i am having serious problems trying to tighten a master airscrew prop. whenever i try the backplate of the spinner just keeps turning so it will not tighten. does anyone have any advice on how to resolve this

I don't really understand your problem but I'll try and help.

Here is how I tighten props.

Put the spinner backplate on . Put the prop on. Wind the propellor on (or push it depending how tight it is). You need to do this until the prop driver (the metal washer on the engine with the grooves on it), the spinner backplate and propellor are all in contact. Put your washer and prop nut on. wind this by hand until all are in contact. Hold the propellor in the correct position (top right blade in two o'clock position at the compression when viewed from the front).

Once there is a little pressure between the prop nut, washer, prop, spinner backplate and prop driver nothing will move. Now hold the propellor in your hand and tighten the nut with your spanner.

I hope this helps.

I understand your problem completely. I have the same problem with some of my older engines. The problem is most likely your spinner back plate is plastic. The plastic back plate is softer plastic than your prop and there for can not "bite" into the prop. There are a lot of ways you can fix this. Some are controversial, some are expensive.
-- The first thing I would try is to clean the spinner back plate and prop with denatured alcohol. (you can get at the auto supply store for cheep) The slipping may be caused by the oil in the fuel.

OR take and make a washer from sandpaper. If you use sandpaper with the sticky back you can fold the sandpaper in half so the two sticky sides come in contact. This will give you grit on both sides of the paper. Cut the paper into a round shape, put a hole in the center. Place the sand paper between you spinner back plate and your prop. --

OR you can turn the engine so the piston is about 2/3 the way up with the glow plug out. Fill the engine with glow fuel through the glow plug hole. then replace the glow plug. This will hold the engine from turning while you tighten the prop. once the prop is on, take the glow plug out, drain the fuel, flip the engine with your starter for several seconds to dry out the piston. then replace the glow plug and your done. This may fowl your glow plug, so if you have one that is no good, us it to hole the presser then put your good one back in before you fly. --

OR you can get an aluminum spinner and back plate with the hash marks in the spinner back plate to bite your prop.

I hope one of the ideas will be helpful.

Try to roughen the hub or glue some sand paper to the spinner back plate.

There's a problem with putting anything (even fuel) in the cylinder to prevent it from turning, when you go to tighten the prop you can damage the wrist pin or the connecting rod. I wouldn't recommend it.
Dennis-

There are two official ways of 'clamping' the engine and loads of unofficial ones.

a) buy a piston locking tool (these screw into the glow plug recess). a lot of manufacturers don't like this as they say it damages the piston.

b) buy a crank lock. You'll have to take the backplate off the engine and insert a tool that clamps things in place. This is O.S. preferred method.

c) other ways I would not reccomend involve shoving things in the exhaust port, or removing the carb and shoving things down the hole.

Sounds like you will solve your problem without resorting to any of these.

You shouldn't need a piston lock or any other device for keeping the engine from turning while tightening the nut. These are needed for helis where you have notheing decent to hold when tightening the nut on the clutch/fan assembley. If you grab the fan wrong you knock the fins off of it and have to get a new fan. You are not going to have that problem with the prop.

NEVER stick something n the exhaust port to prevent the engine from turning while tightening or loosening the prop nut. That is a good way to break a piston ring or to put a dent inthe ede of the piston... this will result in destruction of the engine.