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Old 03-28-2017, 08:17 AM
BruceMe's Avatar
BruceMe BruceMe is offline
Join Date: Feb 2005
Location: Shawnee, Kansas
Posts: 713
Default Engine Mount Cracking

I've owned An RV-3,4 & 6 all have the combined engine/gear mount. I've had to do this twice now on the -3 and the -6 (the -4 is not old enough yet). It's common/engineered fatigue issue and RV with this mount configuration should inspect for cracked mounts around the landing gear on every annual and I like to look at them every time I have the cowl off. It's really not a big deal to fix actually and you can do it without fully pulling the engine all the way off. If it's simple it should take about a week and $100, two (of both) if you wish it bead blasted to strip it all the way down.

..oh, I personally would avoid "beefing up" these joints. This is a designed break-point. Making it stronger will cause more expensive parts (like airframes/firewalls) to get damaged when you do bad things. It's a _LOT_ cheaper to re-weld a twisted engine mount that rebuild a firewall or throw away a bent fuselage.

My point is that proven and well-engineered designs are built and time-tested for a reason. Alterations, even seemingly well-intentioned and small changes like these have unintended consequences.
Bruce Meacham

Built/Own RV-4 254MM
Owned RV-6 4074V
First RV RV-3 3456B

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Last edited by BruceMe : 03-28-2017 at 08:38 AM.
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Old 03-28-2017, 10:04 AM
Sam Buchanan's Avatar
Sam Buchanan Sam Buchanan is offline
been here awhile
Join Date: Dec 2004
Location: North Alabama
Posts: 3,737

I found a small, hairline crack at the lowest weld on the aft side of both gear leg sockets after a few years of flying my RV-6. I watched the cracks for several years and they didn't spread. In spite of not being able to see how these defects could lead to a gear or mount failure, three years ago I pulled the mount away from the firewall far enough to run a weld bead over the cracks.....just because.

By next condition inspection a small crack had appeared at the edge of one of the new welds. I've decided this is something I can live with since this appears to be a compression failure where the mount is very unlikely to ever fail and I've never seen a crack grow to more than 1/2" long in this area. I will continue to monitor this situation but doubt I will ever feel the urge to repair it again.

At least until some sharp-eyed A&P sees it during a pre-buy someday.
Sam Buchanan
1999 RV-6
1918 Fokker D.VII replica
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Old 09-19-2017, 06:53 PM
N941WR's Avatar
N941WR N941WR is offline
Join Date: Jan 2005
Location: SC
Posts: 11,325

While completing my Condition Inspection I noticed a crack.

The crack is on the bottom horizontal tube, just inboard of the short bracing tube that ties the gear leg tube to that horizontal tube.

My engine mount is circa 2009 and supports an O-360. There are no gussets on this mount.

The current manufacture of the mounts, which is different than the person who made my mount, said he now machines the gear leg tubes rather than nesting and welding two tubes.

The mount will come off and be sent to where Steve will create a jig from my mount to make sure the six firewall bolts and the dynafocal mounts are all in the same place in space. This is important to make sure the mount will bolt back up and the prop/spinner will match the cowl.

Once my mount is in his jig, Steve will cut out the cracked tube and the old gear leg tubes and weld in the new machined gear leg tubes, cross tube, and gussets. Then it will have to be drilled to match my gear legs.

Next time you have your cowl off, please inspect it carefully for cracks.
Bill R.
RV-9 (Yes, it's a dragon tail)
O-360 w/ dual P-mags
Build the plane you want, not the plane others want you to build!
SC86 - Easley, SC
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