It isn't often that I take issue with Lex. His advice is spot on.
Yet, as I slowly transition from a "big bore pistons are simpler and better than complex diaphragms" guy, I'm starting to flesh out why.
My first reg was the Mk10. I'm too young to have started with the Mk5. And I watched the growing pains of the Mk10+ as the knife edge piston was replaced with the same rounded end piston still used in the Mk25.
But one thing I consistently note over the years, which has been the subject of more than one thread, is the "mushy lockup" often displayed. Like this thread: Thread Unexpected IP drift in a newly-serviced first stage
As @James79 points out, small imperfections between seat and piston end allow a leak and increasing IP until there's enough pressure on the seat to finally seal it. That was a huge problem for the Mk10+, with its small piston head.
While an old Mk10 has a delicate sharp knife edge that is easy for ham-fisted techs to damage, the one thing a good knife edge does well, is seal crisply. And the Mk25 and Atomic have been the butt of well-deserved criticism for the results of the attempt to make the piston less delicate: it just doesn't seal as crisply. At least, not for a whole two-year service interval.
Get them both, @Timmyjane ! Play with their differences as you restore the two pistons. The techniques are different. You'll see the advantage of the Mk25 in easy tuning and easy disassembly. You'll see the disadvantage of the Mk10 in now-hard-to-find-parts. But a good sharp-edge piston sure does seal up crisply!
None of the lockups last as long as a diaphragm, I'm coming to learn, but diaphragms have a host of compensatory problems. There will always be two camps.
Sounds like you need to participate in our Reg Tech Seminar. I'm still doing them every six weeks or so, and our graduate list is up to 120 people from the US and around the world. But the waiting list is about six months long, I'm afraid. Now that I've retired from real work, perhaps I can trim that down. But it's frustrating to keep folks waiting, and I hesitate to even mention it. Still, you seem like the ideal candidate.
Hey, @lexvil , you're right! The Mk25 is just as cheap to buy and a lot more fun to work on. Certainly easier to restore the piston. But there's a hidden jewel in the Mk10 that's worth playing with. I didn't mean to pick on you.
. I might have a problem. 
