I am an avid mk5 / mk10 diver.
I know in theory they are not fit for 100% O2, but I O2 cleaned my mk10's and use them with up to 80% oxygen on my stages. I have disassembled them many times and have yet to see either increased degradation or soot because of compression fire.
The tools i use are super basic but help alot in my opinion.
cheap brass o-ring picks:
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I use a bullet tool to not cut the piston or o-ring as mentioned above.
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Started with an IP-gauge, but upgraded that to an inline adjustment tool.
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An adjustable pin spanner wrench
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A first stage holder or a scuba hose which you can clamp between a vise.
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A cheap way to remove and adjust the turret is getting a hollow socket wrench
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(these for 8 euros in a Belgian hardware store)
and a cheap pin spanner for the hp-seat
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If you don't wanna invest in the inline adjustment tool, get an ip gauge and a screwdriver + bucket of water.
It takes more time but is alot cheaper. Worked for me for the first four years of servicing. But damn I would never go back since you can find these inline adjustment tools very cheaply sometimes.
The main thing that i could NOT live without is the internal o-ring tools.
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It's just three plastic or pom pieces that are the best way to put a new o-ring without having to fiddle too much.
For me the easiest way to get started was servicing an old mk2+108
then I went for an mk5 + 109
then mk10+ 109
You'll notice if you combine the above tools with a set of allen keys
and then the next step for a first diaphragm reg I'd suggest something like the apeks ds4. (I specifically don't suggest the DST because I see sooooo many damaged DST's because people use wrong sized pin spanners).
One thing that has been mentioned above, but is somewhat of an unpopular opinion nowadays is :
Don't use Christolube unless you need O2 safe regulators. I feel that greasy oldschool silicone just works sooo much better and doesn't dry/crust up ever.