You need a special 1" yoke nut socket to properly remove the yoke nut. It is a thinner than normal cut down socket made to fit over the nut and within the yoke. Price is about $20.00. You also need to either clamp the first stage in a padded vice or alternatively you can use a first stage handle (another special tool also costing about $20.00) braced against the bench to hold it while you untorque the nut. The padded vice is a lot easier if you have a stubborn nut.
In a pinch, you can remove it with a large cresent wrench (15"). Yoke nuts on most SP regs are torqued to 22-25 ft pounds but are often a lot harder to remove than that after they have been in service awhile and developed some salt deposits and possibly some mild corrosion. And of course, some techs have a bad habit of skipping the torque wrench and tend to consequently over torque them in the first place which can make them even harder to remove.
When you install the DIN fitting it also has to be correctly torqued which requires a suitably sized torque wrench as well as a long reach, 3/8" drive, 5/16" hex key socket (about $15.00).
All of the required tools can be purchased from peterbuilt tools (scubatools.com).
So you can spend $55.00 in specialty tools (plus at least that much more for a torque wrench if you don't already own one) to do it right, or you can use a large cresent wrench and standard 5/16 allen wrench to do it wrong, or you can take it into a shop and have them install it for you. Spending the money on the proper tools and doing it right is more personally satisfying for a true do it yourselfer, but taking it into a shop is quicker and more cost effective for everyone else.
