Engine Mount hole misalignment question

[BlackhawkSP]

Guys:

I started to install my engine mount today. The top two holes lined up well on the QB fuselage. The two bottom corners are a little off but I can pull on them and things should work out OK. The two center bottom holes however, do not line up well at all. One of these holes hole in the fuse is between 1/3 to 1/2 outside the diameter of the bolt pass through on the engine mount.

I have an email to Van's with pictures, but I wanted to know if others have had this problem and what was the fix. How much can I "tweak" the mount with some boards and a bottle jack before I over stress the welds, etc.? Any opinions would be appreciated. TIA

 

[BobTurner]

Mine did not line up. By pulling hard but by hand, no jacks or pulleys, I got the pre-drilled, undersized firewall holes entirely within the mount holes although way off center. Used rope to hold it there, used mount holes as a drill guide. Vans okayed this procedure.

 

[9GT]

Mine were the same as yours pretty much and I did pretty much like Bob did.

 

[RViter]

It is NOT a perfect world ...

Guys:

I started to install my engine mount today. The top two holes lined up well on the QB fuselage. The two bottom corners are a little off but I can pull on them and things should work out OK. The two center bottom holes however, do not line up well at all. One of these holes hole in the fuse is between 1/3 to 1/2 outside the diameter of the bolt pass through on the engine mount.

When looking at the problem of alignment it becomes a matter of an immovable object meeting an impenetrable object, more or less. Start with the premise that there is some flexibility in the portions of the mount, and other points are virtually immovable. On the RV-6A QB I found that the middle two bottom holes had L-R spacing that was determined by the steel tube between the holes but it was possible to match-drill each hole 'a little bit' to the left or the right, but once you chose to align ONE of them and match-drill a concentric hole, the entire mount could shift to that side and almost every hole could then be OFF center. So a little shift on each hole to average the error.

I had decided on a couple of corrective holes on the most immovable points and then used a tow-strap to cinch up a little flex on those that would flex.
So I started in the bottom center, and then bolted them tight before flexing the other holes. I think I also decided that it had to at least maintain pilot bolts (undersized) in every hole simultaneously before match-drilling any hole. Once it was match-drilled I tightened the correct size (street hardware) into the hole to maintain position. BTW, I put the street hardware in backwards (front to aft) and then could use the correct AN bolt to tap out the street hardware. Being careful not to force or bang away and damage the final bolts.

gary

 

[DaAV8R]

Mount holes

I had the same issue on my -8. It doesn't take all that much force to spread the mount a bit. While I could spread it by hand I felt it was important to stabilize the mount at the wider dimension. Trying to spring the mount and drill seemed like a recipe for disaster.

I built a couple of spreader bars from a piece of all thread, a piece of tube and some double nuts. I attached the spreaders vertically, one on each side of the mount. This provided an engine mount which was dimensionally stable.

I spread the mount the minimum distance which would swallow the pilot holes. I pulled the bottom holes with the use of a dremel. The process went much quicker than I expected.

I like to use some bushing material when I drill these large holes. This allows me to start small and work up to the final hole size. The last drill bushing is a piece of brass tubing from the hardware store. Very thin wall and leaves just enough material for the final hole to be brought up to size with a reamer.

Here is a pic of the spreaders, installed on the engine mount with adel clamps.



I don't know why my photos show up full width. I change the width to 320 but they still show up full width

 

[BSwayze]

I think slight misalignments with these holes is a fairly common issue. I don't think I've talked with one builder who hasn't seen it. But the holes are undersized and the fix is easy. I wouldn't recommend messing around trying to force the weldment or bend it. It's not necessary. I started with a 3/8" drill bit and hardware to get the motor mount temporarily bolted to the firewall, and then used two more bits and finally a reamer to gradually drill them out to full size. The final holes will be perfectly centered that way. I have it all documented here, with pictures, feel free to take a look.

Installing the Engine Mount

 

[BlackhawkSP]

Hole Misalignment

Guys:

I "got her done" today. Using a combination of Bruce's method and a little tweaking with a couple of boards in the right pivot points, I got the worst misaligned hole within the mount pass through before drilling the third hole. The others weren't perfect, but once I had the third bolt in the worst one, it was down hill from there. It's solid as a rock now on the firewall. Also, Vans builder assist actually said they saw the replies here and agreed to go that route. Thanks for the replies.