Engine Mount Inspections. service problmes (cracks?)

Rvator51 asked about motor mounts and I miss read. He wanted to know about the rubber isolators. Any way I deleted my post on that thread and decided not to waste it and just post this as information. IF YOU HAVE ANY SERVICE PROBLEMS WITH THE STEEL Weldament post it here:

Metal fatigue and hidden weld defects are always a cause for cracks in steel. Do RV's have a particular problem with engine mounts? No. (with a RV-4 caveat, below)

Inspect it with a bright lights, good eyes, magnification and take your time. If you don't trust your inspection skills get a good A&P or AI. Also strip the wires and tape and clamps away. If you need to, by all means wire brush the paint away and clean it well. Of course repaint it after. If the engine mount is off the plane, by all means strip all the paint off and inspect it. You could even send it to one of the GA plane engine mount OH companies for inspection. They will use Dye Penetrate or Magnetic Particle Testing. You can by a dye inspection kit. One brand is "Zyglo", a fluorescent dye penetrate. You use a black light to expose cracks.

You also may want to up your inspection schedule. Frankly every oil change you should have a good visual inspection at it. Cracks start slow and grow. So it's not like it will fail immediately.

A mount may last in theory forever? Well that is a long time but in GA flight time longer than we will be around. It also depends on the kind of flying. Unpaved operations (aka soft fields that are not really soft) can be tough on the gear. The engine mount of course takes gear loads. If you do aggressive aerobatics, again a factor.

The RV-4 like most tail wheel RV's takes the landing load through the engine mounts. If you have had a hard landing, inception is a must. The older RV's had cracks in the corner bulkhead fittings on the aft side of the firewall, especially the the bottom ones. Changing those out is a bear but can be done. Some cracked and some did not. Again visual inspection. Remove the bolts (one at a time) and have a good look. Theses where only an issue on early RV's and the design was changed for the later RV-4's and all RV-6's and so on.

The answer is hard to put your finger on but there is no LIFE LIMIT on it. There are very very old planes, Cessna, piper, stinson, aeronca, sterman.......basically antiques with the original engine mounts 50 - 70 years old. Some times the FAA puts out AD's in contemporary airplanes for motor mount cracks.


RV's don't have AD's and I personally have not heard of any common or prolific engine mount cracks (except for the corner bulkhead fittings in early RV-4's). They do happen from time to time but its not systemic. Again operations is a big factor. There is always a chance a bad weld could happen, but from my experience Van has pretty good welds.

THE BIG KILLER IS CORROSION, hidden damage and pitting inside the tube. Hard to inspect but unless its a float plane on salt that should not be an issue for at least 50 years? (Trivia: hard core aerobatic planes with a steel tubing fuselage, fill the tubes with inert gas through a fitting. A pressure gauge in the cockpit indicates a crack if pressure is lost!)

I guess a question for Van is where are the critical locations to inspect on the engine mount or where have cracks been found before?

I found some powder coat stress cracks. I am going to remove some powder coat and inspect further. Wondering if anyone else has seen these before on a 6 with 350 hours. I have had some bad landings and also land on some ruff grass strips.

Jim Knight

cracks where?

Jim,

Where are your cracks located? It's hard to tell from the pic. Looks like the underside of the mount, looking at the right-side gear leg?

My mount has cracks in totally different locations, nearly opposite of yours. No harsh treatment of my bird, and I'm wondering if they have existed for a long time.

your worst crack looks like that one inboard of the last weld to the right; by the looks of it, the tube is cracked at least halfway through.

Cracks

It looks like the powdercoat has been stretched. I am sure it is a stressed area. I have seen metal stressed and the paint will do the same thing. The dark places at the left of the photo are where the powder coat is missing. The smaller tube looks bad in the photo but it appears to only be the powder coat. I will be removing the powder coat and looking closer this week. I figure this must be a weak area in the mount. I looked at the firewall and seen no issues. I may try to call Vans

GUSSETS

WHEN I BOUGHT MY 6 IT HAD 1600+ HRS. TT. ON INSPECTION WE FOUND CRACKS IN THE LOWER RIGHT HAND CORNER (ON THE COPILOT SIDE). VAN'S SELLS THE GUSSET KIT TO FIX THE PROBLEM. VERY TIME INTENSIVE AND EXPENSIVE. TOTAL ENGINE REMOVAL, STRIP MOUNT, WELD, REPOWDER COAT, REPLACE ALL MOUNTS AND HARDWARE....WELL YOU UNDERSTAND. BUT ALL OF THAT IS BETTER THAN A LANDING GEAR FAILURE. INSPECTION IS IMPORTANT BUT KNOWING THERE COULD BE A PROBLEM IS IS HUGE. THANK YOU FOR THE POST.
DOUG

Actually, you can fix this without removing the engine. I fixed a crack in exactly the same place on my -6 by lifting the engine with a hoist with it still attached to the mount. Remove the nuts from the bolts holding the mount to the firewall and slide the engine mount forward just enough to get a welder back behind the pad. You obviously can't powdercoat the fix but you don't have to pull the engine either.

