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Now that 7+ months have passed since the original posting a couple of additional comments.

It's worth pointing out for the general reader that the division "Early Alfa V-6 Timing Belt" (trapezoidal) and "Later Euro V-6 Timing Belt" (curvilinear) probably presents the case too starkly, in so much as the trapezoidal belt has evolved since it was first introduced and now has curvilinear aspects in its design. Both Steve Alfisto and Jason have remarked on this, in the Dayco (oem) brand it's call the "SX" profile (replacing the "S" profile) :

http://www.alfabb.com/bb/forums/5167569-post214.html

http://www.alfabb.com/bb/forums/695372-post11.html

Here's the deal—what Jason calls the "trough in tooth and rounded edges"—described by Dayco as a "parabolic tooth design with indent" . . .



A tremendous improvement over the original "S" design which one should avoid if possible . . .



Also take note that the height of the SX belt is greater (+15%), another design move that brings it closer to the newer curvilinear style.

Somewhere in this thread comes up the claim that the 12v and 24v engines don't share belt wrap deficiencies. To me they are equally problematic in the event of a failing tensioner or a slack belt on start-up (the most crucial time for TB skip); while the two cams of the 12v have tooth engagement of repectively 10 & 14 the 24v cams have 12/9 & 10/20.
 

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Well, I have to say that this clear up and causes more questions. My 95 LS has a tensioner not described in this thread (or maybe I just missed it?). It looks like a lazy "t" and is similar to many of the Audi tensioners used....which causes me to wonder if there is a suitable swap out there. Regarding the bearing, I did repack all of my bearings during my first belt change. The stuff in there was thick and not overly clean, like stained honey. Getting the caps off without cracking them was a trick but dooable. Now, if a tensioner keeps a certain amount of tension on the belt at startup and while running, and you cannot find that exact bearing but do find one that is, say, 2mm wider or narrower, what's to stop you from altering the mount point of the bearing holder-frame so that the same amount of tension is applied by the alternate bearing? We're not talking about replacing the stock bearing with a Frisbee sized alternative. Picture of tensioner from Steve's post awhile back. 2nd image is of a Mazda V6 tensioner at +/- $99 gbp.
 

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The 24V tensioner is the same part used on some Ferrari engines. There's a thread somewhere on here discussing it.
 

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Ferrari 157356 R/H tensioner same as 24v 164

The 24V tensioner is the same part used on some Ferrari engines. There's a thread somewhere on here discussing it.
Ferrari F360 157356 R/H tensioner same as 24v 164 tensioner.
 

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Yup. Have one of those, as Carlo told me that most Ferrari owners say, when the car is in for service, I've got lots of money so just change everything, regardless of it's condition. The one he gave me is used but just fine, with only a few thousand miles on it.
 

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Good to have friends, eh! As for interchange (non Alfa-Ferrari parts), even if one were to "fit", unless the tension is the same, it would affect the spacing but I have to believe there is a better fix out there. Logically, but possibly incorrectly, if one were to find a near-fit and manufacture a bracket to mount properly, that substitute should work correctly, assuming that the pressure from the pin were equal to the factory specs. As for the bearing, the eccentric center is used to position the bearing at the correct pressure point but again, if a bearing with 2mm more or less were to be used, the position of the tensioner could be shifted to put the same amount of pressure on the belt. The question of measuring it might be tough as the Alfa tools would not be calibrated for an alternate setup. I don't have the machinery or math skills to do this properly.
 

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Incidentally, step 1 of working a suitable substitute would be to find a new stock one (or perfectly fine used one), measure the amount of pressure at the spring in psi, compare that against an alternate and then manufacture a bracket BUT I read that the Audi versions warn against removing the pin with the unit not in place as that will ruin the tensioner. Not certain if that applies to all brands and types. This might be a rabbit-hole endeavor but it has come to mind recently when trying to justify $400 for a few ounces of metal and oil. And yes, I understand that goods are priced based on need, not materials.
 

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I have bought my 75 / V6, 12-Valve with the mechanical tensioner and had a belt tooth skip incident, so after reading this great post I decided to convert to the original hydraulic. I have found a used one, and already bought the kit and the bearing and am ready to put it on the engine, but I have a question, if anybody could help me. Shouldn't there be a gasket for going between the engine and the hydraulic tensioner ? What is holding the oil passing from the engine to the tensioner from leaking ?
Thanks,
Pavlos Pavlides
Nicosia
CYPRUS

There is really no convenient way to know if the temperature spring is broken. With the mechanical tensioner, feeling the tension on the upper part of the belt between the two camshaft pulleys may not a reliable way of telling. Depending upon where the engine stops with regard to camshaft position, the belt may appear quite tight, or may appear somewhat loose, even with a serviceable tensioner.

