BC Gap & Headspace

[K Harris]

A center fire Smith & Wesson revolver with a recessed cylinder has a BC gap of .008 with cylinder end-shake of less than .001. The headspace is approximately .012. My question is, would it be better to install a .002 yoke bearing and reduce the BC gap or leave it with the .008 BC gap and the .012 headspace? Keeping in mind that the cylinder would still not close on the field gauge with the yoke bearing installed. In the hierarchy of tuning what is more important the bc gap or the headspace, if they are both within spec either way?

 

[armorer951]

Installing a shim will move the cylinder back and increase the barrel/cylinder gap by the thickness of the shim. IMO, your headspace and endshake readings are ideal. With your current spec, you couldn't install a .002" shim.

Headspace is far more important than the front gauge, especially with respect to reliability. (my opinion)

If it were mine, I would leave it be. (also my opinion)

Note the date on the spec chart.....I believe the factory accepted front gauge spec is now .004" - .012".

Carter

 

[Alk8944]

Armorer 951 is absolutely correct. All he didn't say directly is if you try to install any yoke bearing it will result in hard closing and cylinder binding! Leave it alone.

 

[stansdds]

You already have the minimum end shake, so I would recommend leaving it alone. Revolver cylinders need a little bit of end shake in order to prevent binding when the cylinder is locked into the frame.

 

[K Harris]

Ok. Are you guys saying that if I installed a .002 yoke bearing that I wouldn't end up with around .006 bc gap, .014 headspace and continue to have less than .001 cylinder end shake?

 

[armorer951]

Yes sir, that's correct. The end shake shim will move the cylinder to the rear not forward. You don't need a tighter headspace. You're currently at the tight end of the spec, at .012", which is good.

As a member said in an earlier post, some end shake is required, and built into the cylinder/yoke assembly so that the cylinder can open and close, rotate properly, and to provide room for lubricant and fouling. (.001" - .002")

In your current scenario, you can't install a .002" end shake bearing. There's no room in the assembly for it.

Carter

 

[K Harris]

I think we are talking cross purposes. I am talking about a yoke bearing, not a cylinder bearing. With the cylinder having just less than .001 end shake there is no way a cylinder bearing can be installed and still have the revolver function. Unless you trim the yoke barrel face with a piloted face trimming reamer. Doesn't a yoke bearing just move the whole assembly forward?

 

[armorer951]

This is called "end shake on yoke" by the factory. In an assembly using the old design piloted yoke screw, you could shim the yoke forward if there is gauge between the piloted yoke screw and the yoke button. The new, spring assisted yoke screw design and change in the end of the yoke stem eliminates the development yoke end shake, in terms of gauge that would, or could develop between the yoke stem button and the piloted yoke screw in the old design.

Shimming the yoke would move the entire cylinder assembly forward the thickness of the shim installed on the yoke stem, and, in turn, close the front gauge or b/c gap by that amount. This would also result in the headspace and the end shake measurement opening up by that same amount. In your case, shimming the yoke forward by .002" would result in approx .014" of headspace and .003" of endshake.

Frankly, I can't think of a single scenario where you would want to remedy a front gauge deemed "excessive" by doing this. Especially when the gauges you gave are all in spec. Headspace could be adversely affected, which would result in reliability issues. (misfires) To remedy this inevitable result, you would then have to install a shim in the cylinder yoke cavity on the end of the yoke barrel, or peen the end of the yoke barrel back.

The factory repair for excessive front gauge over .012" would be to remove the barrel and set it back in the frame by one revolution (.027") and then relieve the excess material from the barrel extension down to the front gauge desired (.004 - .012") and re-cutting the forcing cone to the correct angle and depth.

To me, the scenario you proposed in your opening thread represents almost "perfect" gauges in the cylinder assembly. Front gauge at ,008" is not a big deal in terms of the proper functioning of the revolver. As a matter of fact, front gauge on carry and service revolvers should be a bit larger than the minimum recommended .004", to accommodate fouling and debris which can form and build up on the front of the cylinder around the chamber holes, which could tie the cylinder up and prevent rotation.

If the end shake you quoted at .001" is on the yoke itself, (old style yoke) then this would be corrected by peening the yoke button to remove this gauge in the interface between the yoke screw and the yoke button. Here again, a bit of gauge or space is required at this interface as well, between the yoke button and the pilot at the end of the yoke screw, so the yoke can open and close properly.

