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Engine frame from VANS - dimensional accuracy?

Steve Sampson

Well Known Member
Has anyone else had problems with the dimensional accuracy of the engine frame on the -4? The 20? in the vertical between the corner bolt centres is spot on, but the 23? across is significantly less. Perhaps 22 ??. (I need to re-measure to be sure.) It matters at the lower corner because it is starting to become tangled with the firewall where it turns forward to accommodate the rudder pedals.

In the event that I push VANS for a new one is that a simple replacement or will I then have a problem matching the gear legs to the frame?

I am amazed to have this problem. The model I had as to how you would weld one of these up, was a jig with 4 bolts standing up to mark the corners. Clearly not the case. What has happened when people change the engine frame from the short gear legs to the longer? Or how would you change a frame.

I guess I am wondering if I have a rogue unit or are they all different?

I would welcome comments.
 
mount fit.

Steve, I didn't measure mine but the fit was spot on. I'll go measure mine this weekend if I get a chance and see how it comes out.
 
Steve,
I just measured my engine mount for you. Easy to do since it's hanging on the wall. I had it powder coated so it is serving as wall art in the shop. The mounting holes are spot on as per the drawing. Two of the tubes are angled very slightly so when the bolts are inserted they bind just a touch. I can still slide the bolts in without using a mallet. Cheers
 
Motormount alignment

What has happened when people change the engine frame from the short gear legs to the longer? Or how would you change a frame

Steve

I rebuilt a wrecked RV4 & changed the existing mount to a longer legged mount...I don't know if the old mount was short or long gear leg version and it did not matter since it was bent & broken along with misc. other "stuff". The geometry of the new mount differs from the old so the wheels will end up with the same "ground" CG. Aircraft have two different CG's...one for inflight and then one for wheel location which effects ground handling. Merely plugging in longer gear legs would upset this wheel location so the entire mount must be changed AND new weldments in the lower corner as well as the two center mounts must be changed as the holes will be slightly different with the longer gear leg motor mount. The upper two holes remain the same.

I replaced the firewall, lower weldments and center pads as well as fwd. skins, bolted the new firewall into the 2 upper bolt holes and back drilled each hole installing proper bolts as I went after shimming for any irregularity. That mount is so strong that I am sure the airframe would twist before the mount during installation from any misalignment so shims were required but without changing the thrust line. Also remember, the pilot is there to correct any out of trim conditions or he wouldn't be needed...Take that with a huge grain of salt!!!!

This may not answer your original question but may shed some light on solving your alignment problem. It worked in my case as the airplane flies just fine.

Dick DeCramer
N500DD RV6 Flying
N149KC RV4 Flying
RV8 in the works
Northfield, MN
 
I have never seen a motor mount that "met specification" exactly. They are welded just as someone above says on four pegs or a jig of some type... but when you weld that much tubing together, then it probably takes a sledge hammer ( :mad: ) to remove it form that jig..... I think that is why Van's tells you to "use the motor mount as the fixture to drill the final size holes through the firewall....".

I would be very surprised not to find variations of up to 1/4" from drawing dimensions on such a complex weldment.
 
Got the frame on today. Panic over!

Chickenlips and others, thanks for your inputs. My inspector gave me a hand and we got the frame on today. All is well. He concurred exactly with your comments that after you pull the frame out of the welding jig it can move every which way.

I think this was an issue of my inexperience with welded items, and VANS instruction to 'match drill it to the firewall." It was a bit more complicated than that!

Thanks again!
 
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