Lifespan of a 76/760 Recoil Spring

[Jim in Texas]

The recent discussions about spare parts got me to thinking about recoil springs. For many firearms, I see replacement intervals for recoil springs that are based on either:

1. round count (i.e. replace the spring after x number of rounds); or

2. length of the relaxed spring (i.e. replace the spring if its relaxed length is not at least y inches).

I have not, however, ever seen a definitive replacement interval for 76/760 recoil springs.

Does anyone happen to have an old Smith & Wesson armorer’s manual that covers the Model 76 and that specifies when the recoil spring should be changed? Assuming no one has such a manual, how often does everyone change their recoil spring and why?

Thanks

 

[The Dude]

I think the reason there are no reply's to this post is there is no hard and fast rule to your question. There are many factors regarding the wear of the recoil spring such as type of ammo that has been used, has the gun been suppressed, age, temps etc. Regarding the S&W 76 which is an open bolt blowback design, I personally use the following method. You want the bolt to travel rearward enough to be caught by the sear when released otherwise you will get runaway. You do not want the bolt traveling all the way back and slamming the rear of the tube/receiver either. I like the bolt to travel halfway between the point at which the bolt is held by the sear and all the way back. You can observe this by filming with your phone. The iphone can do it in slow motion. If you are experiencing feed failures you might want to let the bolt travel a little farther back but still not all the way. I would buy a S&W76 extra power spring from Wolff springs and see how it performs using the phone camera method. That spring may work just fine as is. If the spring is too strong you can trim one coil at a time to achieve desired operation. Ammo is the variable. Pick an ammo and tune the gun to it. I use Winchester 115gr White Box, it seems consistent. Disclaimer, I am no expert but this has worked for me. I used this method to tune my Subguns .22 conversion kit. Initially I was having light strikes but after tuning the bolt travel with a spacer it works great. I had an early kit that did not come with the "break-in Spring".

 

[Jim in Texas]

Thanks for the response. I have been using the same criteria for bolt travel that you describe, but have been measuring it in a more old-school manner, using masking tape over the cocking slot and observing how far back the tape is torn by the bolt handle during firing. This has served me well, but I do not do it every time I go to the range.

I have probably gotten spoiled with military technical manuals that give a specific free length for recoil springs. For example, as I recall, the recoil spring for a 1928 Thompson should be 10+0.25 inches. I was hoping that S&W had provided a similar number that users could check at the cleaning bench or in the shop.

Does Wolff offer an extra power spring for the 76/760? I have only seen one model of spring from them for the 76/760, which I assume is supposed to be factory specification. Although that spring uses the same diameter wire as my MK Arms factory springs, it has 1-1/2 fewer coils than those factory springs. That makes me skittish about the prospect of cutting any more coils off of that particular Wolff spring.

 

[The Dude]

Thanks for the response. I have been using the same criteria for bolt travel that you describe, but have been measuring it in a more old-school manner, using masking tape over the cocking slot and observing how far back the tape is torn by the bolt handle during firing. This has served me well, but I do not do it every time I go to the range.

I have probably gotten spoiled with military technical manuals that give a specific free length for recoil springs. For example, as I recall, the recoil spring for a 1928 Thompson should be 10+0.25 inches. I was hoping that S&W had provided a similar number that users could check at the cleaning bench or in the shop.

Does Wolff offer an extra power spring for the 76/760? I have only seen one model of spring from them for the 76/760, which I assume is supposed to be factory specification. Although that spring uses the same diameter wire as my MK Arms factory springs, it has 1-1/2 fewer coils than those factory springs. That makes me skittish about the prospect of cutting any more coils off of that particular Wolff spring.

My bad, Wolff no longer calls that spring "Extra Power". That is a term they use on some of their springs and they used to on the S&W76. I have not used the tape method to measure the bolt travel, will give it a try. I have also put grease on the bolt behind the charging handle to see how much is rubbed off.

 

[The Dude]

This discussion has got me trying to recall that several years back there was someone working with Wolff to make a recoil spring with proper specs. I wish my old brain could recall the details. Maybe this is why they do not call it an "Extra Power" anymore and refer to it as "original factory specification"?

 

[Jim in Texas]

As I recall, it was PK who headed up the effort to get the proper specs to Wolff to duplicate the factory recoil springs. The factory MK Arms springs have 60 coils, but the Wolff springs have only 58-1/2 coils. I wonder where Wolff got that number. Could the original S&W springs have been slightly shorter/weaker than the MK Arms springs? The 60-coil springs have worked well for me.

On the masking tape method, I find that the inexpensive tan-color tape works better than the more expensive blue tape. The tan tape adheres well to the receiver and tears without pulling off of the receiver, so it is easier to get an accurate measurement.

 

[The Dude]

Well Jim, I started off replying to your post to help you with recoil spring tips. It seems after all you are very knowledgeable and ended up schooling me on the subject!

 

[Slowmo]

I asked a similar question related to Uzis, and member K2 had a pretty clever way to take relative measurements. His method was to fix a suitably sized dowel rod (wooden, brass, etc.) in a vertical position and then slide the barrel down over the rod until it presses against the bolt face with the bolt closed. The weight of the gun should push the bolt back away from the barrel by some distance, which can be measured over time.

The distance itself doesn’t tell you anything, but if you have a brand new spring and a worn out spring, you can take measurements with this method and then use those as your benchmarks to determine where a spring is in its lifespan. Likewise, you could take periodic measurements with the same spring to see if it is noticeably weakening.

 

[Jim in Texas]

This is proving to be a very educational thread for me. I have learned both new techniques and new things to consider when thinking about recoil springs.

 

[Jim in Texas]

Slowmo –

As an FYI, I just tried the weight-of-the-gun technique on a 760. It did not push the bolt back at all.

It appears that technique only works on guns with a certain ratio between the weight of the gun and the strength of the spring. The 76/760 appears to be too far out from that ratio for the weight-of-the-gun technique to work on it. (As I recall, the Uzi is heavier than a 76/760 and its recoil spring is significantly smaller in diameter.)

The underlying principle of measuring compression under a specific load should still be a useful tool, you would just to do something like adding additional weight to the back of a 76/760 for it to work.

 

[Slowmo]

Could you tell if it was close to pushing it back at all? Any idea if your spring is closer to the new or worn-out end of the spectrum?

 

[Jim in Texas]

It did not feel like it was even close to pushing back the bolt. I had to push down on the back of the receiver to get the bolt to move back at all. As soon as I started to release that pressure, the bolt firmly pushed back all the way to the closed position.

This is a factory MK Arms spring with about 5000 rounds on it. The 5000-round number is what got me thinking about the expected lifespan of those springs - I have seen references to Sig and Glock recommending replacement of their recoil springs at 5000 rounds.

The relaxed length of this spring is approximately 1/8" shorter than an unused MK Arms spring, but I am not sure of the significance of that measurement. I have not done the masking tape test in a while, but I have not seen any evidence of the bolt traveling back all the way to the flange on the recoil spring guide or the rear receiver plug.