What really irks me is that the ID ov the props is different for each size!  I have a lathe, and can make whatever I need, but having to have a separate adapter for each engine/prop combo is the pits.  Norvel 3mm, AP Wasp 4mm, Cox 5x40.  What a pain!  And why the big holes in the first place? 

Ha!  Ha ha!  Bwa ha ha ha ha!

"One bush to rule them all one bush to find them.
One bush to bring them all and in the darkness bind them!!!!!"

Item:  Bushing.
Application:  Fixing APC's screwups.
Material:  Metal*
Tolerance:  Yes**

Item to consist of a cylinder 0.230 inches long, with a hole reamed*** to fit match the mounting screw.  Item to be turned to a diameter of 0.1875**** along 0.175 inches of it's length, with the remaining length forming a 0.242 inch diameter shoulder.

Sorry, no drawing -- they're for sissies.

This bushing works in all three props -- directly in the 6x2 and the 5.5x2.5, and in conjunction with the smaller of the two plastic bushings on the 5.7x3.  Mount it from the back on the first two props, and from the front on the last one.

You may find that I've specified it to be a bit long -- mine is .235" long, which is .005" longer than the 5.7x3 prop is thick.  It only works on the Cox engine because the center of the prop driver is relieved for the crank shaft.  Subsequent ones will be built to the dimensions I'm giving above, or maybe a bit shorter yet.

* I.e., whatever you have lying around.  Mine is made of aluminum, but I don't know if it's 6061, 2024, or some other hard, easy-to-machine aluminum.  Brass would work just fine, and be even easier to machine.  Don't use an old 1/4" bolt unless you're really good at putting a fine finish on crappy steel, and don't much care for your reamer.

** I.e., according to your pride.  The propeller holes aren't all that precise, I aimed for 'snug' and was fortunate that I chose the prop whose hole was a hair bigger than the smallest-holed one, 'cause it'll push into that one and still be righty-tighty.

***  You really want to ream it, but if you're too cheap to send out an order to McMaster for reamers, you could get away with drilling it.  Reaming it is just plain nicer -- drilling leaves it with noticeably more slop on the screw.  In fact, I have a slightly undersized 1/8" reamer designed for sizing holes to press-fit pins; I think I'll see how my 5-40 screws fit in that.

**** Well, mine measures 0.186 and fits just fine, but I have cruddy calipers.

I've been using 5.5X4 on a Testors 049 I use a piece  of med fuel tubing as a spacer and it works fine but yeah I have no idea why the hole is so wide, boggles the mind.

Just slip a short length of large fuel line on the shaft, then the prop.  As you tighten it down the fuel line squashes and expands, self-centering the prop.  Takes a little trial and error to get the right length but it's not critical. Works for me. For Cox & others with prop bolts, put the bolt thru the prop, then the chunk of fuel line, then thread it into the shaft.

APC props work great on my 1/2a fleet.  I use the yellow fuel tubing for weedwackers and such for bushings available at your local hardware store cheap, it fits the id of the props perfect and will work on norvel and cox prop bolts. Also the cox 5x3 flexible props from the Cox store for 3 bucks apiece work great and last a long long time!

APC does sell a set of metric adapter bushings.  Part no. APCMPA1.  They might help solve some of your issues.

I couldn't find them on their web site.

Yea, they only seem to list the props only.  Just Google the part number.  A bunch of on-line hobby shops carry them.

Well, I've got the lathe all set up to churn them out, and I have four of them, and they're pretty (it is soooo easy to put a nice finish on aluminum), so I think I'll keep them.  But I'll remember that it's out there.