Cylinder/forcing cone gap

[AZAndy]

I saw a mention of a cylinder gap measurement elsewhere and it occurred to me that I had never actually measured one, other than eyeballing it when checking out a gun at a shop. So I went into full nerd mode, grabbed a couple handfuls of revolvers out of the cabinet, and got to work with a set of feeler gauges I used to use for setting up guitars. Here's what I discovered, listed from smallest to largest:
S&W 640 .002
Taurus 82 .003
Ruger SP101 .004
S&W 64 .004
S&W 325 .005
S&W 36 .005
Kimber K6 .006
Taurus 85 .006
S&W 625 .010

I don't really have a point to make, but I just thought it was kind of interesting. Finally, a use for all that time I spent back in the old days learning to measure point gaps and valve clearances on engines of yesteryear! My biggest takeaway from this is that I see that I need more revolvers, as what's listed above is all I have.

 

[243winxb]

The 2 and 3 thou ones may be a little to tight?
Were measurements taken with cylinders pushed towards the recoil shields? Any difference with hammers cocked?

When shooting lead, i would want a minimum of .004"

Tighter is better, as long as the cylinders doesnt bind when rotating.

Interesting, thanks for the post.

 

[AZAndy]

The 2 and 3 thou ones may be a little to tight?
Were measurements taken with cylinders pushed towards the recoil shields? Any difference with hammers cocked?

When shooting lead, i would want a minimum of .004"

Tighter is better, as long as the cylinders doesnt bind when rotating.

Interesting, thanks for the post.

Didn't try pushing the cylinder back, because I didn't think of it. Next time I get measurement fever I'll try that! Since two have internal hammers, I checked them all uncocked to keep things similar.

I've shot the heck out of the 640 with coated lead, but not bare lead. I'm wondering if I might not get a little binding at some point, but it's unusual for me to do more than a hundred at a session, so no trouble so far.

 

[Riomouse911]

The .002 gaps may clog a bit after an extended session with lead bullets, but they will work just fine if kept clean. I had a Rossi with a slightly cockeyed cylinder that would drag against the forcing cone as the tight side rotated into battery after firing some lead bullets... but that was more the result of crappy QC.

Just for fun, did you check all charge holes for gap, or just one. It would be interesting to see if there is any cylinder wobble in your more used S&W compared to your lesser-fired guns.

This is a good thread! It makes me want to pull some revolvers out and see what mine show just to put a few more out there for comparison.

 

[BSA1]

Did you measure the b/c gap from both sides of the gun?

I have a Ruger Vaquero that measured .003" from one side and only .001" from the other. Imho the importance of the b/c gap is way overrated and is only a concern because gun writers need something to print. For reference .003" is the thickness of a piece of paper and .008" is the thickness of a business card.

Reliability is of upmost importance to me so I prefer a b/c gap of .008".

 

[C5rider]

Buy a Dan Wesson and it can be anything you WANT it to be.

Never checked any of my other guns since I figured that it wasn't going to change things and they all worked fine. Well, except for that one Ruger that had a problem with the cylinder hitting the forcing cone when you fired it. Sent it back to Ruger, who fixed it well beyond my expectations. Didn't even check the gap on that one since zero didn't have need of a feeler gauge. Although I think with this thread, there may be a run on feeler gauges in the future. Might be interesting to see what info filters out of the gun rooms of this forum.

 

[AZAndy]

Just for fun, did you check all charge holes for gap, or just one. It would be interesting to see if there is any cylinder wobble in your more used S&W compared to your lesser-fired guns.

Hey, that's an interesting thought. The most heavily used looks to be the 325-- bought it used, so it wasn't me who put all the miles on it-- and I got it out and checked. 5 are at .005, and one is .004. Of course, that may be due to not cleaning the front of the cylinder very aggressively because it's coated titanium. I rub it with oiled cotton rather than going at it with lead-away cloth like I would do on stainless. So, I got out the 640, on which I do keep the front of the cylinder fairly clean, and on which I've done maybe 1200 rounds, and found all five to be the same. Holding the cylinder back adds .0005 so that I get .0025.

 

[AZAndy]

Did you measure the b/c gap from both sides of the gun?

I have a Ruger Vaquero that measured .003" from one side and only .001" from the other. Imho the importance of the b/c gap is way overrated and is only a concern because gun writers need something to print. For reference .003" is the thickness of a piece of paper and .008" is the thickness of a business card.

Reliability is of upmost importance to me so I prefer a b/c gap of .008".

On the two I got out this morning-- the 325 and the 640-- they were the same from both sides. I agree that it's probably not that big of a deal, unless we're talking a quarter inch or something.

