On my 3W 100, I seem to have stripped the threads on one of the 6 prop bolts. 5 of the bolts tighten down nicely, but one of them will not tighten. The threads extend about ¾ ways through the prop hub, so if I get a slightly longer bolt (55mm) perhaps I can catch a couple threads deep into the hub. Any other ideas? Can I drill out the hole and tap it for a larger diameter bolt? Will this cause out of balance issue?

Try a Heli-Coil. The extra weight of the small Heli-Coil will be much less than that of a larger bolt. If you think it is too much then put heli-coils in all six holes. Then it will be balanced out.

Bob

Bobby Wilson (Cactus Aviation) also recommended a helicoil when I spoke with him today. Looks like something I can do myself. I've ordered a "kit" (tap and install tool with some inserts).

They'll give you enough inserts to do them all.

We use them at work. Make sure you turn them in far enough so that the end of the wire is below the top of the hole. Dont forget to break the tangs off after you install them.

Good Luck
Bob

My fix, would be to replace all the bolts with next larger size english thread. I say this because if one is stripped, you may have others on the way. This would be less expensive and maintain balance. Use a little medium loctite and less torque. Good luck, red

CoralCape

One thing wrong with that approach. If they are 4mm diameter ,.1574", the next largest English size is 8-32 (.168" diameter) this only leaves a .005" thread depth all the way around. That will strip out lickety split. Common practice in industry is to put Heli-Coils in holes in aluminum that will see a lot of screw insertion and removal, like changing props. They will only last so long as they come from the factory, then you fix them "right" with Heli-Coils.

My $.02

Bob

I agree. Don't like to have to remove and replace steel into aluminum. In other situations I would use a permanent stud and capture with locknut. My concern, in this case was the distance between holes enough for a heli? Good luck, red

I like the permanent stud and locknut idea!

The silver locknuts would probably be easier to see than black socket head capscrews if you dropped one.

Bob

OK, I mail-ordered a 5MM helicoil kit and it arrived today. Worked great. Took me a few minutes to drill out the hole and install the helicoil. I have re-installed the prop and all seems well. Can not fly due to bad weather now. Bobby Wilson says to torque these 6 bolts only to 37 inch-pounds. I don't have a torque wrench that will go that low (this is less than 3 foot-pounds). But I would estimate that I was torquing them to at least 5 foot-pounds. I have had trouble with them loosening up and then they all shear off. At 37 inch-pounds it seems to me that they will loosen up within 5 minutes of running. I'm using a wood prop.

Run it for 5 minutes then check it.

Check after each runn until you're confident it isn't getting loose.

Bob

This is from the Troybuilt Website. This is the method I use and I never have to use Loctite or have a problem with Loose Props, sheered bolts or stripped threads. By the way I have a 3w 70


HOW TO TIGHTEN A PROPELLER


When installing your prop, special techniques must be employed. Just tightening up the prop bolts one time won't do it! You will lose your prop on the second or third flight otherwise! See the video page of a prop coming off in flight!


Proper procedure for tightening prop bolts:
1) Obtain thin steel washers and place them under the heads of the socket head screws. The heads of the socket head screws without the washers will gall the aluminum spinner hub and will prevent full tightening of the socket head screws.
2) Tighten the screws a little at a time. Tightening one screw as much as possible without the others being tight will cause uneven pressure on the prop hub. Tighten one screw some, then go to the screw 180 degrees from it and tighten that screw. Go around and around several times. Don't over tighten so that you crush the prop.
3) Fly the plane for 5 minutes. You can leave off the spinner cone (of course have the backplate in place).
4) Land the plane and retighten all the prop bolts. (Not too tight!)
5) Fly a standard length flight.
6) Retighten all the prop bolts
7) Fly one more time
8) Retighten all the prop bolts. If they don't move this time (they should not). Then you have two options: a) leave it alone or b) remove one screw at a time, put on some very light thread locking compound (not too strong!), and reinstall the screws one at a time. It is not necessary to use thread locking compound, but if you lost a prop before and now you are jaded, this is your extra insurance.
9) Check the bolts every once in awhile, though the prop should never loosen if you followed the above procedure.