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  #1  
Old 02-13-2009, 11:30 AM
motzg motzg is offline
 
Join Date: Mar 2006
Location: mason city, iowa
Posts: 4
Default Firewall/Engine mount holes

My Engine Mount is shorter in every direction compared to the predrilled holes in the firewall. Vertical left = -1/8", Vertical right = -1/16", Horizontal top = -3/32", Horizontal bottom = -1/32". If I put 1/4" bolts thru 3 holes I have to hang a 30 pound weight on the open hole in order to catch metal to drill for a circular hole. Van's says to pull it into position and to drill the holes--if you oblong a coulple of holes, don't worry about it! I asked what the criteria (+-) was for the firewall holes and the mount holes. He said there was no criteria? What's up with that? He said every week someone calls complaining about the same thing. Wouldn't you think they would initiate some action--like don't drill the holes in the first place! Or tighten up the specifications at the weld shop for the engine mount. Has anyone had mismatched holes that have been to bad to drill? Input would be appreciated so I can formulate how to proceed.
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  #2  
Old 02-13-2009, 11:45 AM
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Brantel Brantel is offline
 
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Don't put the mount in a bind and drill or you will be asking for a cracked mount later!

Put the mount on there in a position that tries as much as possible to get the mount holes aligned with all the pre punched holes in the firewall. Find the happy medium between them all and go for it. You will either need a helper to hold the mount from moving or some way to clamp the mount to the firewall. Hold it secure because that bit will want to pull the mount around. You can use various bushings to step up the drill sizes to the full sized one.

I think you will find that you can find a combination of position that will allow you to drill out all of the prepunched holes completely.
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  #3  
Old 02-13-2009, 11:52 AM
C-GRVT C-GRVT is offline
 
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Default there are criteria

In the RV-7A manual I recall reading a short description of installing the engine mount, with the advice that positioning the mount is not critical (within 3/8 inch of centered was OK, if I recall correctly, but please check the manual).
I suggest that you position it for best fit on all the predrilled holes, so when the holes are enlarged, as few are "oblonged" and as little as possible. I am sure that you know that holes may be "moved" slightly by opening the pilot hole in the direction you wish to move the hole with either a small round file or a drill, but still within the diameter of the final size hole.

On the RV-6A I am building there are no predrilled holes, so I just centred the mount on the firewall/weldments and angles and drilled - it may be one place where predrilled holes do not make things simpler.
Bill Brooks
Ottawa, Canada
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thinking about the Whirlwind group buy
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  #4  
Old 02-13-2009, 12:16 PM
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Mike S Mike S is offline
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Quote:
Originally Posted by C-GRVT View Post
it may be one place where predrilled holes do not make things simpler.
Bill Brooks
Just a guess here, but I suspect the prepucnhed holes are quite accurate, and the jig used when welding the mount is also.

The problem is that the heat from the welding causes the mount to move around a bit------even though it is held firmly in a jig.
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  #5  
Old 02-13-2009, 12:53 PM
Norman CYYJ Norman CYYJ is offline
 
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Mount the engine mount using just bolt eg. top left corner. Check to see that the other holes are at least touching the edges of the engine mount holes. If they are bolt the mount down tight and drill the adjacent hole and bolt down then drill the remainder holes. I have used this method and it always works well.
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  #6  
Old 02-13-2009, 01:07 PM
Rich_in_AK Rich_in_AK is offline
 
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Location: Anchorage, Alaska
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Try putting 3/16 bolts in all four holes to average out the position of the mount. then remove one at a time and drill to size, putting the full size bolt in each hole as you go. As you drill each one nudge the mount just enough so you can see the full pilot hole before starting. That is the best you can do for it.

Last edited by Rich_in_AK : 02-13-2009 at 01:41 PM.
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  #7  
Old 02-14-2009, 02:50 AM
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kens_cockpit kens_cockpit is offline
 
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Location: Launceston, Tasmania, Australia
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Default A word of encouragement

It is better not to be too precise. I had
some pins made up large on one end to fit the engine mount holes and small on the other end to fit the predrilled holes in the weldments on the firewall. Somehow, clearly because I didn't check enough when drilling, I managed to drill one of the holes so that it looked like it had a pimple on it. I had completely missed the predrilled hole because I was trying to hold everything too accurately in place!

