Engine mounting bolt sheared

[BillL]

Jeff, tough issue - -
You did not ask, but I would select an easy out that could be attached to a 10mm bar to turn it, then make a drill guide sleeve and insert in the hole to drill to an appropriate depth for that tool. Might even see if an extended length of a left turning bit were available to do the job. Then after drilling and cleaning back out the stub. 90% chance that it is not stuck, just fatigued and failed.

I just looked and only found 6" aviation RH bits, but might experiment and silver braze a bit in a long rod for an extension to "make" a left hand bit. Might have to do the same for the screw extractor.

If it is a case bolt, then a good discussion regarding potential case shear and fretting. Even so - -if not leaking and after removing the tip, if the free movement of the crank remains then would leave it. The crank motion should be checked at TDC so piston/ring friction is not included.

May luck be with you!!

[Jeff Vaughan]

Thanks for the great suggestions. Keep them coming!

[azonslow]

Jeff, check your PMs.

[RFSchaller]

Might be a substandard bolt that got into inventory as opposed to in service conditions.

[rgmwa]

Possibly not fully torqued originally leading to fatigue failure?

[Piper J3]

When a bolt is torqued to correct value then any cyclic tension forces, less than the pre-stretched tension, are not seen by the bolt. Conversely, if a bolt is under less than designed pre-stretched tension then it sees repeated forces that lead to failure. This is why cylinder head bolts and connecting rod cap bolts never fail if torqued correctly.

Another possibility is wrong grade bolt was used. Metric cap screws come in different grades. Grade 8 has less tensile strength then grade 12.

[azonslow]

Rotax MMH from September 2022 states that the M10x110 bolt must be of strength category 10.9 and goes on to specify that the bolt is torqued to 44 ft. lbs. using LOCTITE 243. See 71-00-00 page 20

[Bob Y]

Originally Posted by azonslow

Rotax MMH from September 2022 states that the M10x110 bolt must be of strength category 10.9 and goes on to specify that the bolt is torqued to 44 ft. lbs. using LOCTITE 243. See 71-00-00 page 20

Hmm. That’s interesting…. The same size, and 10.9 grade, bolts are used on the 12iS mount, yet the MMH for the iS (71-00-00, p 24) says to tighten to only 29.5 ft-lbs (40 Nm vs. 60 Nm). Big difference. Wondering why?

[azonslow]

I'm confused also. I've looked at the latest MMH for 912iS that I could find and the torque value is 29.5 (sometimes in NM) with no mention of LOCTITE. The iS MMHs are older than the September 2022 912ULS one I cited. This is what the MMH I cited said at the bottom of each page: "Effectivity: 912/914 Series" Maybe in Rotax speak that means all 912/914. Maybe Scott from Vans can help us out here.

[Bob Y]

If we can’t get an authoritative answer here, I can post the question on the Rotax Owners forum.