 

[merl67]

Don't mess with it especially if function is good, and I suspect with your numbers it should be.

 

[K Harris]

Ok. I think I've got it now. There is no end shake on yoke. I have been rereading this section in Kuhnhausen's manual. Unless it is in another section he does not say that using a yoke bearing will increase cylinder end shake, or more likely, I just misunderstood. He does seem to only talk about yoke bearings in relation to yoke end shake, and the section on making the yoke right and correct comes before making the cylinder right and correct. Thank you armorer951.

 

[Protocall_Design]

There is another way to deal with all this, (Warning, Will Robinson!) although it's not on anyone's list of approved way to do things. I have done it many times with good results.

Most people think the only way to get a tighter cylinder gap is to turn the barrel back a thread. It's ONE way. Another way is to put the cylinder shim under the extractor instead of inside the cylinder tube. In effect, you are stretching the cylinder, something that we all know is impossible.

This has several benefits.

#1- You maintain the same headspace. That is set by the round bearing surface in the middle of the ratchets.

#2-You move the cylinder forward by the amount of the shim.

#3- If there is endshake, it takes care of that at the same time. If there is not endshake, you trim the end of the crane barrel by the amount of the shim.

#4- The shims are very thin and often don't last all that long, due to the constant rotation and pounding of the cylinder on them. This way, the shim turns with the cylinder and lasts a long time.

#5- It creates a little space between the cylinder and extractor so the gun isn't as finicky about binding up with debris getting under the extractor from when you eject spent cases.

#6- This can easily be undone - it is not a permanent modification.

Remember- this is not an approved mod, just something I came up with that works. All the flamers can dive in now.

 

[armorer951]

Is there any reliable way of doing this? Sounds like an additional possible remedy if it could be installed properly.

I've experimented with this adaptation, but I could never get a shim that was held in place around the extractor stem to stay properly in place because of the location of the alignment pins. The extractor alignment pins are "in the way" around the perimeter of the shim and will not let the added shim to lay flat.

I'm wondering if having the shim under the extractor could cause reliability issues? Seems as though the shim would act much like debris often does in that location, and cause light hammer strikes because of the offset in the extractor.

 

[Protocall_Design]

I'm doing the newer ones that don't have pins. Haven't tried an older style yet, so hadn't encountered that problem.

You might be able to cut a small V notch for each pin in the shim with small scissors and some super glue. I don't know if that would work or not.

Or maybe make a bit bigger one out of shim stock and push it down over the pins, letting them make the holes, then cut out the center hole with an Exacto knife. Maybe trim around the pins with the Exacto knife, as well.

 

[steelslaver]

The early cylinders have the pins right in ratchet area and those would interfere with pins. later they were moved out to the arms and those should clear. Then they went to the odd end on arms method

A thought. A series of punch marks around the center of cylinder recess for extractor. The punch makes a dip by displacing metal up around outside of mark.

I have now got a couple jigs and experience and can take .0277 of the back of the shoulder and have a whole new starting point for B/C and forcing cone pretty fast, so that is what I would do if the gap was too big.

BUT

I would leave the OPs gun alone. Barrels by the Inch set up a Uberti 1873 Cattleman Single Action Revolver in 357mag so they could set the barrel to be flush against the cylinder, then .001 B/C then .006 B/C and tested different length barrels set at those gaps

With a 6" barrel and Federal 158gr Hydra-shoks they got 1359fps with no gap, 1304 with just .001 and 1290fps with .006. You can not have a reliable revolver with no gap so dump that. But notice that 55 fps was lost with first .001 gap. The difference between .001 and .006 was only 14fps. I would bet over 1/2 of that was in the first .001-.002 but even on a average it is only a bit over 2fps per .001 of gap. Even the 14 fps between .001 and .006 is withing the majority of ammo's extreme velocity spread.

Go here BBTI - Ballistics by the Inch :: Cylinder Gap
click on the ammo of your choice
when page with graph appears scroll down, pick the length matching your barrel and see their actual velocities with 000, .001, and .006 B/C gap

I doubt that closing the B/C gap from .008 to say .005 will gain you 7 fps or any noticeable accuracy improvement even using a Ransom rest. A waste of time effort and concern IMHO