Converted a husky 57cc chain saw engine (partner). I used the original mag flywheel. I cut it down and added a machined prop adapter fastened with three axial bolts. This the scary part, on the first run I was reving the engine (23x8 prop) and suddenly a loud bang. The original mag wheel had disintegrated. Found the mag wheel had a steel center with about 1/4" of aluminum on the outside. The three holes I had drilled and tapped (6/32) were half in the steel and half in the aluminum. Looks like the fastener holes weakend the mag wheel aluminum to steel hub join. Fortunately, no one was hurt. I always insist observers stay behind the prop. Good luck, red

Wow!

Glad nobody got hurt.

Do you have pictures of after and before?

No pictures, wouldn't be worth it. Would only show the steel hub still on the crank, as the rest (aluminum) of the hub is in the grass somewhere. I will make sure in the future to leave enough aluminum hub, so as not to disturb the join. red

Not in my wildest dreams would I ever put a hub on any engine with only THREE 6-32 bolts..
You must have some good tools, it 's damn near impossible to drill and tap holes that small half in aluminum and half in steel....

yeah, going simple and using the threads on the flywheel as the only prop hub attachment has it's advantages, I can't imagine a flywheel breaking that way...

I can imagine the prop hub, prop, and spinner taking leave of the airframe with the soon-to-come failure of those #6 screws on a 57cc engine.

Bedford

I never thought the 6-32 would be too small. I chose them because of the physical size of the original flywheel. There was only approximately a 1/4" of flat aluminum outside of the steel center, beyond that the casting fell away in flutes. I no longer use the one piece hub. Too dangerous in my opion. I have just finished a new two piece prop adapter. It has an all aluminum inner hub and will be fastened with 8-32. BTW, I also have a Husky 62cc that uses the same method. It's over two years old and so far no problems. Anyway be aware that the bolts on the original mod didn't break, the steel hub to aluminum casting did. Thanks for all the info. Good luck, red

Was curious about the three bolt arrangement being too small. I measured the OD on the 6-32 at .131", times 3 = .393". Then I measured the OD on the engine shaft (5/16x24) at .391". Seems the area is pretty close. Also the placement of the three srews is further out from the center of ratation, giving it a mechanical advantage. I am only an amateur machinist9 (Bought a used sears 6" lathe from from my 80 year old neighbor, which belonged to his Dad) and learning as I go. I talk to retired machinists at the field often about the best way to do things, but usually find my own solutions by cut and try. Anyway, I'd rather be lucky than good anytime. red

Skill and experience are the foundation of most good luck.

Your measurement ratio is not figured correctly. First, when considering the 'meat' of the screw, you need to consider the root diameter. For a #6 screw that is 0.1065. Next, when comparing area you don't do it by adding diameters, you need to add up area. The area of metal of a #6 screw is 0.1065sq x 3.14/4 = 0.0089sq in, or 0.0267 for three of them. The area of your main shaft (5/16 x 24), at the root, is 0.26" dia, about 0.053 sq in, or about twice the area of the screws used. Don't forget the pulse loading and severe vibration going on there either. Fatigue failure will be the problem and that is very hard to predict, particularly with those stress risers in the v-groove of the #6 threads. I think most manufactures use at least a high strength #10 screw and that is a good guide.

Keep at it!

Bedford

Thanks for the info. I checked the 62cc engine and I had used four 8-32 on it. I will change the 57cc to #10s. Easy to do as I have alot of hub room now. Appreciate the help. red

If you modifyers look real close on those engine magneto wheels there is a warning molded in that says "do not alter"......

I don't recall seeing this warning, but then I wasn't looking for it. I do know that many of the older gas engines that used mags, have been modified by the manufacturer ( zenoah,quadra,etc.) All used industrial type engines and had been lightened and fins cut off. I suspect original warning had to do with changing timing. Good luck, red

In many fields, a warning suggests bodily harm or death if warning is not heeded. A caution is used when equipment failure could result. Most of the magneto flywheels I have seen have this warning, as do outboard, lawnmower and other small engine flywheels. Consider the forces involved in a flywheel turning at 8000 or more RPMs.

Sincerely,

Richard