So here's the story....

I have 7 flights on my plane In the first 3 flights I had to re tighten and locktite everything around the engine. Now after the 7th flight I have allot of the same bolts loose.

Now I think I figured out that the 1/4" plywood spacers that I had are compressing and allowing the mount bolts to loosen. I am going to use 1/4" aluminum spacers instead. This should hopfully solve that issue.

Now all you guys that are having no issues with this, what have you done to keep this type of thing from being an issue?

I had the same problem with ply spacers but realised it before flying I changed to aluminum used blue locktite, flat washers and lock washers, no trouble with loosening in 31 flights so far, also I did apply a thinned epoxy to the firewall several light coats before I mounted anything it seems to tighten up the wood pretty good.Just make sure to spread the load on each bolt with a flat washer first then lock washer and locktite. Good luck

I've been using a true hardwood for spacers for years with no problems or needed adjustments. Maple, Birch, and Oak are a couple that I normally use, nut never Pine, Redwood, or Firs. Of course, metal works fine, too, but it's either more money or work to get the exact sizes in metal. Weights usually end up close to the same.

Soft woods and plys will always compress after a period of time.

So you're saying you use washers between the stand-offs and the hardwood? Might just buy something hard like silver since i'm just going to cut them into small squares. One more question on engine mounting. The standoffs I bought are bored for 10-32 oe 5 mm hardware. I couldn't find long enough 10-32 at the hardware store in my home town or online so I bought some long 10-32 bolts from Tru-Turn that are for their spinner retaining screw. Will these work as my engine mounting bolts?

They will work, but boy are they pricey! I'm surprised that Micro Fasteners didn't have any long enough.

In a worst case scenario, get a length of 10-32 all thread and cut it to the lengths you will need. Double nut it on both ends with Locktite or something and it is just a good as a bolt.

Too bad I didn't ask about this earlier. I can still use the Tru Turn's though as the Dave Brown spinner screws aren't long enough.

I am going to guess that I have put my foot in it (both my motors 3W 60 and DA 50) mount with 1/4-20 bolts my stand offs are 1" diameter at the base and narrow down to 1/2 at the back of the motor by the size bolt you mention you have neither of these motors. Silver's explanation would best suit your application If your bolts are coming from the engine side into blind nuts at the firewall use a lock nut after you have them tight, If your bolts go from the fuse side of firewall into the engine mounts then put a lock washer on the bolt first then slide on a large flat washer then slide through firewall into engine mount with blue locktite of course I went over board and use 1/4 aluminum plates cut in 1" squares instead of large washers just remember to spread the load of the bolt/nut the wider area that carries the load will not compress as much and anyway there will come a point as you tighten down that the wood will not compress anymore. Don't be shy with the wrench!

The secrete to using the ply standoffs is soaking, and I mean soaking the standoff in thin CA. I've done this with several airplanes when I don't need more than 1/2" or so. I've never had a problem with the motor bolts loosening.