Recoil Spring Question

[JimFromFL]

Over the past few years of gathering spare parts I have purchased a few Recoil Springs and just placed them in the spare parts box.

 

I happen to look and noticed something and thought about posting here for an explanation.

 

When looking at the very end of the spring, the end most tip is usually touching or close to the 2nd coil. And you can tell when a spring is cut as there is a gap between the tip of the spring end and the 2nd coil. (I can post pictures if need be.)

 

All the springs look the same meaning the coil distances and thickness look identical.

 

We know the manuals say spring length should be (10+.25 inches) or 10.25 inches.

 

I have one spring that is 10.5 inches with both ends of the springs touching the 2nd coil and looking like brand new springs.

 

I have about 6 other springs that are about about 11-12 inches in length and these all look like they have been cut as all ends have a gap between the ends and the 2nd coil.

 

How to tell which are "real" thompson springs and which are "just" springs?

Is the 10.5 inch spring a "real" Thompson spring?

Are you supposed to use any spring but just replace once they become less than 10.25 inches?

How do you tell when its time to replace the spring?

 

[PK.]

In spring parlance, the end is “closed” if the last coil is lacking the helix and presents a more or less flat surface against whatever it is pushing. “Open” ends lack this and appear to be cut from a continuous helical length (which in fact they are, as they run off the machine).

 

I don’t think I have ever seen a spring that was traceable as a military part with closed ends.

 

That said, closed ends are generally better

 

It has been said before- the number of coils is as important as the length of the spring; an 11.5 spring with the proper number of coils is alright- it will shorten with use.

 

[JimFromFL]

What is the proper number of coils?

And since some springs can look the same but be weaker than others, how can I tell these are actually for the Thompson?

[Dave Janowski]

SARCO has closed end recoil springs they are GREAT (I made them) I would strongly suggest them ask MG dave for them I have them in my guns and the price he asks is pretty good!

 

Dave

[Hawkeye_Joe]

QUOTE

We know the manuals say spring length should be (10+.25 inches) or 10.25 inches

 

This is a major misunderstanding when reading the manual. The manual and training films say that the spring should not be any less than 10.5 inches.. not that this is the length it is supposed to be or the length it has to be. If it gets to be less than 10.5 it needs to be replaced.

[PK.]

The GI springs I have counted have 69-70 active coils. Some of the aftermarket springs have up to 74, but the closed ends aren’t active.

 

I think the reason they listed overall minimum length for inspection in the military manuals was they never expected a serviceman to cut a spring, and they sure didn’t expect them to count the coils!

 

It would be unusual to find a spring for anything else that fell within the parameters of this part. If it has all the basic features (length, coils, wire diameter & OD) of a Thompson spring, then it likely is.