Perhaps surface rust

N24YW said:

I found some powder coat stress cracks. I am going to remove some powder coat and inspect further. Wondering if anyone else has seen these before on a 6 with 350 hours. I have had some bad landings and also land on some ruff grass strips.

Jim Knight

Click to expand...

I found that the earlier powder coated parts from Vans had inconsistant coatings. You may find that your mount was not prepped properly prior to powder coating. It is hard to detect as the powder coat makes a nice smooth cover over the top, but eventually, it will flake away exposing the bad prep area. I had this on a few of my small parts, early HS and VS hinges. This is what it looks like to me in your photo due to the dark color of the steel. Powder coating usually does not flake off in patches if it was prepped correctly.
Let us know if you find any cracks or if it was just a case of poor adhesion due to bad prep.

Honestly, I plan on removing the powder coat before I install any of my mounts. I feel that the powder coating makes it extremely difficult to correctly inspect the mount. Not only can cracks in the powder coat not be cracks in the metal, but the metal can also crack below the powder coat without showing through. I prefer to strip the mount (media blasting works great if you have a big enough cabinet available) and prime, then paint in a white or black color. White shows up cracks better.

Hey Jim - ?

Jim - have you discovered yet if it is the mount cracking or just the powder coat?

No crack

It seems to only be a crack in the powder coat. It does show light signs of stress but no cracks in the tube. The pealing of the powder coat will be sanded down and painted. I will watch area closely at the next oil change.

Jim

hidden cracks

Jim, a note of caution:

If your cross tube stretched enough to crack the relatively flexible powder-coat, it may have cracked the metal as well. Once the hard-landing stress was relieved, the tube may have pushed back together to effectively hide the crack from view.

DO NOT SAND the paint off. I suggest you remove the paint with stripper, because sanding will smear the metal and further help to hide the crack.

Once the paint is off, use a dye-penetrant kit to check for invisible cracks.

If there's a hidden crack, the next hard landing or gear-stressing event could splay your gear out enough to make a prop-strike. Also, if that bottom tube gives way, you'd probably distort the firewall too.

Fly safe,

G.

Excellent Advice

born2fly said:

Jim, a note of caution:

If your cross tube stretched enough to crack the relatively flexible powder-coat, it may have cracked the metal as well. Once the hard-landing stress was relieved, the tube may have pushed back together to effectively hide the crack from view.

DO NOT SAND the paint off. I suggest you remove the paint with stripper, because sanding will smear the metal and further help to hide the crack.

Once the paint is off, use a dye-penetrant kit to check for invisible cracks.

If there's a hidden crack, the next hard landing or gear-stressing event could splay your gear out enough to make a prop-strike. Also, if that bottom tube gives way, you'd probably distort the firewall too.

Fly safe,

G.

Click to expand...

Looking at where the powder coat sluffed off, and the relative dirty appearance under it, I still believe it was poor prep, not stretching that cause this. A quick dye check would go a long way for your peace of mind though and is easy to do. I would follow that advice.
Keep us posted.

f1rocket said:

Actually, you can fix this without removing the engine. I fixed a crack in exactly the same place on my -6 by lifting the engine with a hoist with it still attached to the mount. Remove the nuts from the bolts holding the mount to the firewall and slide the engine mount forward just enough to get a welder back behind the pad. You obviously can't powdercoat the fix but you don't have to pull the engine either.

Click to expand...

Randy,

Was wondering it you could elaborate on the specific location of the engine mount crack you repaired. I have found a suspicious hairline feature on the aft edge of the bottom weld on one gear leg socket of my RV-6 engine mount (ten years, 940 hrs) and am deciding how to proceed. While assisting with a friend's RV-7 project today, I noticed the new mounts have a gusset at this location, so assume this is a known issue.



(This photo is of my project ten years ago but shows where the "crack" is located)

I'm still trying to decide if this is a crack or minor corrosion where primer didn't penetrate around the weld.

Thanks,