Snapped belt

[Nickj]

I had one snap across it's width at 18 months after fitting, never quite got what or how but it didn't do the SS motor much good.
Oddly that was the horizontal cylinder too.
Mine didn't have that mouse nest look though!!

[Flip]

Originally Posted by Nickj

I had one snap across it's width at 18 months after fitting, never quite got what or how but it didn't do the SS motor much good.
Oddly that was the horizontal cylinder too.
Mine didn't have that mouse nest look though!!

Snapping like that, across it's width suggests to me either a manufacturing fault or an over-stress failure which I suspect the 748 failure was as I am sure it would have been inspected at least between meetings if not races.

Whereas the failure that Nasher is dealing with looks more to be caused by something physical I would think.

I've seen one ripped in a similar way on an old Ford Mondeo diesel which was caused by a plastic tensioner (they weren't a service part apparently) failure but again we can rule that out in this case too.

[Kato]

In contrast I know of a set of belts on an M900 that has not been changed in over 12 years and not a sign of anything failing, the owner of said belts is not at all concerned

[Dukedesmo]

Originally Posted by Kato

In contrast I know of a set of belts on an M900 that has not been changed in over 12 years and not a sign of anything failing, the owner of said belts is not at all concerned

My Son was quite happy, having never changed the cambelt on his car at 12 years - thought he'd saved himself £300 that he was quoted to do the job until it broke.

Actually it didn't break as such, it just stripped off the teeth around the crank pulley meaning it just stopped turning!



At at this point, taking it to a garage would have cost more than the car's worth if it was even repairable? so we (I) did it ourselves, luckily the only damage was (all 8) exhaust valves bent and dinged pistons.

In the end the repair cost £250 in parts, including all new valves, seals, gaskets, tensioners, water pump & camshaft locking tools - so he's still happy having 'saved' £50 over the previously quoted price and reckons he'll leave it until it breaks again!

However it took around a month to get it sorted and it's not a job I want to be doing again - access to the belt being the worst part.

[Nickj]

I love doing those 'we can' kind of jobs

[Nasher]

I pulled the head today to see what the damage is.

There is a definite mark across the straight edge of the Exhaust valve cut-out in the top of the piston:
(Ignore the sideways scratches, that was me with a fibre rod down the plug hole the other day to show when the piston was going up and down)



And a corresponding mark across the Exhaust valve head.
Note the valve has spun round:



The easiest way to see if there was any gaps between the valve head and seat was to turn the lights out and shove a small pen torch up the Exhaust port to see if any light escapes:



Then spin the valve round 90 degrees to see if the light leakage moves, which it did:



And then the light can't be seen with the mark towards the camera.

[Nasher]

The stem of the valve is perfectly straight, but the head is canted over slightly.

I put it in my lathe with a dial gauge and spun the chuck by hand. The result was a 0.65mm runout.
Note that's on the bottom of the valve not the seat area, but it's good enough to show it's bent:





The guide feels fine, with very little slack.

Anyone got a spare Exhaust valve knocking about?

[Dukedesmo]

Looks like you got away with just the valve then?

The mark on the piston doesn't look significant enough to warrant changing it but whilst you're in there it's worth taking it off and cleaning it up anyway - also gives you chance to look at the rod and how it sits/floats on the crank but I would think it's fine down there.

The dings on my Guzzi piston were a little worse and so I took the rods off to check for straightness but they were fine, the big end shells were also good but having removed them it was a no brainer to fit new ones anyway as they're cheap plus the bolts are a one use-stretch jobbie so I changed them as well but, you can do all that on a Guzzi engine just by removing the sump, on a Ducati it's much more involved.

As this is a DS engine presumably the valves might be different (larger) to those on a 900?

[Nasher]

Thanks Dd, with the head off the mark on the piston looks far less severe than it did using my endoscope.

I'm loathe to disturb the barrel and piston really, there is always the chance of breaking a ring when refitting it, so am going to just change the studs I think.

The valve is a 7mm stem, 40mm head jobbie, @95.5mm long.

[Kato]

WOW lucky lucky lucky, Think I have some spare valves in fact I know I do will have a look later if I can find them your welcome to have

[Nasher]

Originally Posted by Kato

WOW lucky lucky lucky, Think I have some spare valves in fact I know I do will have a look later if I can find them your welcome to have

Thanks Mate, that would be great.

[jerry]

Interesting ,, when I did a mates 996 in Thailand that bent 2 valves after a cam belt , adjuster bearing seized and belt broke the damage was far worse than yours he needed 2 valves , guides , and a piston