I have a P320 Max 9mm. It has the 1911 style recoil spring. Can’t find any info in the manual for which end to put on the rod first. All 1911 manuals say to put the closed end of the spring onto the rod. Called Sig and Mike said put the open end of the spring on the rod. I’m thinking if it’s a 1911 spring I should put the closed end on the rod.

The guidance about Classic line SIG recoil springs (P220, P226, …) was always to insert the guide (rod) into the tighter end of the spring so the tight end of the spring ends up at the back of the rod. If we think about that, it makes sense: If the tight end were at the front of the rod, it could be tight enough to interfere with the free movement of the slide, including when returning the slide to battery, and the additional friction of the tight end’s movement could cause more wear on the rod.

I cannot answer your question based on direct knowledge, but without that I would insert the rod into the tighter end of the spring.

^^^ Yes...tight end goes on the rod.

Have used 1911's for decades. Have a P320 FS .40 and GGI full guide rod for 1911 mainsprings (hammer springs). Same as 1911's, tight end on rod.

BTW, Gray Guns developed the combo only for 9mm. He considered the P320 FS 9mm "over sprung". The combo works with the .40 also, you just use slightly different mainsprings.

Welcome to the forum.