I purchased a Wolff spring kit.

The factory equivalent spring is 1/3 inch longer.
Should replace the current spring?

The kit also has a 16, 17, 18 & 20 pound springs.

When would one want to use the heavier springs?

Thanks,
MartyD

__________________
Who are? "They"

The factory recommends replacing the recoil spring at 5 year or 5000 round intervals.

If you don't know the age of the spring, it can't hurt the pistol to replace the recoil spring with the factory original or its equivalent.

If the current spring only seems to have compressed 1/3" from the factory equivalent and the pistol is operating fine you can wait to replace it.

Heavier (or lighter) recoil springs are hardly ever needed unless you are shooting some remarkably heavier or lighter hand loads.

With standard ammo, use a standard spring.

John

OEM factory springs for the model 39 (14 pound rated) are available, oddly enough, from the manufacturer, in this case Smith and Wesson.

Many vendors, including Midway, also carry OEM springs for many firearms, including the model 39......Part # 06048

Smith & Wesson Recoil Spring S&W 5903TSW 5906TSW 5943TSW 39 3904 3906

__________________
Ret. LE, FA Instr, S&W Armorer

The only figures I've ever seen for slide forces in operation are for the 1911. The figures are from, of all sources, Leupold as someone mounted a scope on a 1911 slide and they got curious about what kind of loads are involved.

On a stock 1911 the shock load when the slide is in full recoil is 700 Gs. The force going into battery while feeding a round is 750 Gs. So installing a heavier than stock recoil spring will decrease the recoil force while substantially increasing the force going into battery. Which, in the 1911, is absorbed by the barrel under lug and slide stop. Some other designs will use either the slide stop or a take down latch and the barrel under lug to absorb the force of the slide going into battery.

After watching several slide assemblies go down range after under lug/slide stop failures, I've often wondered why people think they're smarter than the designers.