Do you have a ruler?  My Big Mig 049 is 3mm.  It's pretty easy to tell -- it'll be metric, probably 3mm like my 049.

Standard metric sizes are M3x0.5, M3.5x0.6, and M4x0.7.  Unless those Ruskies are up to tricks, it'll be one of those.  Diameter is obvious if you have calipers and can be squinted out if you have a ruler (3mm is just a hair less than 1/8).  You can figure the thread pitch by counting off ten or twenty threads and measuring.

... same as cox, i believe 5-40 ...

Yes, Cox 049 threads (at least the reedies) are 5-40.  I have two bags of nicely-fitting screws to prove it.

#5 screws are nice because their nominal diameter is exactly 1/8".

Keith, the Norvel .061 car engines have 5-40 threads in the crank for the prop to fasten to, trust me, i have several and fly them regularly and one new in the box.  Ray

Ray is correct. 

Norvel .061 car engines take 5X40 prop screws.

Bill

The Norvel airplane engines .049 and .061 use M3-0.5mm x 20mm, the 0.74 uses M4-0.7mm x 25mm, and the .15 uses M5-0.8mm x 25mm.

The Norvel Cmax car engine .061 uses 5-40 x 3/4", like Cox .049 engines.

Luciano ///