Engine mount hole drilled incorrectly

[kens_cockpit]

[IMG][/IMG] With tears, a sinking feeling in my stomach and thoughts of throwing the towel in on this project I sent the following email to Vans support this morning.

I have attached some pictures of the top left engine mount hole.

I aligned the engine mount using some correctly sized pins in 3 of the holes and drilled the first hole using the mount as a drill guide then put a bolt in that hole and moved to the next one until all four were done. It appears I drilled this hole first or last and the engine mount didn?t line up with the predrilled hole. Anyway the cause of the problem doesn?t matter except to avoid it next time!

The question is what do I do about this. I see a few options:

1. Leave it. I think this is not good. It is one of only 4 attachment points for the weight, power and vibration of the engine. The ability of the hole to work as it was designed seems to me to have been severely compromised. I see the potential for the edges of the original hole to scratch the bolt and lead to cracking. That seems far too risky to me.

2. This is my preferred option if it will be safe and satisfactory. Drill out the rivets holding the WD-802-L engine mount bracket angle and the F-801J filler plate to the firewall and longeron and install new WD-802-L and F-801J. This would leave the hole in the firewall only out of shape. The engine mount sits directly on this and that may mean there is some risk of scratching the engine mount and causing cracks there in the future. However I could deburr and smooth the sharp points on that hole which I anticipate would alleviate that problem. It would then mean that the stainless steel skin would not be providing support to the bolt all the way around but I expect the design is for the engine mount bracket angle and filler plate to do that and not the firewall itself. Also, drilling out the rivets will leave slightly enlarged holes in the firewall (though I would think not as bad as usual as it is stainless steel not aluminum). If this was a problem then perhaps some extra rivets could be installed to create extra strength. I do not think the extra rivets would be required but I will install them if you would advise that course.

3. Some other repair to fill the original hole and fill to the edge of the drilled hole. I can?t see how this could be done satisfactorily.

4. Completely rebuild the firewall and engine mount brackets. I think this would be overkill.

5. Something else you can advise me about.

Can I please have your thoughts?

Thanks
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[rv8ch]

Originally Posted by Ken

2. This is my preferred option if it will be safe and satisfactory. Drill out the rivets holding the WD-802-L engine mount bracket angle and the F-801J filler plate to the firewall and longeron and install new WD-802-L and F-801J.

I'm pretty sure this is what Van's will tell you to do. The small amount of stainless steel firewall material that is not there will not make any difference, but if it makes you feel better, put a dab of flexible firewall sealant on there when you install the engine.

This will be a bit of work, that's for sure. I can't recall, but I think you may need to remove the left side skin as well to get access to all the rivets. I hope not.

Hang in there, Ken. It's just a bump in the road.

[painless]

I agree that option #2 is the preferred route.

Regarding the fuselage skin, it will need to be at least partially removed in the area of the engine mount weldment. You should be able to drill out enough rivets to be able to "peel" it back enough to get a drill and then a rivet gun on the offending rivets.

Don't worry about the hole being enlarged in the firewall. Dress up the edges and apply some proseal on the base of the engine mount before you install it.

As was said before, this is just a bump in the road. You will be back on track in no time.

Regards,

[13brv3]

Originally Posted by painless

Regarding the fuselage skin, it will need to be at least partially removed in the area of the engine mount weldment. You should be able to drill out enough rivets to be able to "peel" it back enough to get a drill and then a rivet gun on the offending rivets.

I also agree that #2 is going to be the preferred way to go, but I don't see why the skin would have to be removed, or peeled back to do this. What am I missing? I just looked at mine, and don't see anything that can't be done with the skin in place.

The hole in the steel weldment could be welded up, and redrilled, but unfortunately, it would have to be removed to do that without melting the aluminum around it. Someone who's really good with a TIG welder could possibly do it in place, but you'd still end up burning off all the paint in the area, and then would face rust problems later. Now if you removed the weldment, then had it welded up, you could reinstall it, and all your original rivet holes would still line up.

The other option I can think of is to find a steel rod that's the same diameter as the original off center hole, and cut a piece just long enough to fit the thickness of the mount area on the firewall. You could glue it in place with JB weld, then carefully file it to fit the diameter of the correctly positioned hole. A large washer on the aft side of the firewall would capture the filler piece, as would the washer that's part of the mount.

I'm not sure how "acceptable" the above "fix" would be, but I'd sure be tempted to try it. The downside is that it will be roughly 500 times harder to replace the parts in #2 once the plane is flying, so you don't want to take the chance of having to do that later if this "fix" doesn't work.

Good luck,
Rusty (electronic technician, NOT A&P or ME)

[szicree]

I say remove it, weld it, put it back. A good welder can fix this as good as new and all the holes will line up perfectly. Like others have said, a small bump in the road.

[painless]

Regarding peeling the skin back.........

Again, I am not sure if the 8 is the same as the 6 in this area, but on the 6, that weldment is riveted to a longeron prior to drilling and riveting the skin on. It is these rivets that will need to be accessed to get that weldment off.

I don't have my plans in front of me, so I may be all wet here.



Regards,

[13brv3]

Originally Posted by painless

Again, I am not sure if the 8 is the same as the 6 in this area, but on the 6, that weldment is riveted to a longeron prior to drilling and riveting the skin on. It is these rivets that will need to be accessed to get that weldment off.

Hi Jeff,

Sounds like the -8 is different from the -6. On the -8, the rivets into that weldment go through the skin, then longeron, then weldment.

Cheers,
Rusty

[kens_cockpit]

Thanks for the input and encouragment. OK, I'll keep going with the project. The annoying thing is with hindsight the technique I used appeared extra careful but had an obvious flaw in it. And one careful look would have avoided the problem. Hopefully others who follow can learn from this mistake.

I was discussing this with an RV6 builder and he raised the possibility of drilling out the hole to a larger size, getting a larger bolt to fit that hole and machining it down so one end would fit in the engine mount hole, threading it and putting a nut on both ends.

Another thought was to drill the hole out larger and use a bush with the correct hole in it. But holding the bushing in place to drill while it is in the airframe so the hole is correctly located might be difficult.

I am also attracted to the idea of having the old hole welded up (after removing the weldment) so I can use the existng and matching holes for the rivets. However in the end I am thinking a new part can be back drilled and then I don't have to worry about how good the welder was.

This is Van's reply to me.

Ken: this is why it's best to use the mount itself as a drill guide -- there is inevitably some variation between welded tube trusses and trying to get them to match on 4 corners is hopeful at best.

In this case, the simplest solution -- and fastest -- is to replace the upper WD-802 bracket inside the firewall. It can be drilled off, and replaced with holes back-drilled thru the skin and longeron. The the mount can be bolted with the 3 fitting holes and used to drill the 4th hole thru the new Wd bracket. Carefully done, there is no reason this should over size any holes.

The stainless portion is not contributing any strength to this structure, so an oversize hole in it is of no consequence.

I think the whole job could be done in a couple of evenings. Then you can fly the airplane knowing it is done right and never have to think about it again.

In the end, the consensus seems to be with Van's suggestion which corresponds with my intial thoughts. I've ordered the new bit today and that's how I'll proceed - carefully!!

Thanks again

[painless]

Thanks for clarifying that Rusty.

See....I AM all wet!! Not the first time I might add......


Regards,

[jlfernan]

How did this finally turn out?