From: bercov@bevsun.bev.lbl.gov (John Bercovitz)
Subject: Re: more questions
Organization: Lawrence Berkeley Laboratory

In article <36919@mimsy.umd.edu> ut-emx!lizard@emx.utexas.edu 
(Tom Linsley) writes:
#In article <36874@mimsy.umd.edu>, magnum@cs.umd.edu writes:
## 1.  Problem with misfires in a S&W 686.  Once in every dozen or so rounds
## a primer will fail to ignite.  (...)
## Jim
#You might want to check the tightness of the side plate screws.  I have
#seen them shoot loose, allowing the plate to move out slightly and inter-
#fere with the hammer.  Otherwise, inspect the hammer, looking for marks
#on the sides where it may be rubbing against the side plate or frame. 
#High spots are usually accommodated by stoning the frame or plate (not
#the hammer), or by installing shims (very expensive thin washers) to
#position the hammer properly within its slot.  A bent pivot pin (usually
#caused by forcing the side plate into position) can cause alignment
#problems.  Make sure the mainspring tensioning screw has not been
#loosened or filed down at the tip.  It is also possible, though unlikely,
#that the mainspring has pooped out (replacements are quite cheap).
#
#Tom

#[Reply:  Thanks for the scoop, Tom.  I've checked for rubbing, and also
  [stuff deleted]
#here is my latest theory:  The gun had been worked on by the previous
#owner, smoothing the trigger and (I think) slightly lightening the pull.
#Its sweet, except for this occasional misfire.  SO... is it possible that
#a lightened pull also translates into a lightened hammer drop by ever so
#little an amount (same spring, after all)?  If the drop is *just* off a
#little and the occasional CCI primer is *just* off spec by smidgeon, then
#the combination gets me a failure to fire.  Of course, this can *still*
#be a bad batch of primers having nothing to do with the gun, but it bears
#consideration.  Whad'ja think, gang, time to buy another mainspring and
#run another batch of rounds through?]

Excellent advice from Tom; here's some more details on springs:

There are lots of things you modify when you lighten up a Smith but when it 
comes to springs which affect trigger pull there are two - the one in the 
rebound slide and the one that runs the hammer.  When you lighten one by any 
of several methods, you have to lighten the other to "keep things in balance"
or else you get some more problems.  If your gun is lightened up too much
you may well need to replace both springs and start over.

Mainspring: To lighten this dude, you can screw the strain screw all the
way in and file a little off the end that bears against the spring.  This
is easy to spot because the stock screw is turned so it has a sort of
"bullseye" pattern of tool marks on the end of it from being lathe-turned.
If you've overdone this little trick, take a spent small pistol primer,
remove the anvil, flatten out the dent with a flat punch, and put it over
the strain screw like a hat on yer head.  If the "hat" is too thick, thin it
with a flat file.  Another way mainsprings are lightened is by draw filing
the edges to make the spring .015 to.040 narrower.  This is easy to spot
because the stock S&W spring has rough edges - looks like the sides were
sheared out.  To reverse this problem, replace the spring and start over.
The two methods give two different force curves.  Narrowing the spring has
the added advantage of lightening the driven mass.

Rebound slide spring:  You can replace this spring with the single-action-
only K-38's rebound slide spring but if you do a little figgerin, you can
buy a spring from an ordinary spring manufacturer that will do.  Also I'm 
sure Wolff et alii sell suitable springs these days.  You can recognize these
springs because they are longer than stock.  You can also feel them in the
gun because the trigger pull is "flat" - it doesn't peak up as you approach
hammer release.  Another thing that's commonly done is to cut a few coils
off the stock rebound slide spring.  This is easy to spot because the spring
is too short and because most folks don't know how to re-close that open loop
so it doesn't look like dog stuff.  Also, the trigger pull is _very_ peaky,
an effect I like but you may not - depends on how you shoot yer PPC - 
straight through pull (good when yer shooting a Colt snake) or pull up and 
squeeze off.  There are more spring things you can do if you change the 
camming between the rebound slide and the hammer and bevel the hammer block,
but these won't affect the reversibility of the spring conditions which
give you a too-light hammer fall.

I test hammer fall by firing primer-only 38 cases which are primed with
magnum small rifle primers.  I figure if they get lit, anything will get
lit.  The primer-only rounds will jam the gun unless you drill out the
flash holes.  Do that at your own risk - I mark the cases well when I do that.
If you don't and make a round with one, pressures will be excessive.
I use Federal primers in all my loads and I don't get failure to fire.

