Measuring Cylinder Gap

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Arquebus357

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I have just corrected the cylinder end shake problem with one of my Mod 64-3s with a cylinder shim.

Before installing the shim I had a cylinder gap of .004". but if I pushed the cylinder to the rear, I had a gap of .008". After installing the shim there is no perceptible end shake and a gap of .008".

My question is...when measuring the Cylinder Gap, should you first push the cylinder to the rear. Even if you have a end shake of .002" don't you need to take up this slack when using your feeler gauges ?

After all, when you fire the revolver the cylinder is slammed to the rear anyway.

I have tried to find the answer but I can't find this covered anywhere. Probably need to get Jerry Kuhnhausen's book.
 
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Arquebus357 said:
My question is...when measuring the Cylinder Gap, should you first push the cylinder to the rear.
Click to expand...

When initially checking your revolver you should start by pushing (& hold) the cylinder forward & then measure the gap between the cylinder face & barrel breach.

Next you push (& hold) the cylinder to the rear & re-measure that gap.

(Admittedly it doesn't matter which you check first. The rearward reading will always be the larger (maximum) reading.)

The forward reading is B-C gap & the rearward reading is B-C gap plus (any) endshake.

Any difference between the two readings is the end shake.

You have to do both checks to come to a meaningful understanding. If you do one without the other you do not have the whole picture or all the details.

.
 
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Bluedot37 has given excellent advice on measuring BC gap and end shake. A BC gap of 0.008" is acceptable and S&W considers 0.012" as the maximum, so you are fine.
 
So reading BLUDOT37's explanation my Cylinder Gap went to .008" from .004" by installing a cylinder shim (bearing). My question would be...wasn't the cylinder gap actually .008" to begin with ?

I thought Cylinder Gap had the effect of allowing ignition gasses to escape out the sides of the cylinder window until the bullet exits the barrel. On firing, the cylinder is slammed to the rear taking up any End Shake. If the main concern is escaping gasses, shouldn't the instructions for measuring Cylinder Gap include pushing the cylinder to the rear ?

The purpose (I think) for installing a shim, or stretching the rod, is to minimize the slamming of the cylinder to the rear when fired. But this does not actually change the Cylinder Gap, it simply changes the position of the cylinder at rest. I'm not real happy with the idea of trading End Shake for an increase in Cylinder gap.
 
You're correct on why one corrects end shake. Having said that, a certain amount of end shake is not only allowable but minimizes possible cylinder binding. If you used one 0.002" shim, you'd be in tolerance and having a smaller B/C gap.

The main concern about B/C gap isn't necessarily escaping gas, but making sure the cylinder can rotate smoothly while minimizing the escape of particulates.
 
I'm not a gunsmith so feel free to correct me if I'm wrong. I would think that to start this procedure you would want to place empty cartridges in the cylinder and then insert a shim of the correct thickness (I would guess ~.002") and then measure the barrel/cylinder gap. Pushing the cylinder rearward without cartridges could give you a false reading and shimming using this measurement could lead to the case heads dragging on the recoil shield. Close tolerances are nice but a gun that works flawlessly for an entire range session without cleaning is more important in my book.
 
WR Moore said:
You're correct on why one corrects end shake. Having said that, a certain amount of end shake is not only allowable but minimizes possible cylinder binding. If you used one 0.002" shim, you'd be in tolerance and having a smaller B/C gap.
Click to expand...

Installing the .003" shim moved the cylinder BACK when revolver is at rest. (springs pushing the cylinder forward) You get rid of End Shake by moving the cylinder Back which has the result of a LARGER B/C gap, with the revolver at rest. (not applying manual pressure to push cylinder forward or back but springs automatically push it forward)

The only way to use shims to move the cylinder FORWARD is to put them UNDER the extractor star which is a no no.
 
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Could he use a yoke shim to move the entire assembly forward slightly?
 
AJR337 said:
Could he use a yoke shim to move the entire assembly forward slightly?
Click to expand...

No...because the yoke would then hang up on the frame. A yoke shim would only be used to take up play, not to move the cylinder. I have no problem with my results after installing the cylinder shim. I only challenge the manner in which Cylinder Gap is measured.
 
What sets the head space or limits the rearward travel of the cylinder is the center portion of the ejector star. There has to be enough space so that the case heads will easily slid into place on recoil shield and tight enough that when fired the brass doesn't get much movement when it is fired and slams back into recoil shield. Placing brass in your chambers should not prevent the cylinder from moving to the rear any differently than when empty. You must have .001 or a bit more between the casehead and recoil shield when cylinder is pressed back or your cylinder will not turn smoothly when loaded. Your barrel to cylinder gap is the distance measured with cylinder pressed back. Endshake is the movement forward and back allowed by the yoke tube to cylinder fit. Pressing the cylinder forward when you have excess end shake and measuring the gap does not give you a true gap reading because of the excess end shake. What is important is the gap when you have an in spec end shake. But, while an end shake of .003 is not that excessive, but, it could cause some drag if you a very tight gap and gunk on you cylinder or forcing cone area.

Moving your yoke forward will not fix a bad B/C gap. All that will do is cause more endshake as the cylinder will need to move more to the rear to have center of star contact the center of recoil shield It will also case increase head space as the rear of the cylinder will move forward as firing pin strikes primer.
 
^^^^^ great post with good information. Cleared up some questions I had.

Many thanks.

Me thinks this thread is about played out.
 
series guy said:
Pushing the cylinder rearward without cartridges could give you a false reading and shimming using this measurement could lead to the case heads dragging on the recoil shield.
Click to expand...

As steelslaver already mentioned this is not needed nor correct. Rear cylinder gage, or gross headspace, is determined by the extractor collar's height/thickness.

If it's correct, then, with an unloaded case in the cylinder (& the cylinder held rearward), the shell head clearance should be ~.006" to the recoil plate. Optimally set endshake will only add .001" to that when the cylinder is free to move forward.
(The centerpin's spring is what pushes the cylinder forward in it's natural state.)

Excess endshake can also allow the cylinder to be too far forward which increases gross headspace & can lead to misfires.

.
 
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