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Discussion Starter #1 (Edited)
This is a primer guide to 1991-1993 164 (12v) timing belt tensioners. This is meant for new owners trying to decide on what type of tensioner to use. If you see anything incorrect or poorly explained, let me know and I'll change it. And a thank-you to all those whom I pirated pictures from.

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If there’s one aspect of the 164 12v V-6 engines that strikes terror into the hearts of it’s owners, it’s the timing belt. Keeping in mind that since these 164 engines were designed in the 80s and the timing belts are of the early trapezoidal design. This means the teeth on the belt form a small trapazoid. More modern belt designs are of the “curvilinear” type which forms a sort of half circle. The trapezoidal types do not have the amount of grip that that the modern curvilinear types do. That said, trapezoidal belts are just fine under most circumstances, given sufficient “belt wrap.” Belt wrap is the how much of the belt grips the pulley which it drives. A trapezoidal belt with lots of “wrap” grips the pulley well and poses no special problem.

Later Alfa V-6 engines use a curvilinear belt. As an aside, early 12v engines can be retrofitted to use the curvilinear belts, but finding the various pulleys/pieces/parts is difficult and must be sourced from Europe.

Which brings us to back to the 164 12v Busso engine. The problem with the early Busso engine is that the belt wrap on the rear camshaft pulley can reasonably be described as “just barely adequate.”

Under good circumstances, given good maintenance, non-extreme conditions, and not allowing the engine to rotate backwards, it works fine. However, given problems and the right circumstances, there is a real possibility that the belt will skip/shear teeth and go out of valve timing. Given that the engine is an “interference” type engine, if the timing slips by more than 2 teeth, it’s almost a certainty that there will be a collision between some valves and the piston. The result of this is a very expensive repair and very often has led to the car being junked as non-economical to repair. And virtually all of this is unnecessary and preventable.

Causes of 12v Timing Belt Failures

The VAST majority of timing belt failures can be attributed to three causes:

1. Failure to adhere to Alfa recommended change intervals.

2. Failure of the temperature sensitive tensioner spring of the mechanical type tensioner. This can either be by simple material failure or by installation error with the installer inadvertently breaking it upon installation.

3. With a mechanical tensioner, allowing the car to roll backwards while in gear, causing the engine to rotate backwards. This could happen while parking on a hill for instance. This is not an issue with the hydraulic de-tensioner or fixed-type (ZAT) tensioner.
 

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Discussion Starter #2 (Edited)
Three Types of Tensioners

There are three basic types of tensioners that can be fitted to the engine.

1. Mechanical tensioner
2. Hydraulic “De-tensioner”
3. Fixed tensioners

Mechanical Tensioner

The mechanical tensioner was developed in response to the chronic oil leaks of the original OEM Hydraulic De-tensioner. The mechanical tensioner contains two coil springs. One sets the basic tension, while the second one is temperature sensitive that increases tension at colder temperatures and loosens tension as the engine heat increases.

This tensioner has garnered a reputation for broken temperature springs, either upon installation or in service. Sometimes, it is not a problem if the belt is not too loose even if the thin outer spring is broken (most of the time one would not know anyway until the tensioner is taken out and inspected). That said, there are thousands in service that have done fine. However, if they do break, there is a serious possibility of a timing belt failure and severe damaged engine.

As mentioned before, most of the stress on timing belts is upon initial start-up of the engine when the engine inertia is nil and the belt maybe very cold and not as pliable as when warm. This is when most belt failures with the mechanical tensioner occur. Belt failures at high engine speed and are normal running temperature are rare but have happened, usually associated with long overdue belt changes, and/or failed mechanical tensioners.

One problem is when a tensioner is getting old, the bearing will tend to freeze at the off-centric shaft due to corrosion (or lack of lubrication) there. It it does, the primary spring will not be strong enough to spring back the bearing freely to take up the slack when the engine cool down. This will also result in a loose belt at cold engine and possible belt slip. A slip of more than two teeth (usually the rear bank of cylinder) will cause valve damage. If after start-up you hear mechanical clatter or rough running and have a mechanical tensioner, you should shutdown and investigate. Hopefully, only one or two teeth have skipped which will not cause immediate damage.
 

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Discussion Starter #3 (Edited)
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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Discussion Starter #4 (Edited)
3. Initial Install/Setup. Initial setup is done by reversing the conversion done by the mechanical tensioner. Remove the plugs and install the hydraulic unit oil feed stud. Once on the engine, the bearing carrier is locked in the up (belt looser) position by a removable pin (initial setting tool). This will require significant upward force to overcome the unit's piston/pin spring. The initial tension spring (located in the picture sticking out slightly on the left side of the tensioner body) is then allowed to push the entire unit INTO the timing belt rendering it tight. The unit is then locked down by the bottom adjuster nut and the initial setting tool removed. When the initial setting tool pin is removed, the oil actuated piston and pin spring is released and pushes down hard on the bearing carrier arm and subsequently, the timing belt. The primary tension spring is then attached and the unit is ready for service.

The reason that the hydraulic de-tensioners were abandoned by Alfa in favor of the mechanical tensioner, was because of oil leaks from the actuating piston & pin shaft seal. However, now there are better seals, and with a bit of RTV, they are very reliable and long lived. They are repeatedly rebuildable by just replacing the O-rings and other rubber parts. The actual bearing, however, is difficult to source and expensive ($180 as of Sep '15). It is not a standard size bearing available at your local seal and bearing supplier. However, it is very robust and lasts a long time.

