hi the hobbie shop sold me a 4 in spinner for a 20 10 prope for a zenoah 45 is the spinner to big ????????????

Spinners are normally not sized for the engine or the prop you are using. The spinner size is normally determined by the plane it is on. Usually to match the shape of the cowl on the plane.

Ken

Definitely not to big. I've used 3.5" on 60 size glow powered planes so 4" on that 45cc is nothing. Like RCKen said, it is the size/shape of the cowl or just plain old appearance that really determine spinner size most of the time.

I find that advice somewhat incorrect. I have a cowl (giant UcanDo) that is sized for a 2 3/4 inch spinner but the engine, SPE 40, installed takes a 20X8 prop. I can't fit my 20X8 wood props inside anything smaller than a 3 1/4" aluminum spinner and will probably have to use a composite APC or go with a visually over sized spinner or none at all.

No, the advice still applies. Very few spinners will fit props without modifying holes in the spinner. You should still size the spinner to match your cowl, and then widen the holes in the spinner so that your prop will fit in it.

Ken

I guess what I'm saying is the prop hub won't fit into the spinner, not the blades. I don't think it's proper procedure to whittle down the hub til it fits, is it?

I think you've got one of those situations that could be considered an exception. I wouldn't do anything to the prop hub; the last thing you want to do is weaken it. I'd just run without a spinner or keep going up in size until you find one that works and doesn't look too out of place. There's a chance that you may find another prop that works but then you may not like the way it performs. You could get lucky though and find both in one prop.