Hi! I got an old OS Max FP .25 but ts discontinued and OS doesn't list the recommended props, any idea?

lovely engine,an 8x6 will be fine.

That is a good engine, had one on a Dynaflite Butterfly with 99" wingspan and it hauled the plane around very well. 8x6 for speed 9x5 will give you a bit more thrust and 10x5 for some scale applications. This engine was replaced by the O.S. 25 LA. Check the Tower Hobbies website .25 size engines for some more prop size suggestions.

Dale

Possibly even a 9x4 or 10x4, depending on the condition of the engine and the application.

All I have laying around is a 9x6. Is that too much to just run a tank through on the bench?

Hi!
The oS .25 FP could handle a 9x6, but it depends on the model what prop work best.
I would go with a 9x5 APC on a low winged sport aerobatic airplane weighting 1.5-1.8kg with a span of...let's say 115-125cm.
If a highwinged trainer at the same weight but with a span of 130-140cm...a 10x4 or a 10x5 APC might be better.
For a smaller faster model at around 1.2-1.5kg with a span of 110-120cm...a 8x6 might work best.
Regards!
Jan K
Sweden