Don't want to take off the optic to measure. Pretty sure it's a 6-32 but not sure if it fine or course thread. Thinking fine due to the size. This is for the front holes. The rear is 8-32 I think?

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Regardless to how you answer my question, I don't know the answer to your question ~ sorry about that!!!

My question is which slide do you have?

I have the original P320 RX Slide which used the Romeo 0, then Sig moved to the Romeo 1 and then to the Romeo 1 Pro.

I wonder if the three different footprints might have different thread sizes???

Sorry, I'm only giving you something new to think about and not providing answers.

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X-5 Legion slide and a Spectra Comp slide.

6-32 and 8-32 are both coarse threads.

6-40 and 8-36 are the fine threads of those sizes.

The numbers 6 & 8 denote the nominal diameter.

The numbers 32, 36 & 40 denote the the pitch (threads per inch) of the thread.

Not an answer to your question, but pertinent info when specifying thread sizes.

Thanks for the info. Just called Sig 6-40 fronts(RMR) holes and M4 rear.

Anyone had a line on 6-40 screws. Very hard to fine. Why would Sig tap a slide with 6-40's and not 6-32 is beyond me. I could re-tap the holes to 6-32 I guess.

That will ruin the holes. It would wipe the threads out completely.

Depending on head style, you should be able to find what you're looking for at McMaster-Carr.

https://www.mcmaster.com/produ...ws/thread-size~6-40/

Thanks again! I sent a note to SIG and asked why and what they suggest - on a purchase.
Just order some from McMaster - completely forgot about them. Actually, have an old in-active account from the old days.

The factory 320 optic plate has M4 x 0.7 hex head (2.5mm) cap screws to retain it. Replace the hex head with a t-15 torx head fastener. Hex heads are misery as the hex pattern often strips during secondary removal.

Really never had a problem with stripping the heads on either. Just need good quality tools - only talking inch pounds of torque with these small screws. Thanks for the input!

All good to go. Here was my problem - had a broken screw from a Holosun mounting a 507C Vulcan. 10 inch pounds of torque and the screw sheared below the base. 15 inch pounds is the spec. Also the screws are too long and protrude into the extractor spring hole/channel. Took a measurement and 27/64 in length is perfect. MCMasters screws didn't fit right - threads were Ok - weird headshape from the factory. Will call them for a few spares.