Your mention of the paper products reminded me that, as a teenager, I used to set the points on my MG with a matchbook cover. It was just right. In case you ever need to do that, they were plain white matchbooks. Printed or shiny ones might not work as well.

 

[ArchAngelCD]

Like the others said the .002" and .003" gaps are on the tight side and I will add the .010" gap on your M625 is on the loose side. If it gets any wider you might want to address it with a shim.

 

[243winxb]

the .010" gap on your M625 is on the loose side. If it gets any wider you might want to address it with a shim.

Where would the shim be placed?

 

[Varminterror]

Most manufacturers - if you dig deep enough to find the right guy who will admit to their specs - will tell you they use a 4-10thou standard for spec. Personally, I don't take receipt of a revolver over 7thou, and I'm happy to accept one less than 4, as cutting OPEN is easy, but closing is more tricky.

It IS quite common to find slightly uneven cylinder faces or off-axis mainpin bores, such one side of the cylinder is closer than the other, regardless of whether it says Ruger, Taurus, or S&W on the side. Nobody really wants to pay for improved precision in these areas, and would rather have base model revolvers start at $500, and the importance of this variance is minimal in the application. Until you commit to line boring and blueprinting, that slight variance is inconsequential.

However, there ARE guys who will face your cylinder true and set back your barrel, or install a new barrel, to run a minimal gap of 0.5-1 thou. Chris Roads of Bayside Custom Gunworks being one of them - which picks up about 6-10% velocity over the average production revolver. His Franken Ruger's really are a sight, and their performance is something to behold.

 

[JohnKSa]

...which picks up about 6-10% velocity over the average production revolver.

That sounds a bit optimistic.

Going from 0.006" to no gap at all gives an average velocity improvement of about 7% for barrel lengths of 2" to 6".

Going from 0.006" to 0.001", the average velocity improvement is only about 3% for that same barrel length range.

http://www.ballisticsbytheinch.com/gaptests.html

My guess is that his figures relate more to the revolvers he is paid to fix than they do to the "average production revolver". In other words, he's probably not getting a lot of "average production revolvers" sent to him to have their gaps reduced. It makes more sense that he would be getting revolvers that have larger than average cylinder gaps and that would account for his numbers being slightly on the high side.

 

[Catpop]

Yum yum getting ready to dig in!!!!!

 

[ArchAngelCD]

Where would the shim be placed?

I know you're being sarcastic because didn't word it correctly but a video from Mr. Potterfield will fix that.

 

[Catpop]

Deleted

 

[2011redrider]

View attachment 764322Yum yum getting ready to dig in!!!!!

You use those chicken legs to measure your gap? they do look good!

 

[Varminterror]

I know you're being sarcastic because didn't word it correctly but a video from Mr. Potterfield will fix that.

Endshake shims/bearings move the cylinder back, opening the gap - the opposite direction you want to go if you're wanting to close an excessive gap.

 

[Varminterror]

It makes more sense that he would be getting revolvers that have larger than average cylinder gaps and that would account for his numbers being slightly on the high side.

Generally, I would say Chris is getting any revolver which someone wants to send him. Google the FrankenRuger - it's a long ways from anything you'd consider to be standard gunsmithing repairs.

I've set back a lot of barrels myself, I've seen the BBTI data before as well, and was surprised by how little loss they showed. I generally see between a percent to a percent and a half per thou in closing the BC gap.

 

[ArchAngelCD]

Endshake shims/bearings move the cylinder back, opening the gap - the opposite direction you want to go if you're wanting to close an excessive gap.

OK, everyone can make a mistake...

 

[JohnKSa]

Generally, I would say Chris is getting any revolver which someone wants to send him.

I'm just trying to make sense of his claims vs the data from BBTI. Another possibility is that he's chronographing primarily with lighter loads and longer barrels.

Here's a comparison between the +P and .357Mag loadings vs. the standard pressure loadings across all barrel lengths.
From no gap to 0.001", the loss for the hotter loads was about 4% but about 7% for the lighter loadings.
From 0.001' to 0.006", the loss for the hotter loads was about 3% but about 5% for the lighter loadings.

Here's a comparison between all the loadings but with different barrel lengths.
From no gap to 0.001", the loss for 2" to 6" bbl lengths was about 4% and about 5% for the 4" to 6" bbl lengths.
From 0.001' to 0.006", the loss for 2" to 6" bbl lengths was about 3% and still about 3% for the 4" to 6" bbl lengths.

Of course, if that's what's going on, the problem with providing specs based on long barrels and light loads is that it's likely the people who want to squeeze every last bit of performance out of a given barrel length are most concerned with short barrels and hot loads.

It would be nice if BBTI could have included data for at least one more gap--say 0.010" or 0.012"--but I can appreciate how much the ammunition costs must have been for all that chronographing even for what they did.