I have been trying to find the propeller nut/locknut assembly for the 91S engine i have without much success. Not sure if it needs one as it didn't come with it but all my OS 4 strokes have them, always a concern with throwing a prop if you happen run it to lean.

https://www.horizonhobby.com/prop-wa...eiijj-sai56135

Here ya go. Your Saito Engine Code is K, just type in Saito K, the thread size is 7mm x 1.

I ordered a lock nut set, it had more letters on the package than what is shown on the one in the link it ended in SAI56135 but was the wrong thread size. It fit my FA-90TS engine so I just kept it. Perhaps the locknut set in the link is the correct tread size. Don't know how I ended up with the wrong one.

I know, a time or two I ordered the combination Spinner Nut and flat lock nut by Dubro, the spinner was the wrong size in a package that was marked correctly, the lock nut was correct.