In order to accomodate a 4 stroke engine in the future, I built my GP 40 Cub with the firewall about 3/4" aft of what the plans call for. Currently using a 45/2stroke.
I used rubber expansion nuts, sometimes called Lord Mounts, to attach a "sub firewall" and attached the engine mount to that.
I could have used blind nuts, but I thought these might take out some "buzzyness".

See quick and dirty drawing

My question is has anyone had any failures with these rubber plugs.

I've used that style with no problems from .50 to .91..

Tom,

I use them on my mid-sized gassers (up to 42cc). I have not had a failure in flight. I have lost the ability to reingauge the brass nut requiring replacement, and I have had the bolt vibrate loose. Otherwise they are fine and I continue to use them ... checking bolt torqure regularly.

Bedford

I believe what you are referring to is normally called a "Well Nut". A "Lord Mount" is a completely different animal. I had a 1.20 mounted with 10-32 Well Nuts. They deteriorated very quickly and would not stay tight. When I removed them, the rubber was in very bad shape. I don't use them anymore.

ilikepllanes,

You are right, I am referring to Well Nuts. In my case, I am running gassers, that may be why mine are not deteriorating while yours are.

Bedford