Hey guys Im installing the motor mount for the OS .91 inverted in my P-51 but hte problem is its a CMP and the instructions dont do diddly squat. Where should I start? There are no holes in the firewall so are there any other things I can do besides trial fit? I know the mount pretty much has to be high on the firewall but are there any rules I can follow?

It's a tough situation, and there's no "Easy Way" that I know of.

If it were me, what I would do is this:

First, determine from the "manual" how far the thrust plate is from the firwall, or mount the cowl and measure it.

Place the rear of the mounts flat on a table. Place the engine between them at the proper distance (It would be good to have an extra set of hands) and mark the drill holes.

Drill the mounts and install the engine.

Now place the assembly on the firewall in what LOOKS like the proper spot and secure it with one wood screw in one of the top mount holes.

Trial-fit the cowl and see how close you are.

If you are off, remove the screw, replace the engine and secure it with a screw in the opposite top mount hole.

Test-fit the cowl again.

IF it's good, drill the other three mount holes and install the blind nuts or whatever method you're using.

Thanks Minnflyer thats what I thought I needed. I already measured the distance to the prop washer. Its the height of the motor mount thats gonna be tough but thanks.

I do what he says but with these changes...


This is given on the CMP manuals so you DO know the firewall to thrust washer distance.


Since you know the distance from the "table" to the thrust washer, you can do this quite accurately.

Make sure that the mount arms are parallel to the engine mounting "tabs" to get proper alignment.

I mark the holes with the Great Planes device and then drill them on a drill press.

I've done this so much that I've built a small jig to hold the mount level to the drill press.


At this point I measure the width of the fuse and of the mounting box.

( If possible I position the fuse so it points straight up ).

Take exactly half of that and scribe a point high and low on the mounting box.

Draw a line between these two points and you'll have your vertical reference line for where the tip of the prop shaft should end up.

Use a T-square to position the engine at this point.

Instead of drilling a hole at this point I put a little bit of CA on the mount to hold the engine while I trial fit the cowl.

Now carry the engine's mid line down to the firewall. This will give you the reference for the engine mid-line.


Once I like the vertical position of the engine and cowl, I use the GP center hole marker to make starter holes where the blind nuts will go.