1911 Mainspring tester

I'm doing a trigger job and put in an ismi 19 lb mainspring. Its MUCH lighter that the original colt. As a "test" I used a trigger pull gauge through the hole in the commander hammer. The colt took 8 lbs to get cocked. The ismi took 4...

So, anyone kmow a way to verify mainspring weights?

GI spec is 22 lbs at a compressed length of 1.312, 29.5 lbs at 1.062. Spring rate is 27.7 lbs/in.

So I'd make a guess you measure the load at 1.312.

Thanks for the info. I built a quick tester... the factory colt came in at 20 lbs. The ismi came in at 11... so, EGW is getting a call tomorrow...

rrpmi on Eslay has listed a mainspring tester in the past.  That is who I got mine from.

I bought a 19 lb mainspring earlier this year and it would hardly ignite anything. Don't recall where I bought it from though. I ordered a couple more from someone else and they were fine.

I just checked it and it measures right around 12 lbs on a Rimac tseter.

I believe I saw a post from Jon not too long ago regarding bad mainsprings that he had received.

It doesn't even look right as far as finish. Must have had the oven run away when in went in for the final bake after forming it.

I use nothing but 19 lb mainspring by Wolff in all my builds..never a problem with ignition. I do like the ISMI recoil springs.

Calculate it and be sure.

k = Gd^4 ÷ (8D^3 N)


k is spring rate in pounds per inch (the value we all talk about)



For G, use 10,600,000 psi (changes very little for all common spring alloys)

d is wire diameter
D is average diameter which is just (outer dia minus wire dia)

N is number of active coils (first and last coils are not counted)

If it got 'over-cooked' it will be soft and take a set after being compressed. So free length being noticeably different should be a clear indication.

OK, upon further investigation and finding a 19# Wolf spring to compare to, the wire diameter differs by a few thousandths. Apparently, the lighter spring that I received was improperly packaged as being 19# spring.

The first thing I do is replace the main spring. With arthritic hands a little hard to rack with factory. I replace with a 17lb.
I may be just lucky but they always go bang. Lighter than that and not so much.

Oh, Wolf is who I replace them with.

I'm guessing you could get a digital postal scale and start pressing down on it with the spring and see what it registers at when compressed.

mpolans wrote:I'm guessing you could get a digital postal scale and start pressing down on it with the spring and see what it registers at when compressed.

It's a tiny bit more involved, since you have to keep the spring captive.  If you want to go that route, here is a description of a method to try:

- Wear eye protection!
- Find a scale that can handle twice the expected weight.
- Find a Phillips screwdriver that can fit in the spring loosely and is a bit longer than the spring.
- Find a food can taller than the screwdriver blade length.  (It's more convenient if the can is empty of the food.%^)
- Put a hole slightly larger than the screwdriver blade (but smaller than the spring diameter) in the center of the bottom of the can.
- Make a cardboard test gauge the correct length of the compressed spring in its intended installation.  (This is not the solid length.)
- Place the can on the scale and zero the reading (or, write down the reading for adjustment later).
- Place the spring on the screwdriver and the screwdriver into the can.
- Carefully keeping everything square, press the screwdriver into the can until the compressed length matches your cardboard gauge.
- Read the scale (and, if necessary, subtract the can weight).

Like Ed said but the above is for recoil springs, same deal

Another way to build a spring tester....
https://forums.brianenos.com/topic/3920-99-cent-recoil-spring-tester/
May need a different scale for main springs