I want to replace the original prop retaining steel screw for the MVVS 2cc Junior diesel with a hex head cap screw. This screw threads into the crankshaft and the original slotted one was "deformed" due to my heavy-handedness! Is there a particular grade of steel I should be looking at while I try to source it locally. I believe it is an M5X25 partially threaded screw. Thanks

You might want to consider a metric allen head bolt these are pretty tough and avaliable in a decent hardware store. It may be threaded all the way but so our our crankshafts
I have one also and the screwdriver thing not really the best idea for the prop bolt martin

Good idea, I will check that out. The reason I was looking for a hex head screw was that it has a larger surface area to spread the load against the prop washer. The local source for metric hardware sells screws in different grades of steel eg. 8.8, 10.9, and 12.9. Wasn't quite sure about the specs and hence my question. The local source I was referring to can be accessed at www.fullermetric.com.

I've replaced the hardened steel prop studs in my son's slow combat Norvel 15 glows with HT 10.32 caps screws.

The standard threaded studs were far too brittle and would simply fracture with a decent wack into the ground.

The High Tensile Steel cap screws still bend though with a decent wack but while I'm happy with that I'm not convinced that a softer (and cheaper) one wouldn't be just as good..

I've turned up some Alloy spinners with a recess for the head of the capscrew to protect it somewhat.

It seems to work out ok

I think maybe a softer one may be better too.  It is likely better to bend and absorb some shock than break something on the motor.  They are easy to replace if they bend too.  A hardened one will always break flush (Murphy's Law) and be harder to get out or impossible to drill out.

M5 in 8.8 standard hex bolt quality is excellent. There is no need to use higher specs. Stay away from lower specs though. They seem to be around.

I appreciate the input from everyone and will try the hex-head cap screw first. I really envy qazimoto's skill in machining that neat looking spinner nut/allen bolt combo.