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Discussion Starter · #1 ·
Hi, I bought a 1999 Monster 750 as a project. The bike was semi-disassembled. The cam belts were missing as well as some other bits.
I continued disassembly and found the vertical cylinder dropped the intake valve. It doesn't seem like a total belt failure caused this since the key and keyway on the cam are damaged indicating the belt was still driving the cam. However, the belt could have jumped timing and caused the problem. As mentioned, the belts were already gone when I bought it.
Two questions: 1) Has anyone seen a similar failure and determined the root cause? 2) In general, how fragile are these engines? This one has 28k miles and other than the valve failure seems OK.
Thanks
 

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Incorrect belt tension can cause them to skip a tooth.
Engines that have sat for long periods of time without being turned over can have the belts form themselves to the shape of the pulleys causing problems as well.
Replacing belts and doing valve clearances are the 2 "must do" service issues.
How badly did the valve hit the piston?
What are you planning to do to fix it?
Replace just the valve or the whole head?
Need pics mate, of the bike and the carnage.
 

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Discussion Starter · #3 ·
Sorry, I tried posting pictures, but this forum makes it too difficult. I can't simply paste them in.

I've built engines for about 50 years, everything from small 2-strokes to marine diesels. All dropped valves look about the same. The valve head breaks off, gouges both the head and piston, usually imbeds in the piston, the bent stem is stuck in the valve guide but the cylinder is usually OK. The differences lie in the amount of collateral damage. In this case, the cam drive key and keyway are damaged. That is unusual and indicates the belt was good and still trying to drive the cam. Although this may be attributed to the desmotronic system. The cam is trying to close the valve, but can't because the bent stem is jammed in the guide.

To fix it, I bought a used vertical cylinder, matching piston and head assembly. I have a collection of Starrett, Mitutoyo and older Fowler micrometers, DI's and DBG (none of that shoddy Chinese stuff). I inspected all the fits and clearances of the used vertical cylinder and the bike's horizontal cylinder. Everything meets Ducati book specifications including the valve clearances. The odometer reads 28k miles so it's seems modern materials and oils work real well. I'll put it back together with new seals, o-rings and gaskets.

Back to my original questions 1) Has anyone seen a similar failure and determined the root cause? 2) In general, how fragile are these engines? This one has 28k miles and other than the valve failure seems OK.
I've ridden a bunch of bikes, but never a Ducati. I like the size and feel of the bike just sitting there. Hope I feel the same once it's running and riding.
 

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In general, how fragile are these engines?
It should take a fair beating IF the belts and valve clearances are OK.
The dropped valve shouldn't happen if it's maintained but,,,,,maybe
A. It threw its valve collets, most likely a maintenance stuff up.
B. Incorrect fitment of timing belt, most likely a maintenance stuff up.
C. Has it got adjustable cam pulleys? maybe the outer slipped.
D. Someone put 900 cams in it and stuffed the cam timing.
E. It was ridden without the belt covers and a rock got in there.

Poorly tensioned belt causing it to skip a tooth is the usual suspect.
The keyway is definitely damaged?
There were offset key steels for timing adjustment on earlier bikes without the adjustable pulleys.
 

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Sorry, I tried posting pictures, but this forum makes it too difficult. I can't simply paste them in.
Not sure about a phone, but it's simple on a pc. No, you can't just paste a picture on the page.
When you start a post, after typing etc. (or before) press enter, to create a new line.
select the "insert image" icon just below. To the right of a paperclip, left of a camera icon.
A box opens with 'drop image (or click here)'. just click on it and next, select from any recent downloaded pictures, or select, 'show all files'.
This will take you to where your pictures are stored, just pick the one and click 'open'.
It then will be uploaded to the post.

Rectangle Font Line Screenshot Red
 

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Discussion Starter · #6 ·
Many thanks Ronski. When I first opened that "insert image" icon, there was only upload or url. Now it has the option dropping one in?! Anyway here are the relevant parts.

The key is shown sitting on top of the head. It is damaged. I've used both stepped and offset keys in other engines. This engine made an offset one all on its own. Amazingly, it's almost halfway through the key. The cam sprocket could move back and forth, but the sprocket keyway is OK. The cam sprockets are the fixed design rather than the 3 bolt ones.

I got the bike in pieces. I'm missing some important clues to the failure like the old timing belts. I also don't have any info from the previous owner about the "event".

I going with some kind of belt failure due to lack of maintenance. The belt selection for this engine seems reasonable. The belt is almost as wide as Fiat 1600's , but only drives one cylinder rather than four. Since all the rest of the working surfaces are within tolerance, I'd say it's a pretty good engine.

Automotive tire Bicycle part Gear Vehicle brake Rim

Wood Gas Auto part Nickel Hardwood

Automotive tire Gas Bumper Auto part Motor vehicle
 

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Glad it worked, (pictures).
Most times I've found, and in the very few cases I've seen a dropped valve, yes, usually a failed belt. But there is another possible (rare) cause.

