1998 FLHTC

I just finished re-installing my 4th bolt in the last 10 months. I keep shearing the same bolt, it's the left front bolt that bolts the engine to the rubber mount (just below the wires that come out of the engine from the stator.) I'm using grade 8 bolts and a nylon nut, but I can't any info in the manual about these bolts. I can't figure what I'm doing wrong, and why it keeps happening.

 

Is it hitting something? Are the mounts loose? Maybe you need a new mount, especially the front. I seem to recall some threads regarding the front mount being loose and a stiffer one available from HD. Also try a Grade 5 bolt for the hell of it and don't torque it down to gorilla specs.

 

I don't think it's hitting anything. Mount is not loose but will check again, and I'll look into the stiffer mount from HD. I haven't been a gorilla on these nuts just getting to tight. I'll see about finding a grade 5. Thx for all the good ideas.

 

I hate to disagree with Dr. Hess, But!

Going to a higher grade bolt will just transfer the problem down the line, ie: the next thing to go might be the engine flange the bolt secures.

If you can't find the bolt spec. go to HD and ask them for one. I know it will be expensive but not as expensive as an engine case.

USE A TORQUE wrench and torque to the proper specs, with the correct nut and washer combination.

I could justify this, but it would take way to long and involve a lot of helicopter references that you might not understand anyway.

Plus a dissertation on vibrations and resonances in metals. Tension, compression, single shear and double shear loads on bolts with a follow-up on elongation of holes in metal components.

Check and make sure the hole in the components you are fastening are in fact round. Don't want to stress/point load the fastening.

Just my .02

 

You should probably have the clinometer check done, to ensure the motor and tranny are sitting upright in the frame and not over stressing the bolt. I would also look for clearance between the mount and the frame as this could cause the bolt to shear.

 

Remove the front mount and check it Bad ones can be hard to spot.

 

I have gone to HD and it is a grade 8 bolt, 1 3/4 in length, with a nylon nut. They haven't had one in stock for 70 months so I got some at the hardware store. I haven't been able to find anything at all in the manual, let alone torque specs.

My plan is to get the new, stiffer front motor mount, and to keep digging thru the manual page by page. I will ask the dealer for bolt torque specs when I buy the new motor mount.

 

What condition is the other bolt in and have you replaced it also?
Think this through: the mounting bolts hold the motor mount in the frame; the motor is “Bolted” to the “Motor mount”. The torque required to hold the motor mount (sorry I don’t have the specs.) to the frame can’t be that much.
Have you checked the motor alignment?
Why haven’t you ordered the bolts and nuts from another dealer?

 

+3 on check alignment. If the bolt is shearing, then something is stressing that bolt out. In this case Xanax won't do the trick.

 

Something had to cause the origional failure, current bolts breaking are from the base problem, not which grade and torque of bolt is going in it. The front engine mount has to be flexing enough to go to a metal to metal condition with the safety washer hitting sending vibrations into the mount.

Peace of mind would to check for other loose mounting bolts which would cause misalignment as members are mentioning. This condition isn't the bolts fault, it is being stressed past it's limit by torsional vibration or movement in other areas.