S&W 640 Timing

[roklok]

Hello everyone, new poster on this site. I am looking for a little experienced input. I have a S&W 640-1 that has a timing issue. On 3 out of the 5 chambers, the hammer falls before cylinder locks up. The two chambers that lock up before hammer falls are next to each other (subsequent).

Is this indicative of a hand problem or an extractor problem ? It seems to me that if it was a hand issue, none of the chambers would be timed right. I am wondering if a new extractor assembly would solve the problem.

Another question, where is a source of oversize J-frame hands ? Oversize hands are available from numerous vendors for bigger frames, but not for the J-frame. I did find an oversize J frame hand at Numrich, but received it today and it is a POS. Looks like it was made by cavemen by shaping it with rocks. I think a better example would come out of Khyber Pass. The pins are crooked, and it is actually smaller than the original hand. I installed it, and none of the chambers locked up.

Thanks in advance for any input.

 

[BBBBill]

Could be either or both. Could also be a slightly tweaked yoke contributing to the problem. Could be an enlarged hand window in the frame. Do you have Kuhnhausen's S&W book? If not, I recommend that you get it as it will explain the operation and interaction of the parts in detail.
Some heat and offset swage the tip of the hand. Ron Power used to make a bench block that had that feature. I have one, but don't remember whather it will accomodate the J frame hands. Some heat/bend the hand in the middle. The ratchet pads can be swaged (with proper tooling) to move a little metal. I use the old Jack Gutridge tool for that. Any of the above should come only after verifying the source(s) of the problem and correcting each as appropriate.

 

[Jim K]

How are you testing for lockup? Many revolvers won't fully lock up if the cylinder is held back forcibly as some "experts" advise. Does it fail to lock up when fired normally, or is it so far out that the firing pin strike is not centered in the primer?

That gun could be worn out, but it only dates to 1996, so it would have had to have been shot a lot in that period.

Jim

 

[roklok]

Thanks for the replies. I test for lockup with no load on the cylinder whatsoever. I have owned this revolver since September of 1995, and it has seen a fair bit of use with magnum loads. I own a second 640-1, and it positively locks before hammer drop on all chambers. I am concerned that I may crack the forcing cone if the issue is not fixed. I may just end up shipping it back to S&W, but really would just rather fix it myself if at all possible.

edit: I just examined 50 spent primers fired in this revolver, and approximately half were hit off center by the firing pin.

 

[roklok]

Just wanted to let those interested know that I fixed the timing problem in my 640. My suspicions were correct, the problem was the "star" on the extractor assembly. I closely examined the individual "hooks" on the "star" and was able to predict which chambers would lock up by looking at the star. There were three hooks that were visibly shorter than the other two. The side of the hand pushes against these hooks to rotate the cylinder the last bit for lock up.

I bought a new extractor from Brownells and installed it today. It fit the cylinder perfectly, no fitting required. The individual hooks on the star needed filed to lock up correctly. I took my time, filing with a needle file until I could pull the trigger until hammer drop with no resistance. I did not force the trigger, at first the trigger would stop well before hammer drop. After I got a hook fitted, I would mark it with black marker and move onto the next one. It took me about two hours to file all five hooks for correct lockup.

I am including some pictures of the extractor assembly with three of the hooks fitted (the ones with black marker on). My file is pointing to the tip of the hook that needed filed. The empty casings are in the cylinder to fully support the extractor to ensure it is in the proper position for timing.