Engine mounting bolt broke...again

Hi all,

Hi have been putting some miles on my commando. A few 1000 km the past months, and they were 100% trouble free until this morning.

The bottom cradle to engine bolt broke and the 2 other bolts vibrated loose. Not really sure what happened first. It did exactly the same last year, and under the same conditions. When doing a bit of wide open throttle riding. The bike is not vibrating harder than it should and isolastics were adjusted recently.

Not really sure, but i thought i replaced the nuts by lock nuts.

Anyone else having a problem with self loosening mounting bolts?

Is your frame and / or cradle powder coated?

If so, the powder coating often squashes, allowing fasteners to become loose. Frequent and 'energetic' tightening is the only answer for this until the coating squashes to its minimum.

Yes, the cradle is powder coated. Good point! Thanks for the advice.

dimitri said:

Hi all,

Hi have been putting some miles on my commando. A few 1000 km the past months, and they were 100% trouble free until this morning.

The bottom cradle to engine bolt broke and the 2 other bolts vibrated loose. Not really sure what happened first. It did exactly the same last year, and under the same conditions. When doing a bit of wide open throttle riding. The bike is not vibrating harder than it should and isolastics were adjusted recently.

Not really sure, but i thought i replaced the nuts by lock nuts.

Anyone else having a problem with self loosening mounting bolts?

Click to expand...

I have seen that problem from time to time -even without powder coat.

If you use stainless hardware there they will not stay tight with regular use. They can not supply enough clamping force.

I have gone to using ARP 180,000 psi studs and eliminated the problem completely. Jim

Jim, are you talking about the ARP stainless bolts like Matt @ CNW a is using/selling or plane steel (relative term) bolts?? Thanks, Glenn T.S.

I had the same problem this year. Original paint on frame. I still do not know what happened. The bottom bolt was gone.

I had to take the exhaust off to get the new bolt in. I would like to make sure this never happens again.

I was wondering about the Rocky Mount SS bolts too. As I understand, they're bolts and nuts, not studs and nuts. Wonder how a grade 8 set would fare there, if you could find it. I worry about mine, but I don't put a lot of miles on. Maybe Matt will chime in.

No question there will be an issue if any powder is left on either side (inside or outside) of the cradle after powder coating. The coating is relatively thick and even though you will get a torque reading, that soft powder will start giving after putting some miles on the bike and once the bolt is loose, breaking is very possible

Remove powder to bare metal (washer size) on the outside and trace the shape of the engine cases on the inside to ensure that all powder that may rest against the case is removed or it may give you a false tight.

You can touch those bare metal spot up with some regular paint to prevent rusting

There are of course several different types of fasteners to secure the engine to the cradle and front iso but I have been extremely happy with my ARP fastening kit.

It's a 300 series stainless steel (ARP exclusive) that is conservatively rated at 170,000 psi. Bolts and nuts. Fine thread (3/8-24 & 5/16-24) with 12 point wrenching. This hardware is far better than the high grade stainless I used in the past and I am finding that it doesn't loosen up like most other hardware does over time

Matt / Colorado Norton Works

http://www.coloradonortonworks.com

gtsun said:

Jim, are you talking about the ARP stainless bolts like Matt @ CNW a is using/selling or plane steel (relative term) bolts?? Thanks, Glenn T.S.

Click to expand...

The 170,000 PSI stainless ARP kits that Matt has available is definitely up to the task.

Since looks are not the concern, I just use the 180,000 PSI black oxide ARP studs in my bikes along with maybe a stainless nut. Jim

Very useful info here. Thanks!

I borrowed the bolt and nuts from my other commando. The bike is at work. Hopefully i can get the old bolt out and replace it to get home and properly take care of the problem. Last time, with the bike on the center stand and an extra hand for leverage i managed to get it replaced with everything else in place.

Surface finish of the cradle seems pretty critical at more than one place. My swing arm developed a slight leak, probably from a sharp edge of coating damaging the o-rings.

Did anybody try other surface finishing options for those parts where thickness is an issue? Like parkerizing maybe?

Hi dimitri.
Re your "...The bottom cradle to engine bolt broke...".

Neither powdercoat nor resulting loose nuts will break bolts!
Where and how did the bolt break?
Perhaps post a picture.
Ta.
PS: stainless steel is maybe ok for mudguards but not any critical components i.e. not at all for fasteners.

needing said:

Neither powdercoat nor resulting loose nuts will break bolts!

Click to expand...

Thats not what the evidence is pointing to.....

The causes may need some serious investigation though.
Once a nut loosens, the engine can jump around and hammer the other bolts ?
So its not the one coming loose that breaks (?).
You do see the alloy crankcase bolt casting tunnels broken on more cases than you'd expect.

Since stronger seems to solve the problem, maybe the bolts are simply underspecified to begin with ?
So need to be super strong (or larger) and kept tight.
???

I bought the bike several years ago a a complete rebuilt bike. Previous owner did not ride it one mile. The cradle was freshly powder coated at the rebuild and i never did a retorque of those bolts. I had other bolts come loose so it was not a surprise the cradle bolts also loosened. I can only guess what happened first, but i do think as Rohan said the nuts vibrated loose first, and the jumping engine hammered the bottom (thinnest) of the 3 rear bolts into 2 pieces.
Both times, it happened at high speed (+140km/h), noticing my feet slipping off the pegs by vibrations that was getting worse within several miles. On releasing throttle when finally telling myself something is wrong and probably not wise keeping speed up that high, vibration was suddenly replaced by hard shaking what can indicate the bolt broke on throttle release. By the time i came to a stop, the thin bottom bolt was in half and the other 2 rear bolts were missing their nuts.
They broke both times at the nut side, at the end of the threaded part. I'll check later if the place where it broke matches the place where the cradle is bolted to the case.

No signs of stretched metal.
The bolts fitted were steel AN. Definitely not stainless after the first time it broke.
As soon as i can, i'll inspect everything

Thanks dimitri.
I was thinking sheared rather than necked.
Ta.