Vans suggested drilling out the wledment and reriveting a new one in place which I did. I actually redrilled the hole for the bolt on a drill press making sure it was all lined up accurately and square. Sounds improbably but it's worked. It all fits nicely now.

Anyway, just shows how trying to be too smart doesn't always work, especially if you rely on your supposedly foolproof plan and don't check to make sure it works on the way.

The important thing is just to make sure that where you are drilling the big hole is going to cover the little one. It's OK to elongate the little hole but I suspect its not a good idea to have the final large hole elongated!

So there you go. The worst that can happen even if you get it wrong is to replace the wledment that was drilled wrong. But better not to have to do that in the first place!
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  #8  
Old 02-14-2009, 06:18 AM
RV7ator RV7ator is offline
 
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Location: Boise, ID
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Default For Future Reference...

There's a lesson here for those sifting the achieves...

This is one place following the instructions makes for greater difficulty. If you are a slow builder, order the engine mount with the fuse kit, if not the whole finish kit (there's stuff in there nice to have as the fuse progresses). When you assemble the firewall (one of the first processes for the fuse), that's the time to drill it to the engine mount. Free of the fuse, you can reach around the edges for proper clamping, allowing jockeying around for best positioning (within 1/8" of the prepunched firewall hole is just fine) with respect to the mount. Later, when drilling the firewall to the longerons, temporarily bolt the mount in place so you can accommodate whatever firewall distortion there is in drilling to the fuse. Final assembly is merely bolting together.

Sorry, quick builders.

John Siebold

Last edited by RV7ator : 02-14-2009 at 06:22 AM.
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  #9  
Old 02-14-2009, 10:54 AM
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rvbuilder2002 rvbuilder2002 is offline
 
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Quote:
Originally Posted by Mike S View Post
Just a guess here, but I suspect the prepucnhed holes are quite accurate, and the jig used when welding the mount is also.

The problem is that the heat from the welding causes the mount to move around a bit------even though it is held firmly in a jig.
Mike has it exactly right.

Quote:
Originally Posted by RV7ator View Post
There's a lesson here for those sifting the achieves...

This is one place following the instructions makes for greater difficulty. If you are a slow builder, order the engine mount with the fuse kit, if not the whole finish kit (there's stuff in there nice to have as the fuse progresses). When you assemble the firewall (one of the first processes for the fuse), that's the time to drill it to the engine mount. Free of the fuse, you can reach around the edges for proper clamping, allowing jockeying around for best positioning (within 1/8" of the prepunched firewall hole is just fine) with respect to the mount. Later, when drilling the firewall to the longerons, temporarily bolt the mount in place so you can accommodate whatever firewall distortion there is in drilling to the fuse. Final assembly is merely bolting together.

Sorry, quick builders.

John Siebold
John's idea is also a good one but not always practical for people building on a budget, or for Q.B. builders as he already mentioned.
A way to make it easier is to scoung up four very long #8 screws (in many instances #10 screws will also work). Use them with some nuts and large washers to hold the mount in place on the firewall using the pre-existing holes. If you are able to use #10 screws, they will automatically align the mount enough that you will clean up the hole entirely when you drill. If you use #8's you will have to keep moving the mount until it looks good enough in all four corners.
Then drill the holes one at a time and install the AN6 bolt to maintain alignment.
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  #10  
Old 09-29-2009, 12:27 AM
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sglynn sglynn is offline
 
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Location: Anacortes, WA
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Default Drilling 3/8 hole over the 3/16 hole

Jeff, I am getting ready to drill my 7A engine mount holes but I don't have the mount yet. I just want small 3/16 holes for now. For a rotisserie. And so your suggestion of using these smaller holes to bolt the mount in place and then drill the larger holes sounds good. But one question. If when I drill the larger holes they are not centered on the smaller holes, won't the drill bit wonder and walk over to the smaller hole and try to center in it? How do you prevent this? Is the mount stiff enough to hold the 3/8 bit so it won't wonder but just cut into material and engulf the smaller hole? It sounds like the mounts will flex so will this process really work?
thanks
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