Re: Mark's timing belt tensioner

[Bruce Murray <bmurray@domain.elided>]

At the risk of reigniting flame wars, I would like to offer opinions on the 
mechanical de-tensioner and compare it to the hydraulic design.

The benefit of the mechanical design seem to be that it doesn't leak oil. I 
don't see any other assets to the design.
The down sides are that it is finicky to set up properly, and if the engine 
kicks back the timing belt can slip. It is, to my mind, a rather fragile 
construction.

The benefits of the hydraulic design are that 1) it is ruggedly 
constructed, 2) even if improperly adjusted the timing belt cannot slip
The down side is that it can leak oil.

Both designs require mechanical skill and meticulous attention to detail to 
implement properly.  This is not an "All Foreign Car Specialist " type of job.

The mechanical de-tensioner needs to be most carefully set up by skilled 
technicians otherwise it may break a spring and/or lead to belt slippage.
I think that the several instances of early de-tensioner failure are 
probably due to poor installation.  I have one on one of my 164's that was 
fitted by a well known dealer about 12,000 miles ago and it seems OK so 
far.  My fingers are crossed and I will probably revert to a hydraulic unit 
soon.

The hydraulic design has a very difficult seal to install on the piston rod 
and if not done well will leak in time.  It is however much more 
straightforward to adjust on the engine than the mechanical type.

To me, the bottom line is that the mechanical design can fail rapidly and 
lead to catastrophic mechanical damage, whereas the hydraulic design will 
fail more gracefully and leakage can be seen and corrected in time before 
the belt is weakened to a point that it fails due to oil saturation.

Bruce

164S   49,000 miles with mech unit
164L 180,000 miles with hyd unit
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