hydraulic t-belt tensioner rebuild

[Joe Elliott <jee@xxxxxxxxxxxxxx>]

Although my Zat tensioner is doing fine after 25,000 miles, I'm still 
thinking about putting a rebuilt hydraulic unit on the car when I 
replace the belt.  So I got out my hydraulic tensioners to try and 
rebuild one, but I can't figure out what all goes on the shaft of the 
thing.  I have copies of the relevant pages from the Alfa Sei shop 
manual, but they don't give too much information, and only refer to a 
couple of washers on the shaft.  I thought there several washers and 
at least one o-ring.  I'm talking about the portion of the shaft that 
extends out of the oil-filled tensioner body.  I'm kind of curious 
why there are any parts on that shaft at all though.  The boot I can 
understand, but it's NLA as far as I know, so it's just going to be a 
bunch of washers, etc, loose on that shaft, so I have to ask, what's 
the point?

I also have to ask how that silly device works.  If the oil were on 
top of the piston, that would make perfect sense--higher oil pressure 
at cold temperatures and high RPM would increase the force applied to 
the piston and consequently increase the force applied to the belt. 
But the oil is UNDERNEATH the piston, so higher oil pressure would 
seemingly decrease belt tension.  What the hell?

Why doesn't someone just make a tensioner with a freaking spring? 
(Or is that what Sperry does for $$$?)  The hydraulic one leaks, the 
thermomechanical one breaks, and the Zat one has no accomodation for 
the thermal expansion of an all-aluminum engine.  Don't other 
manufacturers just use a spring?  It seems to me that that would work 
beautifully, because the engine doesn't expand enough for delta x in 
Hooke's law to be appreciable, so the force exerted by the spring 
would remain rougly constant as the motor expanded, and consequently 
belt tension would remain roughly constant.  At least a lot more 
constant than with the scary (albeit reliable, thus far, knock wood) 
Zat device.  And a big ol' spring probably won't leak or break 
either.  Make it a pair of springs if you're too worried about 
springs breaking.

Joe Elliott
'82 GTV-6
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