M

mattsmith

Well Known Member
I am getting ready to install my motor mount and the holes are about half off from the fire wall to the engine mount. The question is do you drill the under sized hole on the fire wall and bend the mount to fit the holes or do you use a drill bushing and move the holes over to line up with the mount? moving the holes over with a drill bushing seems to be the correct answer since the directions say to match drill, but will the drill bushing keep the new hole from wondering into the 1/4 hole in the firewall that is already there?
 
One Data Point

I recently did this on my 7.

After much hand wringing (holding mount in place to see how far off things were).
Most holes looked like they lined up pretty well.
I decided to drill a couple of the "good and close" corner holes 1/4" and get the mount hanging under its own weight.

I used some aluminum fuel tubing as bushings to center up the mount on the 1/4" bolts and also as drill bushings for the other holes.

It worked out pretty well. the majority of bolts slide in with light finger pressure and the rest just need light persuasion or jiggling.

YMMV. Good luck.
 
I measured my mount holes and compared to the firewall pre-punched holes. I could get any two I wanted to fit and the others would be about a half dia (motor mount hole diameter) off. So I chose opposite corners and drilled those pre-punched to final size and mounted with correct hardware. I then manually spread the mount to see if I could get it to spread centered on the remaining undrilled pre-punched holes. It was easier to spread than I thought it would be. I removed the mount and drilled the remaining two holes to final size and re-installed pulling into final location as needed. Turned out to be a fairly trivial event.
 
Ditto

Had the same issue, but it was a bit more than 1/2 a hole. I built a little jig to slightly spread the mount. The pilot hole in the fuse was still off center on the bottom two, but were cleaned nicely by match drilling.

Robert Grigson
RV-8
 
Drilling firewall for engine mount

Your thread title made me raise an eyebrow...!

I had exactly the same concern as you and in the end its really not a problem. THe mismatch arises because the mount is welded up on a mould, but once it cools and is taken off the mould it distorts and hence no longer exactly matches the holes in the firewall.

You can move the holes slightly to take up some of the mismatch. IIRC we marked the holes in the desired positions using the best available fit, used a small round file to elongate the holes into the rough final position and then final drilled to get the holes round and the correct size.

You might well still have some mismatch but should just be able to "spring" the frame into place with your fingers. Vans advice was that there was no problem so long as I wasn't using a car jack, big crowbar, or other means of exacting excessive force, and that after 10 hours or so of engine running the fit would be perfect.

I have since been told by an A and P with warbird experience that getting the mounts for the Merlin engines in eg Spitfires and P51s often entails the use of a very large hammer and lots of brute force.

Chris
 
after measuring the fire wall and where the mount will be if I use the pre-drilled holes it should be pretty close. I can move the mount by hand, at least I can on the ground but it takes a lot of muscle. My concern is if doing it this way would it put to much stress on the components, but after reading above post I think it is close enough once I run the engine (did not think of that.) Thanks for the help.
 
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