Cracks in exhaust U tubes on XT912

[Bluey]

After 440hrs of operation, I have detected two small cracks in the the U tubes on my XT912. These followed the detection of wear marks found on the exhaust lugs on both the front and rear horizontal pipes that attach to the muffler. In total, 4 lugs out of the 8 that attach the above mentioned pipes to the muffler showed wear. This is the second trike that this has occured to. In each case, no noticeable vibrations have been noticed. With the first trike, 2 lugs underneath the engine that connected the horizontal pipe to the muffler showed significant wear. In that case, the front U tube cracked.

 

I don't think that it is a coincidence that wear on the lugs is independent of the tubes cracking. I believe that the cracking is probably due to a poorly aligned exhaust. When I pulled the latest exhaust apart for repair, I noticed that the horizontal pipes were very lose and not in contact with the muffler. This would have led to very large horizontal forces being applied to the vertical U tubes by the springs attaching the horizontal pipes to the muffler. Over a 440 hr time period, the stress and vibration that would have occurred has caused the pipes to crack and the lugs to wear. The other example of pipe cracking on the previous trike occurred at 453hrs (this was when it was detected). The ball joint of the horizontal pipes and the connecting socket of the muffler show no wear at all implying little to no contact. The joints that connect the U tubes to the muffler show some wear as expected as these parts are in constant contact (the wear is not at all excessive given the hours).

 

From my perspective, it is clear that the exhaust components must be in contact when the springs are attached to elliminate the horizontal forces acting on the U tubes. All operators need to inspect the lugs for wear as this is an indicater that stress/vibration is present in the exhaust system. The remedy would be to loosen the exhaust bolts to allow all the exhaust parts to relax and hopefully elliminate the stress.

 

With the previous U tube failure, an exhaust speciallist was able to weld it but unfortunately, the part didn't even last 50hrs before hairline cracks were noticed in the heat affected zone of the weld.

 

It was suggested that placing a plate over the exhaust would be better but the exhaust specialist claimed that this was a no better solution than welding the crack. He also stated that he strongly believed that the original cracking was caused by prolonged and severe stress on the parts.

 

No other parts of the exhaust or engine mounts show any evidence of excessive stress.

 

Bluey

 

 

[alf jessup]

G'day Bluey,

 

Strange that it has cracked, i know of a few others that have had the same problem on some 912's.

 

I must be a lucky bugger, done 820 hrs on old 001 and had no problems at all with her other than i replaced a gas strutt on the left rear shock as it was losing about 100kpa and making it squat slightly left low at 450hrs.

 

I had a few knockers when i bought it 5 years back with 287hrs on the tack, proved them wrong hasn't it

 

All i can say is Airborne might have just put a little more love into the mule 001 i got to get it certified for casa.

 

Hope you can get it sorted and i hope the grandad of all XT912's keeps giving me trouble free flying for another 820 hrs.

 

Cheers

 

Alf

 

 

[Bluey]

Thanks Alf, just out of curiosity, have you ever checked the prop pitch or the engines carby synchronization?

 

Bluey.

 

 

[alf jessup]

Yeah mate done both, have pitched the prop myself and assisted with the carb sync both a number of times

 

with my bike motor mechanic mate, he was trained by BMW and knows the carbs well as BMW used them on their bikes.

 

Cheers

 

Alf

 

 

[Bluey]

Thanks Alf, all the best with 001.

 

Bluey

 

 

[Bluey]

I forgot to ask Alf, when you checked the prop and carbys, did either need much if any adjusting?

 

Bluey.

 

 

[alf jessup]

Bluey,

 

Prop never moves in the times i've checked it, I set it up with a (digital) inclometer as per airbornes directions in the manual, the digital one is far more accurate i feel than the one airborne use.

 

The carbs are never usually out by that much but there is always some minor adjustment to the cables to get them balanced.

 

I usually take the trike base to the shop on the trailer hooked to the car and do the adjustments on the trailer, i find it is a lot harder to walk into the prop whilst being in the trailer frame forward of the prop.

 

Cheers

 

Alf

 

 

[Bluey]

Thanks Alf, I've never found the carbys out by much either. I or anyone else I know have never checked a prop. I think I'm about to find out how mine is though.

 

Bluey.

 

 

[alf jessup]

Only needs one blade to be out a degree or less and it will have some form of harmful vibration.

 

Alf