In recent years I have been using metal plates under the engine lugs to get more bearing surface and not dig into the mounting surface of a plane.  Then it hit me, since I'm mounting the engines to plates now or the plates to the engine, why not make them so I can use different engines on a plane by just removing four screws to change the engine plate.   Some of my engine even though they are the same displacement have slight differences in mounting pattern.  So first off I got a strip of aluminum.  I wanted 1/8 X 1/4 but all my local Ace Hardware had was 1/8 X 3/4.   So I leave with a strip 4 foot long.   Using my band saw I rip it down the middle.  Then I cut the strips into 2 inch pieces.  I glue the piece to the engine lugs allowing for the area to drill for mounting to the plane.

After marking the aluminum pieces I break them loose from the engine,, be sure to mark the pieces as to which side of the engine, the top side and the front.  Once drilled turn them over and use a chamfering bit to allow the flat head screws to be flush.  I use 4-40 flat head screw with nuts with nylon insert to hold it all together.   Last thing is to mark and drill the ends of the plates for mounting to the plane.  Now to get the pictures in order.

Hey, Doc--that'll do 'er!

We do this quite frequently on racing planes--which I suspect you are doing here, otherwise, why the Nagmum Bloohead? (Putting your parts on top of a Fury box is some neat misdirection, though)

Most times, we use 3/16" thick pads and thread them. Some 2024-T6 works nicely. Then the pad-holder bolts are countersunk to get them out of the way of the engine lugs.

My only reservation about the Home Depot and similar aluminum is that it is generally not very high alloy and therefore not that strong. You have eliminated the biggest issue with it since you are not trying to thread it, at least on one of your installations. If you have an Ace Hardware that stocks aircraft grade aluminum, I may start thinking about moving to the Jayhawk state.

Divot McSlow

PS--Gonna need to shorten up the needle on that Nagmum or yur gonna break it off wit' cher flippy finger....

What you have there will probably work OK but I would keep an eye on things.  The 1/8 thick stock is thinner than I would prefer and after countersinking, there is not much material left.  The edge distance is also a bit short.  As stated in a previous post, if you are not tapping holes that will help but the aluminum stock found in hardware/home improvement stores is usually in an annealed condition and not very strong.

I mount engines in a similar fashion but I attach the engine to the plates by tapping holes in 1/4 inch thick 2024 alloy from McMaster Carr.  There is quite a selection there for thickness, width, material, etc.

I have been using 1/8 thick aluminum in a U shape tapped for the engine mounting for years , This includes Saito 62, Ro Jett 67 , La .46 , and others.
Never had a problem with engine mounting screws coming loose . the U shape is attached to the fuse /motor mounts with 4,  4/40 screws inserted in to
the brass inserts . Never had a problem , I use the blue loc tite on the screws .

I still have a couple of engine pads/mounts that Brodak had.  Yes they were for 35 size engine with off set molded in.  Need to look and see if he still has them.  They were U shaped.