pretty straight forward....degas the gun, remove the plenum gauge on left side, check the o-ring to make sure its good, put the gauge on the end, then just using appropriate size allen wrench, tighten the whole plenum on to the gun. I reused the o ring. I recall it seemed that I felt like I was over tightening but that was just me. Regas the gun. You'll know pretty quick if its tight enough or not.
I put it back on the gun side between the plenum and the gun and didn't use one between the gauge and the plenum....maybe would be good to put some teflon tape around the gauge threads. Im pretty sure the only leak I had initially was from the rifle side and not the gauge though so I just tightened the plenum down a bit more.
I was looking at getting the side plenum but you couldn't get them in the states at that time and it would not fit in the classic stock with out modifying the stock in a very visible location. Being fairly new to PCP's myself I opted for the "out the bottom" version which was stupid easy to install as well and the stock was only modified on the underside and you cannot really see it, it turned out pretty good tho...lol.. If its the side plenum it should be as simple as degassing the gun, unscrewing your reg gauge (shouldn't be very tight) and then installing the plenum in that hole. it may need indexed...if your version can be, in which case thread it in till it stops and then un screw till orientated properly then tighten the indexing nut. Then your new gauge simply threads into the hole facing you.... Most everything should be hand tight or just "snugged" up a little. Hopefully this helps you at least a little bit...lol. If not I think both sellers, C88 Machining and Alien airguns and bargains on ebay are both now selling those/similar side plenums and may be able to help you as well.