The closer shims are set too tight, and I mean 'way' too tight. After changing, some forget to double check after putting in a larger set to take up previous loose ones.
In almost every case I've done, It requires some additional sanding or such to get it right. There should be next to no play what so ever on the closers, BUT, you MUST be able to rotate the closer shim by hand (fingers) easily once done.
If too tight, the valve head is slammed into the seat way too hard. Usually the shims (or collets, some call them) will break first, but not always.
Hard to see in your picture, but have a really close look at the inlet seat, It's pretty evident if it's been slamming down hard, the seat will be deformed evenly some of the way, if not all the way around. I've removed heads just to do valve adjustment when noticing the closers were too tight, sometimes having to get the seats re-cut at machine shop. Same thing happens if they are too loose. The seat gets burnt and does not seal properly, this is the easier fix, just need to lap valve in by hand.
Everything else I can think of has been covered, Please, if I were you, check the con rod as well, valve head stuck in piston top can bend the rod, you may be lucky and the piston gave way or the valve head shattered into smaller, less destructive bits.
The big ends are same as most engines, soft metal caps, and can get destroyed / damaged by impact on the piston /con rod.
Your choice really in how far your willing to go into the engine.

Hope this helps.
 

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Discussion Starter · #8 ·
I thought about the closer clearance as the root cause. It seems normal running and wear would increase closing clearance leading to a burned seat. It seems incorrect setting of the closing clearance is the path to this type of failure. The valve seat in the failure looks OK so it's most likely the belt, but without all the evidence or history, we can't be sure.

Speaking of the belt, the Ducati design does have a few advantages. It is wide at 18mm. Compare that to the Fiat 1600 four cylinder at 19mm wide. The belt doesn't have to work against valve springs to open the valves. It has more than 180 degrees of wrap on the sprockets. However, there are some big drawbacks. The sprockets are small and bend the belt in a tight radius. In general, engine speeds are about double automotive applications. These are probably the reason belt life is less than half automotive applications.

I did a close inspection of the con rod. It looks OK. I tried a dial indicator set up to measure big end clearance, but couldn't get a good reading. The bearing felt smooth by hand. In my other experiences with valve failure, the big ends and rods survived OK. I decided to roll the dice and leave the bottom end alone.

You are right though, rod failures can happen. A couple years ago, I had a Yanmar marine diesel that hydrolocked. It bent the rods rather than blowing the head gasket. Guess Yanmar top ends are really strong. I've got another Yanmar in the shop right now. It has no compression and I suspect bent rods.
 

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For your interest,
years ago Chris Kelly developed a belt for Ducati's called Exact fit. He owned California Cycleworks and was a former racer.
He claimed that original belts, were not in 'spec' at all, with the teeth being spaced just slightly wrong.
He set out to develop the exact fit belts, making them much stronger than OEM.
They quickly became the 'industry standard' here.
These guys made videos of how they tested many belts, I remember the amusing one using a fork lift and belt to lift a van off the ground.
just scroll down the page to see the tests.
 

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For your interest,
years ago Chris Kelly developed a belt for Ducati's called Exact fit. He owned California Cycleworks and was a former racer.
He claimed that original belts, were not in 'spec' at all, with the teeth being spaced just slightly wrong.
He set out to develop the exact fit belts, making them much stronger than OEM.
They quickly became the 'industry standard' here.
These guys made videos of how they tested many belts, I remember the amusing one using a fork lift and belt to lift a van off the ground.
just scroll down the page to see the tests.
Thanks for the share. Nice to hear the story, the fun tests to boot.
 

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Discussion Starter · #11 ·
Thanks for the info on the Exact Fit belts. I already bought some Flennor belts. They are a division of Flender in Germany, a very good manufacturer of industrial power transmission equipment. The belts fit well. The tooth spacing seems accurate with equal gaps between the belt and sprocket teeth on the non-drive side.
I can understand the suspicion of the original belts. Italian rubber and plastic products generally seem inferior. Every hose on this Ducati is dry rotted. Most of the plastics are faded various shades and brittle. This matches my experience with Fiats and Alfa's. Yes, the bike is a 1999, but Japanese, American and German stuff of the same vintage is in better condition.
Thanks for all the info. I may fire the bike up for the first time by the end of the month.
 

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Discussion Starter · #13 ·
Things are progressing. Since all the pistons, rings, etc. are already run in, I did a cold compression test by hot wiring the starter. Both cylinders are just a little over 125psi. It's a bit lower than the manual spec of 130-160, but over the 115 minimum. I expect the actual compression will be a bit higher when the engine is warm.
It is interesting how close the two cylinders are in compression since they are a random pair. The horizonal is original with 28k miles as far as I know. The vertical is a used set (piston, rings, cylinder, head and valves) bought off Ebay with unknown history. On the other hand, all the clearances measured up within Ducati specs and the measurements were pretty equal between the two cylinders.
Since I got the bike in pieces, it's now just sorting out the fuel lines, vacuum lines and wiring in order to fire it up.
 
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