Guys I have a saito 91 and use master airscrew glass filled props. 14-6 size. I have a lot of trouble getting them tightened down on the base plate of my spinner with out it slipping. When I think I have it tight it comes loose in flight causing my recent crash. Does anyone have any advice on how to tighten these down to were they will stay tight??? Thanks in advance guys.

Your real problem is most likely that you are running that engine too lean. The fuel/air mixture in a glow engine controls the engine's ignition timing (along with compression ratio and the "hottness" of the glow plug). When the mixture is too lean, it fires too early which causes the engine to get hot which in turn causes the engine to fire even earlier until it causes the prop to loosen. Get a tachometer and use it to tune your engine. Adjust the needle valve for maximum RPM and then open the needle valve to lower the RPM by 500.

I found that the standard cross wrench sold at hobby shops was too small to tighten the nut. I now use an adjustable wrench with a longer handle. Care must be taken since there is no limit to how tight you can make it. Suggest you start by using your current method. Then use a longer wrench and tighten it just a little more. Test it, etc.

I had the same thing happen to me the other day... I had a 10X5 Master Airscrew on an O.S. 40 AL. The outside air temp was in the low 30's, I started it, adjust the needle valve to high RPM's and then backed off a quarter turn (like the manual says) ... yet the prop came loose on take-off (Mostly minor dammage but the wing will have to be replaced). This is the first Airscrew I've used, I usually fly APC or wooden props... and I've never had a probelm with a prop coming loose before yesterday.

If it's slipping on the backplate of the spinner, usie a disc made of sandpaper. Cut about the same size as the prop hub, glue two discs together so the sandpaper is on both sides, drill a hole in the middle and put it between the spinner back plate and the prop. No more slippage.

I agree that the engine is probably too lean and may be backfiring and kicking the prop loose. Richen it up and tighten it up with a GOOD wrench.

I'll have to agree with everything said.

1) Make sure you are not running too lean

2) Make sure prop is tightened with a good wrench and has a locknut

3) If both of those don't work, you might be bottoming out the threads. Use a sandpaper disk as a shim

I use the same prop with my OS91fs, when I install a prop I will use a box end wrench and really crank the main nut down then tighten the lock fairly tight have not had any problems with loosening with the MAS.
the nut needs to be tighter than you would think necessary on 4st engines.

Thanks Guys