Seized Allen bolt on 280 out drive prop. Need some new ideas

[1981fiberform]

Need someone who really understands relationship between prop cone and the screw on the end. Here is what I have learned/broke so far. Last time I changed the prop it took like 5 minutes. I am on day 5 this time! Put pb blaster around head and even filled slot in cone that screw passes through with penetrant by plugging one side and set over night twice - nada. Head is now completely stripped so I converted it to a flatblade slot with a dremal tool. Biggest screw driver with wrench can exert a lot of force before it slips but because blade is just a little small for job I can't get it to break loose before it slides to one side and slips out of groove. Bigger 1/2 inch screwdriver on its way. In the mean time I put my king size wrench on the cap and it turns! Only went an eighth of a turn but it turns. Screw moves with it??? So does screw just screw into cap and I can take cone off with screw in? That makes no sense. I thought there was a pitch difference that would prevent them from coming off together. If so if I tighten the cone that should make the screw easier to remove? The amazing part is it turns- which I am interpreting as the threads are not seized - so why won't the acre turn by itself. I have tried ever bolt loosening word I know. Please help

 

[Scott Danforth]

prop nut threads to outside of prop shaft. locking screw threads to inside of prop shaft. worse case, cut head of bolt off. different thread pitches make the system work.

 

[1981fiberform]

Following up to my old post for the benefit of someone searching topic. I wound up taking my Dremel tool and cutting a slot across the head of the allen bolt (short cut on each side). Burned up a bit or two but gave me giant flathead bolt instead of a stripped allen bolt. Still couldn't budge with giant flatblade and wrench but impact driver got it loose. Bought two new bolts so I had a spare. Have made several prop changes since with no issues. I just snug it tight now rather than really tight.