measure it accurately by selecting two or three gauges that give the appropriate thickness.
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Old 12-20-2014, 11:00 AM
HogBob HogBob is offline
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Originally Posted by bountyhunter View Post
Do you mean headspace, the gap between the cylinder and the breechface? I recall .060" - .064" was the nominal value or around there.


If you have a set of auto feeler gauges you can measure it accurately by selecting two or three gauges that give the appropriate thickness.
All the replies seem to indicate the headspace (flashgap) is consistent as the cylinder rotates through six rounds . My 686-1 varies from .008" to .006" as the cylinder rotates. Is this variation normal? Any info that might help explain this revolver's erratic accuracy would be appreciated.

Last edited by HogBob; 12-20-2014 at 11:04 AM.
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Old 12-20-2014, 12:26 PM
Hapworth Hapworth is offline
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Originally Posted by HogBob View Post
All the replies seem to indicate the headspace (flashgap) is consistent as the cylinder rotates through six rounds . My 686-1 varies from .008" to .006" as the cylinder rotates. Is this variation normal? Any info that might help explain this revolver's erratic accuracy would be appreciated.
Welcome to the forum, HogBob.

You raised a pretty old thread here; next time feel free to start a new one when there's a question that can't be answered with a search.

Some terminology issues in play. "Headspace" is measured between the rear of the cylinder (on the upper right side) and the recoil plate.

What you and thread starter are talking about is barrel to cylinder space, or b-c gap. It's correctly measured by manually holding the cylinder as far back as it will go, and inserting feeler gauges between the barrel face (sometimes mistakenly called the "forcing cone" which is a part of the barrel face) and the held back cylinder. You can measure either from the left or right sides, but sometimes one side can be slightly tighter than the other (if, for instance, the barrel face isn't perfectly true), in which case measure from the more open side. Accurate measurement is when the feel slide snugly in but isn't forced.

Generally, barrel to cylinder gap is even through cylinder rotation, though it is very common to have mild variance. If you've measure gap correctly, .006" to .008" variation is mild and personally I wouldn't worry about it; both measurements are within old school factory specs.

Possible explanations include the cylinder face isn't perfectly square (cut flat and even), the yoke could be sprung (bent), or both at the same time.

Erratic accuracy can have various causes -- the two most common are user error, and either cheap or poorly reloaded ammo, with mechanical issues third. For mechaical explanations, indexing, or the proper aligning of the charge hole with the barrel face prior to ignition -- a function of correct timing -- can contribute to accuracy issues, but thtere are many other possibilities.

Are you spitting or shaving lead when firing? Any other abnormalities?

Best first thing to do to determine accuracy problems is get factory match loads, use a rest, shoot single action, and have an informed partner to observe and also shoot.

As for possible mechanical issues, best first step is have a qualified revolversmith perform a basic set of spec checks based on your experiences and concerns with the revolver. Finding a sprung yoke and the like is easy, quick fixing and inexpensive.

If the cylinder face is measurably untrue, I'd leave that be given that you don't report any binding issues; .002" variation is minor and it's a more involved machining fix.
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  #8  
Old 12-20-2014, 03:14 PM
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Waywatcher Waywatcher is offline
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Quote:
What you and thread starter are talking about is barrel to cylinder space, or b-c gap.
@Hapworth: The original poster back in '09 was measuring headspace. HogBob is measuring b-c gap.

@HogBob, your b-c gap of 0.006" to 0.008" is acceptable. Hapworth's method of measuring it is correct, as is the rest of what of Hapworth has written (except the b-c gap/headspace mix up that I corrected--that was probably a typo.)
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Old 12-30-2014, 10:54 AM
HogBob HogBob is offline
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My thanks to Hapworth and Waywatcher for their helpful info. As noted earlier I am a "Newbie" to this forum and revolvers. Most of my experience is with rimfire pistols and rifles and air rifles. I really like my S&W revolvers, but the 686 has baffled me a bit ever since I bought it new. At 75 years, I still enjoy learning new things and this S&W forum is really helpful. Thanks to all.
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Old 01-01-2015, 10:56 AM
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chief38 chief38 is offline
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Originally Posted by falcon195 View Post
I just checked the air gap on my 642 and the .070" shim is slightly tight and the .060" shim is slightly loose. So I guess my air gap is .065". From what i have read this is within toleraces. But how good is .065"?
You posted .060 - .070" which means 60 - 70 THOUSANDTHS which would measure about 1/16 of an inch - HUGE! I do believe what you meant to post is .006" - .007" which IS 6 to 7 THOUSANDTHS. [ to the right of the decimal point comes: TENTHS, HUNDREDTHS, THOUSANDTHS, TEN THOUSANDTHS, etc.]

The OLD S&W acceptable max was .006" but over time and in an effort to save money the Company upped the maximum allowable B/C Gap to .012" which in my own personal opinion is way too much. Anything between .004" and .006" would be about perfect, between .006" - .008" would be maximum in my opinion, and anything over .0085" would be non acceptable to me personally. I'll bet by increasing their maximum allowable B/C Gap they drastically cut down the number of returned out of spec guns.
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Old 01-01-2015, 04:29 PM
Hapworth Hapworth is offline
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Originally Posted by chief38 View Post
You posted .060 - .070" which means 60 - 70 THOUSANDTHS which would measure about 1/16 of an inch - HUGE! I do believe what you meant to post is .006" - .007" which IS 6 to 7 THOUSANDTHS. [ to the right of the decimal point comes: TENTHS, HUNDREDTHS, THOUSANDTHS, TEN THOUSANDTHS, etc.]

The OLD S&W acceptable max was .006" but over time and in an effort to save money the Company upped the maximum allowable B/C Gap to .012" which in my own personal opinion is way too much. Anything between .004" and .006" would be about perfect, between .006" - .008" would be maximum in my opinion, and anything over .0085" would be non acceptable to me personally. I'll bet by increasing their maximum allowable B/C Gap they drastically cut down the number of returned out of spec guns.
I, too, thought that was a typo or error by OP and presume he's talking about b-c gap, not headspace, and meant to be calculating in the thousandths.

One caveat, though: I think original (not current) S&W specs allowed for out to .010" b-c gap, but aimed for .004'-.006" as ideal. Kuhnhausen notes a performance loss starting after .008".
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Old 01-03-2015, 12:49 AM
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Back when I was a "gung ho" collector in the early to mid 80's (bought a new gun almost every month) Smith would repair any gun that was over .007" B/C gap - no questions asked. When I walked into a LGS to make a purchase I'd always carry a Feeler Gauge with me to make sure the gun was not out of spec. Back then most were around .004" - .006". Back in the day most LGS around here had a few guns of the same model and you actually had a choice of which one you wanted. I can not remember ever walking out empty handed because I could not buy a gun within spec.

These days it's a whole different story and since every thing is so darned expensive and there are so many shipping rules and regulations it's just a whole lot better NOT buying a gun with pre-existing problems.
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