I'm having trouble with the 6-32 bolts holding my nylon engine mount to the firewall... Despite a lockwasher they like to back out.

Does Loctite work with blind nuts? How best to get the darn blue stuff through the firewall and into the threads of the blind nut?

Also, I drilled/tapped the nylon mount itself for 6-32 bolts to hold the engine on. Any reason to loctite those? Or does nylon tend to lock it up sufficiently.

Saito 91 btw.

-Greg

Yes, Loctite works with blind nuts. I would use the stronger red version. Insert all mounting bolts fully through the engine mount and put a drop on the end of all bolts that are sticking out the other side and just screw them into the blind nuts.

No reason to loctite the bolts that hold the engine on since loctite doesn't work real well between nylon and metal. Just use long enough bolts with lock nuts for extra security.

Hi Greg,
applying loctite in the normal manner to the firewall screw threads will permit enough fluid to engage the blind nuts. When tapping nylon mounts I use a size or two smaller than recommended drill. Additionally, I use a tapered tap and only tap till the tip just partially penetrates the bottom. This tapered bottom, undersized hole will keep the screws in place. Loctite and some plastics are not compatible, so try without first.

Locktite will work, use the blue though. If red is used, you may need to heat the nut to get the screw out. FWIW, Mike

I use the blue locktite, and run a tap through the blind nuts to clean them out after I remove the bolts. It is no fun putting a bolt in and pushing the locknut out. I also solder a brass plate or something over my blindnuts to be sure they don't come loose or turn. That's on ones I can't get to should they come loose.

Jim

It sounds like you're going to treat the symptom and not the cause. The lockwahsers should hold. You are probably having a problem with the firewall itself. If it is lite ply, as opposed to aircraft ply, it may be compressing and resulting in the bolts appearing loose.

If you are not going to fix the cause by inserting a plate behind the existing firewall, or laminating an aircraft ply firewall to the front of it, you will be better served to check the tightness of the bolts each morning you fly and tighten them, instead of using locktite which may cause more problems if the firewall is compressing. You may not be able to lossen the bolts at all.

Tapped fiberglass works OK... for a while. If you wind up removing and reinstalling the engine several times the threads in the glass will likely strip out. If you feel that happening, replace the bolts with longer ones and use lock nuts and washers.

Just a thought, but is your propeller balanced?

Yes, but I am still breaking in the engine and it idles pretty rough.

The firewall is doubled 1/8-inch plywood. I seemed to be crushing in the small flat washers I had used to shim for right thrust - may have limited tightness. Replaced washers with thin shim of aluminum all along left side of mount, allowed more torque/tightness.

I put a small amount of blue loctite on each once also... if they don't hold now I'll be surprised!

Thanks all for the advice.
-Greg

Just a dab of blue on the threads of the bolt going into the blind nuts. Do NOT use red unless you are putting the bolts in permanently. Red is basicly a bearing mount type. Do not use lLoctite on plastic or fiberglass parts. It will attack them. I use a little 5 minuit epoxy that I put over the blind nut to hold them in place. Just keep it out of the threaded portion. good luck with your model.

Mark Shuman