Since mechanical tensioners are the factory approved retrofit since the mid-90s, chances are your 164 has one installed. Retrofitting the mechanical tensioner involves putting in a solid (not hollow oil feed) stud and blocking the oil return hole behind the tensioner. The mechanical tensioner does not use engine oil for operation like the Hydraulic De-tensioner does. The mechanical tensioner comes as a complete assembly including the bearing and costs about $175. There is no recommended service life, although most experienced owners change it every other belt change, at least. Some change it every belt change. As mentioned before, the mechanical tensioner is a little tricky to install correctly. If installed incorrectly, the temperature spring can break, rendering the tensioner unsafe for operation.

Hydraulic De-Tensioner

The Hydraulic De-Tensioner was developed by Busso specifically for the Alfa V-6 engine. It is simple in operation and has almost no problems with belt slippage or breakage.

Theory of Operation

The Hydraulic De-Tensioner, despite it's name, IS a timing belt tensioner. The "de-tensioner" part comes from the only active mechanical action that it does. The hydraulic portion is run off of engine oil pressure. Since the most stress on a timing belt is on initial start-up, the Hydraulic De-Tensioner provides extra tight tension to the timing belt when the oil pressure is below 20 psi or so (as in initial start-up), but as oil pressure builds, that extra tension is removed and normal belt tension is applied. Also, it provides for thermal expansion of the aluminum engine block as the engine heats and expands and tensions the timing belt slightly. The pictures below illustrate how it works:

1. Initial Start-up. At initial start the oil pressure is zero. A strong internal spring pushes down on a pin that pushes on the eccentric bearing carrier arm, thus pushing the tensioner bearing into the timing belt greatly tighening it. This pin greatly overpowers the normal tensioner spring. The oil pressure necessary to overcome the strong internal spring is about 20 psi. As the oil pressure builds after start-up, the pin retracts and the normal tension spring takes over.

2. Normal Running. With the oil pressure at normal running and normal idle (>20 psi), the primary tension spring provides belt tension. It can stretch to loosen/tighten belt tension as necessary. If you watch a hydraulic de-tensioner in action on a running engine, you will see the bearing carrier arm juddering slightly as the primary tension spring does its job. This is normal. In the picture above, the primary tension spring is the lower spring covered with a rubber tubular.. The primary tension spring is very robust and not known to fail. To see the tensioner in action at idle speed, click on the following URL. At higher engine speeds the bearing carrier stays fairly steady.

This video shows start-up. Although you will note that the piston/pin is retracted with engine off. That is not normal and must be a sticking piston in the de-tensioner. Just as the engine is jarred with the starter motor, the pin extends to tighten the belt until oil pressure builds and the pin retracted. That is not normal. Also, note at shutdown that the de-tensioner pin is very slow to extend when the oil pressure drops. At any rate, it does show the action of the de-tensioner at startup.

This video shows de-tensioner action at idle:

This video show de-tensioner action at high rpm:
 

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There is a hollow stud with orings that the hydraulic tensioner slides over. You will need one if not currently installed. It might be a bit of a trick getting the plug out that was installed to put in the mechanical tensioner.
 

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"Shouldn't there be a gasket for going between the engine and the hydraulic tensioner ? What is holding the oil passing from the engine to the tensioner from leaking ? "

No gasket necessary, the tensioner would not be secure. The return oil is sealed by the chubby -010 o-ring, and the intake oil as vintagemilano says by two oil rings, a -012 and -011. When the thing does leak it's usually from the actuator shaft (which is sealed by a u-cup). Glad to hear you are reverting to the original design.
seal kit.jpg
 

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There is a hollow stud with orings that the hydraulic tensioner slides over. You will need one if not currently installed. It might be a bit of a trick getting the plug out that was installed to put in the mechanical tensioner.
Thank you very much. Yes, I have the stud and was wondering where it goes.... Thank you again...
 

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"Shouldn't there be a gasket for going between the engine and the hydraulic tensioner ? What is holding the oil passing from the engine to the tensioner from leaking ? "

No gasket necessary, the tensioner would not be secure. The return oil is sealed by the chubby -010 o-ring, and the intake oil as vintagemilano says by two oil rings, a -012 and -011. When the thing does leak it's usually from the actuator shaft (which is sealed by a u-cup). Glad to hear you are reverting to the original design. View attachment 1605251
"Shouldn't there be a gasket for going between the engine and the hydraulic tensioner ? What is holding the oil passing from the engine to the tensioner from leaking ? "

No gasket necessary, the tensioner would not be secure. The return oil is sealed by the chubby -010 o-ring, and the intake oil as vintagemilano says by two oil rings, a -012 and -011. When the thing does leak it's usually from the actuator shaft (which is sealed by a u-cup). Glad to hear you are reverting to the original design. View attachment 1605251
Thank you Pinino fr sharing this info with me...
 
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