I got a hold of some Pachmayr Signature grips for my P226-Mk 25. I tried to install them but discovered that the backstrap didn't seem to fit properly. I was looking around the internet for a vid or write up on how to install but couldn't find anything. Though discontinued, the grips themselves can be found for sale pretty easily (sometimes with backstrap, sometimes without), but not an instruction manual. The packaging I have says it is version "April 89". A date instead of an actual number, interestingly enough. So I'm not sure if these are supposed to ony work on SIG frames from that time or any P226.

Has anyone installed these on their P226 before and can give some explanation on how to put these on?

First off the grips were designed for the ARCHED trigger bar return spring not the COILED trigger bar return spring which came along later. This will effect the right side of the grip. While the right side may show to be sitting flat against the frame it will be binding the coiled spring and most likely cause the trigger bar to be restricted thus not allowing the trigger bar and trigger free movement.

Secondly the back strap piece was designed for the older style all metal hammer strut assembly not the newer two styles of plastic based hammer strut assemblies. The back strap piece can most likely be modified for the newer style plastic based HSA.

Your problem will be with the use of the coiled style trigger return spring. The metal which is embedded in the grip halves doesn't allow for easy hollowing out to gain clearance for the coiled style trigger bar return spring. While possible to hollow clearance out it would be a pain and risk deforming the metal and rubber as well as burning of bubbling the rubber from grinding the metal embedded in the rubber.

If you are really determined to make these grips work first remove the trigger bar return spring and hammer strut assembly then mount the grips and back strap. Make sure the old hard rubber will still flex enough to properly allow the grip screws to seat. The older p226 used a grip screw frame bushing similar to what a 1911 uses which would have been a shoulder or rise which would have held the grip sides in place with the front strap rubber stretched without screws being installed. By not having the grip screw bushings to hold and center the grips around the grip screw holes you risk cross threading and stripping out the screw threads in the frame. The aged hard rubber material also makes stretching and holding in place while starting and securing the screws harder as well. Also the older hard rubber has been known to split on each top side of the front strap material where it touches the radius at the top of the front strap to trigger guard.
Often though if the grips were used and were installed for years the needed stretch will be done and set which may make mounting easier and rubber splitting less likely. But if your grips are NOS that were never mounted then getting the old rubber to stretch without splitting and to gain proper screw hole centering can be a pain.

If after test fitting the grips without the trigger return spring and MSH assembly installed and you like the grips feel and fit and the screws can be installed easily then re-install the removed parts and proceed with fitting. Going with the old style ARCHED trigger return spring would simplify the side issue but be aware that the older style arched spring were known to break so replace them after several thousand cycles.

I personally have tried the Pachys on several P6/225 & 226 and didn't care for the grip texture or the added weight of the grips and in the end went on to the old factory grips or for me the absolute best grips ever made the black Hogue G-10 Piranha texture or the vheckers for a more traditional look.

Good luck and let me be the first to welcome you to Para’s SigForum!