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  #1  
Old 08-17-2018, 10:22 AM
RyanB RyanB is offline
 
Join Date: Sep 2017
Location: South Africa
Posts: 36
Default Firewall question

I have just completed my firewall and am ready to mount it to the jig. My question is that obviously the top mount holes are there as they were already made in the firewall, but at what stage do I drill for the bottom bolts of the engine mount? Do I wait to use my engine mount as a guide or do I just drill them as per the plan now?

Another question please is that the holes for the engine mount are now only at 1/4 inch, at what stage do you open them to accept the 6/16 engine mount bolts? The reason I ask is that surely if you open them up once everything is riveted together, you will not be able to deburr the holes properly in all the material?

I hope this makes sense

Thank you for al the help
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  #2  
Old 08-17-2018, 11:34 AM
David Paule David Paule is offline
 
Join Date: Dec 2009
Location: Boulder, CO
Posts: 3,525
Default

While the RV-3B is different from the RV-4 one in that area, I'll mention what I'm doing.

I have all six engine mount holes drilled in the firewall, but only to 1/4". I'm assembling the fuselage now and when it's ready, plan to remove the front of the jig and drill the holes out to 3/8".

But only one hole, at first. Then with the engine mount held there, I'll use that as a guide to ensure the alignment of the remaining holes, using drill bushings. I expect to be able to level the mount or at least adjust it in that direction a small amount if necessary.

Then I'll remove the engine mount, deburr what's possible, and install the mount.

Following that, I'll install and align the main gear while the fuselage is still on the jig.

At least, that's my plan. We'll see how it goes....

Dave Paule
RV-3B
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  #3  
Old 08-17-2018, 11:54 AM
RyanB RyanB is offline
 
Join Date: Sep 2017
Location: South Africa
Posts: 36
Default Drilling

Thanks so much for the info Dave, that does sound like the best way forward!Man building a 3 or 4 is definitely not the easiest, but will be well worth it at the end!

Maybe using a reamer to open the holes to final size will leave less burrs?
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  #4  
Old 08-17-2018, 04:45 PM
Christopher Murphy Christopher Murphy is offline
 
Join Date: Feb 2007
Location: colorado
Posts: 766
Default What I recall

I am pretty sure I did not drill any engine mount to firewall holes until I had the engine mount to use as a jig. i doubt your holes will line up if you drill them to the plans.

Maybe the kit is more refined now since I did mine almost 20years ago.

Cm
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  #5  
Old 08-17-2018, 05:53 PM
Tom Martin Tom Martin is offline
 
Join Date: Jan 2005
Location: Ontario, Canada
Posts: 1,392
Default

If you have the engine mount there is no reason not to drill them to size now. I did the last HRII this way and had the engine mount bolted to the jig and firewall for construction. In place of the supplied bolts I used threaded rod with nuts on both ends. I extended the length of the jig so that I could use the long threaded rod through the jig, the motor mount then firewall. Using the longer threaded rod allows you to perfectly square up the firewall to the fuselage in the jig by moving the four corner nuts in and out.
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Last edited by Tom Martin : 08-17-2018 at 05:56 PM.
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  #6  
Old 08-17-2018, 06:56 PM
RussellT RussellT is offline
 
Join Date: May 2016
Location: Albany Western Australia
Posts: 33
Default

I pretty much did mine (also a 3B)the way Dave has described. If I ever do another I would do it as Tom described.
The holes you drill as per the plans/instructions serve the purpose but will not be perfectly aligned unless you get real lucky. Don't go larger than 1/4 for the firewall to jig bolts and measure very carefully if you go this way. I used a reamer to finish the holes.

Russell
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  #7  
Old 08-17-2018, 08:14 PM
rv4ross rv4ross is offline
 
Join Date: Dec 2005
Location: Locust Grove, GA
Posts: 49
Default

On my 4, I waited until I had the fuselage rolled out of the jig and then used the engine mount as a guide to drill the holes. That way I could install temporary bolts to hold the mount in place without having to crawl under the fuselage jig to install them.

Ross Scroggs
RV-4 #3911
Finishing stage
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  #8  
Old 08-17-2018, 08:51 PM
Christopher Murphy Christopher Murphy is offline
 
Join Date: Feb 2007
Location: colorado
Posts: 766
Default Did the same thing

Quote:
Originally Posted by rv4ross View Post
On my 4, I waited until I had the fuselage rolled out of the jig and then used the engine mount as a guide to drill the holes. That way I could install temporary bolts to hold the mount in place without having to crawl under the fuselage jig to install them.

Ross Scroggs
RV-4 #3911
Finishing stage

I also didnt drill the engine mount until the fuselage was off the jig and upright

Cm.
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Pitts S1S "Mexican Red" sold and missed
Mr. Twister Airshows
Rocky Mountain Renegades
the mission... have fun.
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  #9  
Old 08-17-2018, 11:55 PM
RyanB RyanB is offline
 
Join Date: Sep 2017
Location: South Africa
Posts: 36
Default Thank you

Thank you so much for all the replyís, I have thankfully already got my engine mount, so I am really liking the idea of using that now as a jig for the holes. I shall turn up some bushes and use them as a drill guide through the mount and then open those holes up to size.
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  #10  
Old 08-18-2018, 09:40 AM
chris mitchell chris mitchell is offline
 
Join Date: Jul 2006
Location: near Harrogate, England
Posts: 366
Default

I drilled the four corner holes using the mount as a jig, as Dave Paule suggests.

I then used the mount to ensure the firewall was square. I drilled the two extra, central, holes later. I used various bits of brass tube as a series of bushes and then used a reamer to get the holes to final size. There is no give in this mount so accurate drilling is essential if itís to fit and not to have ovalled holes.

One point worth noting is that the few RV-4s Iíve looked at, the builders have all had to use some AN970 penny washers as shims for the central holes 1-2 per hole. I think that just reflects the difficulty of getting the correct angle on the lower firewall section.

Chris
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