Yep, not one to throw cold water on a deal, but . . .
Sizing the seal can be done, and even if you end up buying 2 or 3 before you get it right, it's not like seals cost an arm and a leg. However, don't plan on dropping an R9 spring and custom guide into a B26 with a narrow ID tube. I tried that and failed. The R9 custom tune kit spring guide has a metal bushing on the end opposite the piston that is too large OD to allow the spring to be compressed and put back in the gun. Ooops. OK, well, lets carefully size that down until it fits . . . No joy. Even though I could compress and re-assemble the B26 with the re-sized bushing, the problem was that, in compression, the OD of the spring expanded just enough to bind and grind inside of the piston. When I reassembled and tried to cock the gun "crunch". So, Crap! Out comes the R9 spring and custom guide, now with an undersized rear bushing on the guide that will be sloppy if I try to drop it in my Beeman R9. I had to go to the lower power spring and the stock guide. OK, but not what I originally had in mind. So, I was 3 tries getting the right size seal and two tries at the right spring before I could get the gun back together, and then with a different spring and a loose stock spring guide
Well, I said I wanted to have a learning experience with the BAM B26, and I got it! It does shoot OK now, but I can entirely understand why Jim M can't offer a standard "kit" for the gun, then be expected to suffer the slings and arrows of dissatisfied customers when it doesn't fit. Life is too short for that stuff. And the good guys who make springs and do the custom tunes aren't getting rich doing it to begin with.
So, here's a suggestion: if you're a rookie and you want to consider working on your own air gun, the first quality tool to buy is a set of digital calipers. If you measure the exact size of the tube ID, factory seal OD, piston OD, spring OD and ID, and guide OD and accurately communicate the size info, you have a much better chance of getting it right parts the first time. But when the manufacturers significantly change the size and tolerances of their airguns frequently, It's not reasonable expect the Jim Maccari's of the world to jump to change with them, or to refund your money on the used parts if you have an odd size gun and screw something up.
