When to drill the fire wall for the engine mount?

[Aero_Octaveus]

Quick question for you. Has anyone drilled the firewall for the engine mount before you ordered the engine mount? How close was the engine mount to those pilot holes in the firewall?

I ask for the following reasons;
1. The firewall has pilot holes for the engine mount already. 2. It is extremely easy to drill those pilot holes on a drill press now before I mount the firewall to the fuse. 3. According to some build logs, it seems folks have used the firewall pilot holes as a mounting point for the rotisserie. 4. I really, really don't want to try and drill those holes by hand down the road.

Thanks.

[N804RV]

I plan to drill undersized pilot holes for the rotisserie. Then, ream them to size to match the engine mount later.

[jcarne]

While many others have done it before I really don't recommend it but that's just me. My weldment barely captured all the pilot holes. It is significantly easier to drill a hole later than it is to meticulously file your pilot holes until they are centered in the mount holes. Van's even sends drill bushing with the new nosegear that makes drilling even easier.

[Draker]

I waited. The holes ended up very close to the "pilot" holes on the firewall but not exactly. With the engine mount, it was a piece of cake.

[DaveO]

I would wait until later to drill those holes. My weldment did not fit and I had to weld shut one hole and redrill. Not a good problem. my 2 cents worth

[Richard Connell]

Even if tolerances on the mount construction have improved since I built my -7 there?s still no way you could drill your firewall/fuse and guarantee it?s close enough. You really need to drill your unique mount to your unique airframe.
I?ve just done my RV10 Mount. It?s close but still not close enough to not match drill it.
It?s an important part of the build. There isn?t any ?better way? or shortcuts to be had to get it to fit perfectly other than match drilling it. I suspect if you asked the factory this question you would get a very curt reply ?no - follow the instructions?

[rv8ch]

Originally Posted by Aero_Octaveus

... I really, really don't want to try and drill those holes by hand down the road. ...

Literally thousands of other builders have drilled them using the engine mount, and it worked ok. It's not hard, just have good bits, a good drill, lots of oil, go slow. I'm sure you are a better builder than at least one of us who have done it before successfully!

[rmartingt]

Drill it using the engine mount as a guide. I thought I had pictures of my process but I guess not.

On mine, I clamped it to the firewall and nudged it a bit until the pilot holes in the stainless firewall sheet were about as evenly distributed and close to the tubes in the engine mount as I could get them. You pretty much have to eyeball it.

Then, using a couple of spare 7/16 hardware store bolts, I turned a pair of drill bushings (one 3/16, and one somewhere between that and 3/8).

I drilled the first hole at 3/16 with the bushing, then swapped the bushing and drilled the next hole. Then I removed the drill bushings and used a drill bit just smaller than 3/8 (23/64 or something like that). Finally, I ran a reamer through to get a tight, clean fit. Blow out the hole, install a temporary hardware store bolt, and move to the next hole.


Overall, take your time and triple-check things (and remember the advice from the section on drilling the aft spar attach?when you think you have it, walk away and come back later to check again). I found the engine mount to be less pucker-inducing than drilling the aft spar attach or the empennage attach bolts.

[Aero_Octaveus]

Ok, excellent advice all, thank-you. I will drill them later!