     JHBercovitz@lbl.gov    (John Bercovitz) 

From: gmk@falstaff.mae.cwru.edu (Geoff Kotzar)
Subject: Re: more questions
Organization: Case Western Reserve University

In article <36943@mimsy.umd.edu> bercov@bevsun.bev.lbl.gov (John Bercovitz) writes:
#In article <36919@mimsy.umd.edu> ut-emx!lizard@emx.utexas.edu 
#(Tom Linsley) writes:
##In article <36874@mimsy.umd.edu>, magnum@cs.umd.edu writes:
### 1.  Problem with misfires in a S&W 686.  Once in every dozen or so rounds
### a primer will fail to ignite.  (...)
### Jim
#
#There are lots of things you modify when you lighten up a Smith but when it 
#comes to springs which affect trigger pull there are two - the one in the 
#rebound slide and the one that runs the hammer.  When you lighten one by any 
#of several methods, you have to lighten the other to "keep things in balance"
#or else you get some more problems.  If your gun is lightened up too much
#you may well need to replace both springs and start over.
#
#Mainspring: To lighten this dude, you can screw the strain screw all the
#way in and file a little off the end that bears against the spring.  This
#is easy to spot because the stock screw is turned so it has a sort of
#"bullseye" pattern of tool marks on the end of it from being lathe-turned.


I miked some new mainspring strain screws and the lengths were: 

        SS:   .471 inch  (2)
        BLUE: .491       (1)


An "n" of three is not very large but if your strain screw measures below
the .471 dimension by very much you probably need to shim it as John
suggests or replace it.


#If you've overdone this little trick, take a spent small pistol primer,
#remove the anvil, flatten out the dent with a flat punch, and put it over
#the strain screw like a hat on yer head.  If the "hat" is too thick, thin it
#with a flat file.  Another way mainsprings are lightened is by draw filing
#the edges to make the spring .015 to.040 narrower.  This is easy to spot
#because the stock S&W spring has rough edges - looks like the sides were
#sheared out.  To reverse this problem, replace the spring and start over.
#The two methods give two different force curves.  Narrowing the spring has
#the added advantage of lightening the driven mass.


There is a second method for removing metal from the mainspring that thins
down the thin dimension. The advantage to this procedure is that less
metal has to be removed to effectively reduce the spring force. A cantilever
beam under a fixed displacement exerts a force that is proportional to the
cube of the thickness of the beam but only linearly related to the width. This
can be detected by looking for longitudinal striations along the spring. The
stock spring has a flat random "pebbled" appearance. Because less metal has
to be removed from the spring to effect the change it can sometimes be hard
to determine if the spring has been modified by thinning by measuring the
thickness but the appearance of file, grinding, or sanding marks is a sure
sign.


#Rebound slide spring:  You can replace this spring with the single-action-
#only K-38's rebound slide spring but if you do a little figgerin, you can
#buy a spring from an ordinary spring manufacturer that will do.  Also I'm 
#sure Wolff et alii sell suitable springs these days.  You can recognize these
#springs because they are longer than stock.  You can also feel them in the
#gun because the trigger pull is "flat" - it doesn't peak up as you approach
#hammer release.  Another thing that's commonly done is to cut a few coils
#off the stock rebound slide spring.  This is easy to spot because the spring
#is too short and because most folks don't know how to re-close that open loop
#so it doesn't look like dog stuff.  Also, the trigger pull is _very_ peaky,
#an effect I like but you may not - depends on how you shoot yer PPC - 
#straight through pull (good when yer shooting a Colt snake) or pull up and 
#squeeze off.  There are more spring things you can do if you change the 
#camming between the rebound slide and the hammer and bevel the hammer block,
#but these won't affect the reversibility of the spring conditions which
#give you a too-light hammer fall.
#
#I test hammer fall by firing primer-only 38 cases which are primed with
#magnum small rifle primers.  I figure if they get lit, anything will get
#lit.  The primer-only rounds will jam the gun unless you drill out the
#flash holes.  Do that at your own risk - I mark the cases well when I do that.
#If you don't and make a round with one, pressures will be excessive.
#I use Federal primers in all my loads and I don't get failure to fire.
#
#     JHBercovitz@lbl.gov    (John Bercovitz) 

Excellent information.

Geoff Kotzar

From: gmk@falstaff.mae.cwru.edu (Geoff Kotzar)
Subject: Re: 686 update
Organization: Case Western Reserve University

In article <37070@mimsy.umd.edu> magnum@cs.umd.edu writes:
#For those of you have followed and donated ideas on the 686 repair saga,
#here is the status as of this weekend:
#
#1. The strain screw mic'd to .564 (dear earlier poster ... did you really
#mean .471 was the norm for those you checked? What's a .1" among friends...)
#
#Stay tuned ...
#
#Jim

Yes, the number I measured was .471 inch. S&W uses two different mainspring
strain screws: one length for the "K"-frame and another for the "N"-frame.
My understanding is that your "L"-frame gun has the same grip frame as the
smaller "K"-frame. That was the number that I thought I quoted you. Sorry
about any confusion. The other strain screw that I had was indeed about
100/1000's longer.

Geoff Kotzar

[Response:  Ahh, makes sense.  I might just pick up a new strain screw to
start with anyway, can't be all that expensive.]