broken motor mount bolt

[Slyfox]

I thought I would share an experience I just had. while doing my yearly inspection, I found a broken bolt for the motor mount. I sunk. at first I thought it fell out. I looked closer and it was broke clean off. the location was the left upper rear. I removed things to get the mount off. intake, water hose to cylinder. looking at it, it wasn't going to be fun. it was broke off about an 1/8 inch inside flush, not only that the cylinder was in the way for a straight on attack. I took a carbon burr and did the best at putting a nice debit and a nice spot to center punch, which I took a long flat punch and put a point on it. center real close now.

now to understand this the original bolt is a 10mx1.5x50. 10.9 grade. so it is hard steel. I than got some drill bits that were 8 inches long. now I can get a straight shot with no bend. didn't have to remove the cylinder. I used a 3/16 bit and got about 1/8 cut. than I took my extractor and tapped it in and than took a 1/2 wrench and carefully started turning, tight at first and than it came out. feeeeew I was done getting that out.

next step installation. I read a bulletin I found online for a skyranger. lots of good information, seems they are having a bunch of problems with this very bolt breaking. had in there to install a new 12.9 tensile strength bolt and torque to 27ft lb. also they mentioned grinding down the fins on the cylinder so there would be a better straight on with installing the bolt, so I did that.

to install this side on my 4 it also took two washers to make things center for a good install with the rubber mount against the block. so I put to washers there. nice line up and install. I checked all other bolts for 27ft lb and found the lower bottom on the same side loose. so now that's tight.

I guess what is needed is for everybody to check these mount bolts and torque them good. I now have a stronger bolt on that location. I have the 912uls. I do fly hard, I have a medium IVO prop and hang on the prop for landing short field. plus hit it hard on take off. I'm sure that doesn't help for this type of thing.

[DesertFox4]

Thanks for the heads up Slyfox. I'll be checking mine next time I'm at the hangar and proper torque on all the mount bolts. I have the same setup only with the Patriot Ivo.

[TahoeTim]

Is this the ring bolt to the block or one of the rubber mount bolts between the ring and the motor mount?

[Slyfox]

bolt that goes into the engine

[TahoeTim]

thanks, the comments about rubber mounts. I get it now. Should we be looking at locktite, nordlock washers, safety wire, etc?

[SWeidemann]

So there is a history of broken bolts on this same location in other aircraft using the 912ULS? You replaced it with a super hard bolt, so now I'm going to be looking into doing the same thing with mine before I have a broken bolt. Where can I get those bolts?

Skot

[rv9ralph]

I will preface this with I am not an engineer, nor do I play one on TV... however, if you replace the broken bolt with one that is harder than that specified, there are several considerations. First, harder bolts are usually more brittle and will fracture. Another consideration is with a harder bolt so it won't fail, will the case then fail. I would rather check and replace bolts that have a case crack.

Just thinking... Ralph

[PapuaPilot]

Ditto on rv9ralph's comment. Harder isn't always synonymous with better. I would be suspicious of vibration from the engine of prop.

[Dave S]

From my meager knowledge of metallurgy (and "fastener-ology") I have to agree with Ralph & Phil. Tensil strength is not the only concern....toughness of the metal is also; and, success is a balance of the two. And that is just the characteristic of the bolt, before considering installation.

DF 4's earlier comment about checking the torque on these bolts periodically is especially important as the engine mount bolts are subject to a whole lot of vibration; and, bolts can and do lose torque on occasion due to face wear on the bolt or contacting washers or if some paint/powder coat is crushed between or if a too soft a washer is involved .........when torque is lost, that's when the fastener begins to get abused. The engine mount does not have dowel pins to take the shear; and, the bolts would take it all. We check the torque on our propellers frequently; but, I wonder how often we check these particular bolts? Both applications are subject to a lot of direct vibration.

I am thinking DF-4's suggestion rates pretty high on preventing this from being a problem. I have always used torque seal on applications like this but that only tells a person if the bolt has rotated, not if it has lost torque due to wear. Re torquing these bolts on condition inspection might be a good point.

Since a person can't know what he torque was before the bolt broke, using a higher tensil strength bolt is only a guess; and, if torque is lost on a harder bolt, it will likely crack easier than the original bolt if it loses torque.

I recall reading a technical article on fastener failure once......almost always due to installation issues such as torque, etc, was real high on the reasons for failure

Sincerely,

Dave S
KF 7 Trigear
912ULS Warp Drive

[Slyfox]

So there is a history of broken bolts on this same location in other aircraft using the 912ULS? You replaced it with a super hard bolt, so now I'm going to be looking into doing the same thing with mine before I have a broken bolt. Where can I get those bolts?

Skot

I got them here in Spokane at a place called fasteners. the head is allen head, which makes it a whole lot easier to install. plus I ground some of the fins off the cylinder to make it easy to install. DO not put any locktite or anything else on this bolt. also for the bolts for the rubber mounts, I torqued those to 18ft lbs and than went to the next hole for the cotter pin.