Well I'm a bit confused by your statement that the 2 1/2" is too small for the 16" prop...

I run a 2 1/2" Tru-Turn on my YS 110 with a 16x6 prop, but it does require that you order the "wide" version of the spinner which has the cutouts for the wider props. It will also fit the 16x4W prop, but that prop would not load my YS enough to keep it below 10,000 RPM and Dave Shadel the YS importer, recommends running the engine between 8500 and 9500 RPM over on the manufacturers support forum for YS. The 16x4W is also a bit thin in the hub area and you have to watch out that you don't bottom the threads on the crankshaft with the prop nuts and not tightening the prop to the engine completely.

IF you're talking about the length of the bolt that actually holds the spinner on, I had to shorten that bolt to get it to tighten up.

edit info:

FYI, I'm using the Tru-Turn adapter kit for the YS engine. It has the two prop nuts which lock against each other as well as the small bushing to make the backplate fit the YS crankshaft diameter.