I can buy 'em... I can fly 'em- but I have never installed or un-installed a DF set-up [X(]

I am removing my OS-77/ Dynamax from my JHH A-4 (putting 91 in-) I removed the hex-nut and housing but it looks like there are 3 Allen head bolts in a plate on the shaft- It would appear I have to remove them to get the unit out- is that correct? and if so, where would I find a foot-long angled Allen wrench for that? [X(]

Please, someone walk me through it if you have a minute---I thought I would remove the hex-nut and plastic hub, pull off the fan and grab the engine out the other side...

You need to remove the entire fan unit from the airframe. The fan unit is probably held by 4 bolts, 2 on each side of the fan shroud. Once you have the fan unit out, you should be able to see that in the center of the hub, where you see the 3 small allen head bolts, there is a nut up inside there. It should take a 1/2" deep socket to get to it. That nut is threaded onto the crankshaft of your motor. You will need a way to hold the fan blades from turning while you try to loosen this nut. Do nut hold the blades by sticking a screw driver or something between the blades! Chances are very good you will damage some blades and a set of them is $55. JMP makes a tool that is just a piece of flat metal that has 3 holes in it that match up with the 3 bolts on the hub. You might be able to make something that will work. Since I was a auto mechanic for 20 years, I use a small 3/8" drive air powered wrench to remove the nut. I hold the fan blades with my fingers wrapped in a shop towel. Once you have that nut out you can remove the fan rotor assy. Now all you have to do is remove the 4 bolts that hold the motor to the fan.
Install the .91 in the reverse order above. Make sure you use blue loctite on the rotor nut and the bolt that holds the spinner on. Don't over tighten the spinner bolt, as you can break the spinner.

Good Luck and I hope all this makes sense to you. I could show you in a lot less time than it took to type it.

OR.....you could do what I did when I realized I didn't have the "fan holding tool".....I just took out the glow plug, made sure the piston was just higher than the ports, filled up the cylinder with after run oil, put the glow plug back in, and loosened the fan nut while the hydraulic lock kept the fan from moving!!!.....worked fine for me, still do it now
lee.