Hi,

I'm in the process of remounting my motor, a BME 116.
It was on 4 x 2" aluminum leg type standoffs and I am remounting it to a 4" square of 2" thick end grain balsa.

While my plan is to go to lighter bolts, I intend to through bolt it first with 1/4-20s. While these are heavy, the plan is they are strong enough to stop the motor falling off should the balsa block compress more than antisipated.

Once its been established that the mount wont compress, I am thinking of going to a smaller gauge titanium bolt, and there in lies the problem.

I want to use 6-32, but cant find any that are 3" long. Any ideas?

An idea that has been suggested is to use the H9 4-40 titanium servo pushrods. Seriously light, but geesh, they look kind a spindly :-)

So, any ideas on the 6-32?

Many thanks,

Branwell

i wouldn't use balsa as standoff material, use arcraft ply or a nice piece of maple, 10-24 bolts would be ok , 1/4 20 is what i use ................. i would not use 6-32 , they are too small IMO, thats what i use on my small glow stuff

Who ever told you that is full of ****, using4-40 regardless of what its made of to mount a big gas motor would be foolish

Rethink the whole thing. I'd build a lite-ply engine box then cover it with carbon-fiber or Kevlar, Kevlar being my preference because it would be quieter.

I suggest you do as Rich recommends, as any thought of using balsa as an engine mounting material for an engine of that size and power is a bad idea and is asking for trouble, as it WILL compress plus it has very little torsional strength.

Karol

No balsa. About the longest 10-32 bolts I've been able to find have been limited to 4". All thread will work if you need longer. The next size up takes you to 6mm and those can be found in somewhat longer lengths above 4". [link]http://www.mcmaster.com[/link] Look under socket head cap screws.

I get my Kats titanium, Prop bolt stud, landing gear bolts, motor mount bolts, here, http://www.yoyodyneti.com/

No need for 1/4 inch, I have been using much smaller with no ill affects yet. No way would I use 4-40 though. As for the balsa, I have never tried that.
Here is a picture of some long standoffs I used. After running the engine I decided to tie the top two and bottom two together with light ply, glue and screws which is not shown in the picture, also placed larger fender washers at the firewall.

Just reading the title of this thread made me stop dead in my tracks. Are you seriously considering doing this? Surely not.....if you have never run a gasser at all, I can see where this conceptually might seem like a reasonable thing to do to save weight. If you have run a gasser, then I think you're pulling our collective legs.....

1. Mounting bolts for ANYTHING larger than 20cc needs to be 5mm or larger, if not for any other reason than stronger thread strength in shear. No question in tensile strength.

2. There are effective methods of attaching boxes to firewalls using angle aluminum stock, bolts, etc. I'd do that before building a balsa contraption with long bolts. Building a box and attaching a new "firewall" would permit shorter bolts and standoffs, and may address some of your needs.

If you are intent on using balsa and tiny hardware, at least don't fly around anyone or other equipment until you have maybe 300 flights on it...... it was scarier to go back and reread. I think you're messin' with us.....

Hi,

You guys would never have gotten to the moon....

Seriously, on the end grain balsa.
There is a guy running a DA150 on it. He also says he ran a DA85 single on it. If it can take a single, I think it should be pretty good for what ever I can throw at it.. Apparently the key is to toughen it up with thin CA and cap the engine side with thin ply. I have done this to a hollowed out block and it is lighter than the aluminum standoffs. Will be interesting to see how it works in practice.

On the 4-40s.
When I was flying a lot years ago, I used 6-32s to hold 100cc motors on. Had no issues after many hundreds of flights.
I've run many 100cc motors on the test bench on 4-40s with no problems. Would they hold up to the gyroscopic loads of aerobatics and 3D? I don't know, so I'll wait for some other super genius to figure it out.

Soar,
Why would a ply box be quieter?

Alt and TOM,
Thanks for the links.

p.s. Got a nice present in the mail today. See pic.
The things are so pretty I feel like mounting them on the side of the plane like the pipes on old Corvettes.

Branwell

Hay Bob,

I live in the back and beyond and fly the new stuff at my house before flying at a club. I'm not mad you know

Branwell

So you are the branwell that has been around for awhile....pretty pipes.
Glad you're pushing the materials envelope for us. Good luck with it!

I enjoy a light plane like the next guy, but I think for the amount of weight saved vs the risk I can't see how sound mind applies here. Use a water bottle for a gas tank and save 5-6 oz. That would be less time, cheaper, and safer.

A ply box covered with Kevlar would be quieter that one covered with carbon fiber. The carbon fiber will ring if you tap it, the Kevlar will not, it may have something to do with how hard the carbon is compared to the Kevlar. Maybe there is a engineer on the forum that can explain why, I just know a sheet of CF rings, where a Kevlar sheet sounds dead.