Friday, February 20, 2015

Desmoquattro Valvetrain Condition at 20K

Here we have the pinnacle of race engine cylinder head technology, circa 1955.
Physically huge, large included valve angle (for a 4-valve), obsolescent desmo valve drive, ball-bearing supported cams, it's all there.
Actually it's a cylinder head from a 2005 Ducati ST-4 with roughly 20 thousand on the clock (although that doesn't make the above statement any less true).

The valvetrain geometry of the Desmoquattro forces tremendous side loads on the valve stems during the opening sequence.  As one might expect, this is a very tough environment for valve guides and stems.  In a race engine that is serviced as often as it's ridden this isn't really an issue since race engines are expected to have a short lifespan.  This is precisely why using this engine in a road bike isn't the best idea and using it in a touring application is patently absurd.  Make no mistake, the Desmoquattro is a race engine, designed in the 80s to win WSBK races.

In the following photos and video you will see the effects of mileage upon this design.  Keep in mind that although 20K isn't a lot of miles on a street engine, it is multiple lifetimes for a racebike-derived engine.

Testastretta engines are far more tolerant of street use precisely because their valvetrain geometry imposes far less side-loading on the stems due to their narrower included angle and much-improved rocker arm geometry.

It isn't just the valveguides that suffer.  Here are the closer shafts and springs.  Note how the closer springs have worn notches into the shafts.  Also note the wear to the spring wire itself.  This wear is exactly where the springs will fail.  The notches in the shafts are also stress raisers and will result in cracking and failure of the shaft at some point.

In this video the amount of valve guide wear is evident.

Since this is an obsolete engine, there is no aftermarket support except for shims and exchange rocker arms to correct the notorious chrome flaking.  This means that one either has valves custom made or you pony up the 150-200 dollars PER VALVE for OEM.
Buyer beware.

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