I changed out the bearings in an O.S. Hyper 50 today. When re-assembling, the threads in the engine casting where the backplate bolts on stripped out. It's not the bolt, but the aluminum threads in the engine casting itself. The other three torgued down just fine. I hand tightend all four then tightened them with an allen wrench opposite corners first, they did fine until the last one just kept turning. I backed it out and the aluminum threads are on the bolt. I didn't tighten that one any harder than the others.

Is this fixable? If so what is the best way?

I have torn down a lot of engines and never had this to happen.

Stick a piece of thin strand wire in the hole with it sticking out and folded over.

Put the screw in and cut the excess.

If you have a local hobby shop that does in-house repairs, take it to them and see if they can retap the stripped hole to accept a slightly bigger bolt. If they want to charge you for it, get an estimate, then go to the Home Depot, or Lowe's, and see if they have a tap and die set that small and the bolts to fit it. It may be cheaper to just get the stuff and do it yourself. I have never actually dealt with your exact situation before, as I don't own a nitro mill, but I am a shadetree mechanic that grew up around mechanics that is working on becoming a professional mechanic, and that is how I would handle a stripped bolt hole. Maybe someone else can offer a better solution, if so, then it would be a learning experience for both of us! I am trying to learn as much as I can about the care and maintenence of nitro mill before I actually get something that has one. Hope my answer is the right one, and hope you get it straightened out!

Ah! See, BarracudaHockey beat me to it, and his answer is a much cheaper fix than mine! You can tell I am a former avionics tech, I seem to have picked up the military's way of doing things the hard way that also costs more money! LOL

With the wire repair, would you recomend a little thread lock?

Thanks, Bugzilla, I've tapped a lot of things, especially motor mounts. I just hated to get into that one oddball metric screw that didn't match the other.

Thanks BH, a small piece of soft wire retrieved from a zip tie pulled it in nicely, without the complications of tapping it out. Now if it doesn't suck air i'm back in business. I'll probably get a chance to at least run it up sometime tomorrow.

Everything in the engine looked great except the rear bearing, there was a lot of varnish and a little rust on it, and it felt a little rough. Glad I went ahead and changed them out. I've been 33 yrs in planes and with good after run oil I have never had a bad bearing, even with sleeved engines. These heli engines run a lot harder during flight and i've put 2 real hard years on this one, so i'm trying to stay ahead of things on maintenance.

It's nice to have a lot of experience online.

Glad I could help. I wouldn't worry about thread locking it, but I would take a peek after the first flight or two to make sure its staying put.

Copper wire from phone wire works great, its soft, takes the thread, and wedges the bolt nice and tight.

Thanks BH, the backplate screw seems to be holding. It's not sucking air and staying in place. I put a tank thru it today.

If that ends up not holding, a new crankcase is around 55 bucks or so.

Thanks, so far so good. I only have one tank thru it since the repair. It's been cold here for awhile.