Wrestling with the engine mount

For a bit of light relief this evening I studied the engine mount and firewall. The predrilled holes in the firewall look to be pretty accurate compared to the drawing on drawing number 49. The mount itself is not so accurate.

Across the top it should be 33 1/2" - its actually 33 3/8", across the bottom it should be 29 1/4" and is 29 3/16 but most alarmingly the vertical dimension which should be 19" is only 18 11/16". It is possible to "spring" the mount a bit but I was uncomfortable with the idea of it all being "pre-stressed". I appreciate however that welding that kind of structure is not going to be easy to build completely accurately - so are these dimensions acceptable? I'd be interested in other experiences. May be I'm worrying too much (he typed, hopefully).

Thanks

Chris

i match drilled

When I "test" fitted, I noticed the holes were a little off too. My motivation for the test for was that I acquired a fuselage rotisserie (yea, great tool). Anyway I wanted the actual firewall holes to match the engine mount not the rotisserie brackets.

Engine not hung yet, and the engine mount is hanging on the wall, but the firewall holes match it.

$.02

Dave

So, did it match?

Hi Dave

I missed whether your motor mount matched the pre-drilled holes or whether you made a modification. I am about at that same spot, but have yet to match things up to see how it fits.

Michael Wynn
RV 8 Fuselage
San Ramon

advance search

did an advance search "engine mount alignment" one year and older.
this may help!!

fire wall holes didn't match the mount -

Hi,
My firewall holes did not align 100%. Diagonally it looked like maybe 1/16 +/- off. I picked top right (pilot side), drilled it, slipped a bolt through to hold, then did lower left and so on.

I have the fuse on the rotisserie right now. I did have to tweek the previously used rotisserie to firewall angles to match the firewall holes that I matched to the engine mount

I did not stress on figuring dimensions, sorry lack of data for you.

It was necessary to insure lower firewall and fuse skin clearance first. My project is tailwheel, if nose wheel skin & firewall trim would not have been required.


did not research "engine mount alignment" on VAF, didn't occur to me, mainly didnt see an option.

Dave

I looked at "aligning the engine mount" and found a bit more info - it does appear that my mount is at the upper range for being out of size. I don't have the space to move the holes the full 5/16 in the vertical range as the difference in size between pilot holes and final holes isn't great enough. Also its not really possible to clamp the frame in place and I don't lie the idea of drilling through the frame itself. I think I'll make a little template with a hole the final size, positon it so as to give maximal movement in the direction I want and then do some gentle filing before final drilling the hole - hopefully thereby minimizing the the difference i dimensions between holes and frame. I don't think this will be too stressy....

chris

I don't think this is uncommon...

Here's a trick I learned from another multiplane builder, it may or may not work for you as yours does seem to be pretty far out-
Try bolting the mount in place with some long #6 or #8 screws and washers. Drill the firewall pilot holes only big enough for these to fit. Once the mount is bolted in place, remove one screw at a time, drill to size and insert the proper size bolt. Doing it this way will ensure you take in the pilot hole as the hole is enlarged to the final size.
I've used this method on several planes and it has worked out great for me each time!
Good Luck!!

enlarging engine mount holes

Bill, I like your suggestion of drilling pilot holes in the pre-existing holes on the firewall and then enlarging them to the full size hole using the engine mount to match drill them. I want to drill my firewall holes to pilot size now for a rotisserie and I do not yet have my engin mount. Am I'm thinking later I'll bolt the mount to the firewall with skinny bolts and drill them one at a time.

But if the pilot holes are not centered within the mount holes, won't the drill walk or wonder over to the pilot hole rather than simply engulfing it? How do you drill it with the pilot hole off centered without the drill bit moving over? Will using the engine mount holes as a guide for drilling hold the bit in place? Won't that ruin the mount holes? thanks

chris mitchell said:

It is possible to "spring" the mount a bit but I was uncomfortable with the idea of it all being "pre-stressed".

Click to expand...

Another thought - If you can spring the mount into alignment by hand, that is a rather small stress compared to the stress it will experience from the 300+ pound engine hanging off of it all day every day, let alone when you're pulling some g's. Build on.

it's really not a problem

Funny to see this thread pop up gain. February 2008 seems a long time ago now - the aircraft is flying, got 32 hours on it and in between there were a lot more things to worry about than the problem I thought I had with the engine mount.

Van's attitude/advice was that if I didn't have to use a huge lever, smash on the mount with a sledge hammer or use a car jack to lever it into place, there wasn't a problem - a bit of hand pressure would be normal and acceptable and after a few hours with an IO360 on the front it would fit just fine. My inspector then told me that getting a mount to fit on a Spitfire (he has first hand experience) often took many blows from large hammer. So we just filed the holes a bit in the right direction to minimise the stress, drilled them and hung the mount. A gentle squeeze with thumbs was all it took. Not a problem.

Chris