99 cent Recoil Spring Tester

[shred]

I threatened to do this in another thread, but figured this engineering masterpiece deserved it's own..

Here's how to make your basic recoil spring tester:

Pop by your favorite hardware store and acquire an 8-inch 5/16" bolt and fender washer.

Drill a hole crosswise in the threaded end that will fit the hook of a handy pull scale (somewhat more than $0.99 at fishing-supply stores, but multi-purpose). If you've already got a pull scale that won't reasonably fit, you can do what I did and twist up a coathanger into an appropriate adapter wire.

Mark two marks on the shaft of the bolt. I made mine at 2" and 3.7" from the shaft-side of the bolt head. These numbers came from some measuring I did on an STI, and if I did it right, are approximately the length of the spring with the slide closed (3.7") and also at slide-lock (2").

Here's a picture of the parts:

To use it, slide a spring onto the bolt and follow it with the washer. Attach your scale and pull until the washer is even with one of the marks. Record what the scale says. Repeat with the other mark.

Here it is ready to go:

As an experiment, a brand new Wolff 15 lb variable recoil spring measured 8 lbs at "slide closed" and 13 lbs at "slide-lock". With one coil chopped off, it measures 7 & 13 lbs. Missing two coils, 6.5 & 12.5 lbs. With 4 coils gone, it comes in at 6 & 12 lbs.

(Edited by shred at 9:31 pm on May 13, 2002)

[Nolan]

Great idea.  I've got the SDM recoil spring gauge, but I like your blue scale MUCH better than the generic scale that it came with.

Now I'm all motivated to make a magazine spring tester, a firing pin spring tester, and a mainspring tester.

[warpspeed]

Shred,

OUTSTANDING!!!

Looks like I have a project for this weekend

[shred]

Yeah, the SDM tool is the "Pro" version of this. The scale I got in the fishing aisle at MegaMart. The recording feature is handy for this sort of thing.

And for anyone with only a push-scale like a bathroom or mail scale, you can make this work by sticking a bit of anti-skid tape to the bolt head, turning the whole works vertical and pushing down on the washer. I don't suggest doing this in the produce section of your local grocery store, however..

[dave pasos]

So what your saying is that if you have a used spring in the gun you can figure out how worn out it is at lock lock?   sounds cool!      But should a 15 lb spring at slide lock rate at 13 lbs?   Or should it be at 15lbs?    thanks

[shred]

While I'm not sure exactly how recoil springs are officially measured for "poundage", I believe it's at full compression, which is closer to 1.4" in length than the 2" I measured at.

A while ago I ran across This Article on building and using a recoil spring tester and just re-read it (it's worthwhile). In there he computes the maximum in-gun compression distance to be closer to 1.5 or 1.6" instead of 2". I may need to recheck my math and measurements..

(Edited by shred at 2:06 pm on May 14, 2002)

[Nolan]

You'll probably have to measure your OWN gun because I'm sure there is a lot of variation depending on who made your spring plug, what type of spring plug, slide and probably the phase of the moon.

The full compressed length in my Open gun is different than my Limited gun.   The Limited gun uses a collared reverse plug and my Open gun uses a shouldered reverse plug.

I measured the inside depth of each spring plug with the tail of a dial caliper and SUBTRACTED .10 for fudge factor.  At coil bind I make sure each spring is shorter than that measurement.  The collared reverse plug was a little shorter than the shouldered reverse plug.  

[uscbigdawg]

Hey Nolan,

Just read the article/threads below. I don't know if you've built this thing, but the scale is only like $25-30.

I'm hittin' WalMart tonight!!!

Rich

[Richard Jones]

Have you seen this?

http://www.brownells.com/aspx/NS/General/D...?f=bt002003.pdf

Rich

[uscbigdawg]

Hi Richard,

Thanks for the info. The SDM piece is what Nolan was referring to. It's like $60-75. The design that shred came up with is like $15 (including the scale, which in doing a little digging found one for $12).

Simple in concept and it makes sense as far as functioning.

Rich

[CenTX]

SDM has discontinued manuafacture of their Recoil Spring Tester and Brownell is out of stock. It looks like if you want to check your springs, you will need to make your own or borrow one.

Shred, you are welcome to borrow my SDM to test your springs or to check the measurements. I think Dawson has several as well.

Bob

[Kory]

While I'm not sure exactly how recoil springs are officially measured for "poundage", I believe it's at full compression, which is closer to 1.4" in length than the 2" I measured at.

The official way of determining a spring's weight is to compress it 20% and also to 80%, then average those two weights, then divide that by the distance you compressed the spring. The number is often given in pounds/inch

I think those numbers are relative to coil bind length(100% compression), your actual working length being the most important.

ie: you cut your mainspring and the DA is less. The poundage of the spring actually INCREASED by cutting coils, but the force it exerts at it's installed working length is less.

Gun springs must fall under a differnt rating system. Otherwise it would take 30+ pounds of force to rack the slide. Anyhow, direct measurement of them can only help.