Well, that was a fun hour.

It's a 7/32" bore, on an OS Max 25 (not FP or FS or any nonsense -- just a cross-flow OS Max).

Things calculated out to needing something a hair bigger than 7/32 -- I'm going to finish this one, then make one with a 15/32" bore, and see what I can see.

Spraybar will be 1/8", or a hair under.  I was kinda surprised -- the carburetor hold-down screws are obviously M3, I expected smaller holes.  Note that I'm not complaining.  I just got a 5-44 tap & die set from McMaster; the thread's probably a bit coarse but I'll find out!  If the needle's too hard to set I'll try scheming on a finer thread (there are M3 x 0.35 tap & die sets out there, but they're spendy) or I'll experiment with less slope on the needle itself.

I fantasize about making a #5-64 tap -- but that's probably best left a fantasy.

We'll see how it works.

Uh---------Tim-------uh----  Do you REALLY mean 15/32s???   Or perhaps 15/64ths?  I would think that 15/32nds would be more than just a "tad" larger.

Well I'll just open up the crank to match this venturi and -- oops!

Yup -- 15/64ths.  I want a drill index full of number and letter drills for Christmas.

It's done!  It runs!  (well, at least it got through an ounce of fuel -- more to come).  Now to run some more fuel through, just to make sure.  This is an engine that I bought at a rummage sale in the early 1980's, along with the all-styrofoam RC Cub to which it attached, and ran just once or twice since (mostly long enough to find out that I couldn't readily transition to a transmitter that used one stick for elevator and another for rudder -- the plane had a short life). 

The engine just got the whole crock-pot/antifreeze/scrub off the residue treatment, and a brand new venturi, of course.  I'm not all that experienced at engine teardown, but it looks to me like it's probably broken in but not at all worn -- which should be perfect.

And I saved what -- $15? $20?  For only four hours of work!