How do I figure out what diameter spinner to get for mt OS40 LA?

Well, first of all, to answer that question, it would be nice to know what plane you want to put this engine on.

Most kits either provide you with a spinner, or tell you what spinner to use on the plane with your engine.

CGr.

I bought a Sea Star 40 from Nitro. Chinese ARF. Most of their trainers were out of stock. It looks just like an Avistar, but a little bigger. 63" span. I have it about 40% complete. It looks pretty nice, but I didn't really understand 63" until I put the wing together. I'm going to have to buy a new vehicle if I stay in the hobby, which I definately plan to do.

The spec sheet says ARF weight is 3.3 pounds. I'm guessing that doesn't include the gear I have to provide to complete it. All of the comparable trainers I've looked at all say 5 -6 lbs.

I have to wait til payday (Thursday) to get fuel and field gear to run the engine. I'm considering ordering an Aviastar .53 instead of the .40. From what I've read about the .40LA, I think I might want more power in reserve.

I'm going to guess you are correct on the total final weight as the weight they gave can simply NOT include the engine and radio gear. That is a very poor way to advertise a plane and the first I've seen done like this.
As for the spinner, yes, most kits suggest a size and it's usually geared to blend into the cowl. But trainers don't really have a 'cowl' so just use your best judgement. It's probably going to look best with about a 2" or 2 1/2" spinner. Or, you could use one of the metal 'safety spinners' offered by Great Planes or Harry Higley. These all come pretty much in the same size only depending on the thread of you engine prop shaft. They are fool proof and crash proof and easy to install. With the safety spinners or hubs, you should use the matching locking nut or 'prop nut' which gives you a double lock on your prop.

Enjoy your plane!

<span style="font-family: Arial"><span style="font-size: medium">Thanks, I'm sure I will once it's complete.
The manual that came with it was next to useless. I've never been a modelersoI'm going very slowly on the build. I found links on tower to manuals for similar trainers which helped me figure out what I was seeing in the Chinese manual I got. That has helped a lot. I'm ready to start installing the Rx/servos and connecting the linkages. I hope that goes well.</span></span>

2"

Photo shows three types of spinner nut. The one on the engine has the shape for most electric starters. The other two are Harry Higgly nuts, the brass one is used for planes that are a bit tail heavy. Cheap and easy to use, they just replace your stock engine nut. A trainer really doesn't need a real spinner, just something for the starter to grab onto. A good spinner cost between $20.00 and $30.00, a nut is just a couple of bucks. These are much better then any of the plastic spinners.

Thanks Gray, that looks like exactly what I was looking for.

There was a statement in an old Aeromodeller Annual (1949?) to the effect that a spinner 1/5 the diameter of the prop was aerodynamically most efficient. So, 2 in spinner for 10 in prop.