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.
Van's Air Force
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Firewall/Engine mount holes
- Thread starter motzg
- Start date
Brantel
Well Known Member
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.
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.
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
RV-6A in progress,
thinking about the Whirlwind group buy
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
RV-6A in progress,
thinking about the Whirlwind group buy
Mike S
Senior Curmudgeon
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.
Norman CYYJ
Well Known Member
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.
Rich_in_AK
Member
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.
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kens_cockpit
Well Known Member
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!
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!
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
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
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Mike has it exactly right.
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.
sglynn
Well Known Member
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
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
Rick6a
Well Known Member
Steve,
What you are really asking is "How do you relocate a hole?" The question you ask is valid and there is a solution. This is done all the time in the production environment, I've done it myself countless times. The problem for the amateur homebuilder is, the procedure to do it successfully on the firewall may require tooling you do not possess. If you are creative you might be able to cob something together that will work. See this tip: "How to pull a hole" to understand the fundamentals.
http://www.vansairforce.com/community/showthread.php?t=11769&highlight=pull+hole
chris mitchell
Well Known Member
See also
http://www.vansairforce.com/community/showthread.php?t=26356
Van's advice is consistent! Knowing what I know now, seeing the apparent discrepancy you have measured between firewall holes and mount, I would move the holes to minimise the stress in the mount but I would prefer to have round holes of the correct final size that take in all of the predrilled holes, rather than oblong/oval ones.
Chris
http://www.vansairforce.com/community/showthread.php?t=26356
Van's advice is consistent! Knowing what I know now, seeing the apparent discrepancy you have measured between firewall holes and mount, I would move the holes to minimise the stress in the mount but I would prefer to have round holes of the correct final size that take in all of the predrilled holes, rather than oblong/oval ones.
Chris
Noel Simmons
Well Known Member
save the engine mount!
I take a piece of the aluminum tubing that comes with the kit. the OD is 3/8 and the ID is ~1/4 and a 1/4" drill bit will fit in it with some help. This is your drill guide.
I take a piece of the aluminum tubing that comes with the kit. the OD is 3/8 and the ID is ~1/4 and a 1/4" drill bit will fit in it with some help. This is your drill guide.