ADVANTAGES:

1. Very sturdy and reliable.
2. Rebuildable
3. No worries with reverse engine rotation or cold starts

DISADVANTAGES:
1. May be difficult to source a complete salvage unit.
2. Must rebuild at every belt change.
3. Bearing expensive
4. If engine already converted to mechanical tensioner, must convert back.

CONVERSION BACK TO HYDRAULIC DE-TENSIONER FROM MECHANICAL TENSIONER

When the OEM hydraulic De-tensioners were replaced with mechanical ones, the oil fee
passages had to be plugged. This consisted a threaded set screw in the oil return port and installation of a ordinary stud in place of the hollow oil feed stud of the hydraulic de-tensioner.

Converting back should poses no huge problem unless the block oil ports were damaged in the initial conversion. The picture shows the oil ports with the mechanical tensioner conversion done.
 

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Discussion Starter #5 (Edited)
FIXED-TYPE TENSIONERS

Fixed-type tensioners consist of either the "ZAT" style tensioner or a Hydraulic De-Tensioner modified to act as a fixed tensioner. Some critics of the fixed tensioner say that it doesn't compensate for thermal expansion. However, in actual operation, owners have found them very safe and reliable.

ZAT

The ZAT tensioner was manufactured by Tom Zat as a fix to the leaking hydraulic tensioners and the high rate of failure of the mechanical tensioners. With a fixed tensioner, the belt tension is set statically upon installation and things like increased tension at start-up and thermal expansion of the block are not compensated for. It is a very simple device with the only moving part being the tensioner bearing. The ZAT unit uses a bearing carrier and bearing from a hydraulic unit.

Although simple, these tensioners have proved reliable and sturdy and maintain enough tension to prevent belt skips with inadvertent reverse engine rotation /cold start-up.

Readjustment of timing belt tension must be done every 15,000 miles.

CENTERLINE STAYBELT FIXED TENSIONER (centerlinealfa.com) see picture below

This tensioner is a remake of the ZAT fixed-type tensioner featuring new parts and plated against corrosion. With the original ZAT unit, you had to reuse the bearing carrier from the old OEM hydraulic de-tensioner. The Centerline tensioner comes as a complete, ready to install unit, with a new bearing.


MODIFIED HYDRAULIC TO FIXED-TYPE

Some owners have modified they hydraulic de-tensioners to a fixed-type by blocking off the oil feed stud with brazing and locking the bearing carrier into a fixed position. The initial-tension setting spring can still be used to help gain the correct tension.

In the example to the right, you can see that the bearing carrier is locked in place. The fine tension adjustment mod pictured is unnecessary because satisfactory tension can easily be set manually. Assuming the oil feed/return ports are blocked, the internal spring and pin can just be removed.

CONCLUSION

Deciding which tensioner to use is a matter of personal choice and availability. The following is an estimation of the cost of each tensioner (not including installation):

Mechanical: $175
Hydraulic: $375 [$75 for salvage tensioner, $98 rebuild kit (although can source separate
parts cheaper), $200 forbearing]
Fixed: $289 New Centerline Staybelt Tensioner (complete unit ready to install with new
bearing)
$300 [$75-100 Zat (IF you can find one) or salvage hydraulic body/carrier, +$200
for bearing]


END
 

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Please note the above applies to 12v motor not 24v motor. While some of the principles do cross over to 24v the implementation is very different ( but not the "barely adequate" belt wrap!!!

I would also say that many are hesitant / fearful of this whole timing belt thing but needlessly so. It's a belt like any other with a few subtleties. If u can change the plugs on a 24v car you can change the timing belt if u have the right tools. No super special knowledge needed other than a healthy dose of patience and common sense!!
 

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Discussion Starter #7
Thanks. Omission on the thread title. Downloadable copy specifies 12v engines.
 

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Looking at your photos (very good, btw), it is sure clear that there should have been a small pulley, similar to the tensioner pulley, on the back side of the belt below the rear bank exhaust pulley to increase the number of engaged teeth.
 

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Roadtrip this is an AWESOME primer for 12V t-belt tensioner! I am a visual guy myself and seeing photos is so much better for me than descriptions -- but both together is even better! Thanks for doing this -- maybe my next belt change on the LS I'll do some detail photos like you have and post them

cheers, bob
 

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Discussion Starter #10
The purpose of this one is just to help guys understand the differences of tensioners, strengths/weaknesses, so they can make an informed choice on which to use. I didn't want to inject bias in there, although I think the mechanical tensioners are problematic, at best.

Unfortunately, the hydraulic ones are hard to find complete and then there's that cost of the bearing . . . which is insignificant compared to crashing your valves. I've had a hydraulic one on my engine for over a year now and it's been working flawlessly. And with the engine off, you can feel the TB between the cam pulleys, and it's tight. With mech tensioner, sometimes it'd be kinda tight, and sometimes not so. I just felt like it was a hand grenade with the pin pulled.
 

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John - you, as well as Steven and others have put together so many of these downloadable guides I'm wondering if it might be possible to create another "sticky" thread to park them in. Because, although I have all of them on my computer, I see that you often update them with new, revised or corrected content so they are almost like the controlled documents in aviation that need to be obtained from the source where the latest edition is.

As I am currently using your A/C upgrade guide I had to do a search to find it and check. Sure enough I had version 3a and there is now a version 3b. I think a sticky where all downloadable guides of these sorts could be kept would be helpful, ideally one that doesn't contain discussions, just the files themselves, or links to them.
 

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I wish I had the computer skills and writing ability of both John and Steven to be able to do study guides in pdf like they can do.

We maybe able to get the admin group to add another maintenance sticky for them or if not maybe I can add a post to mine and then I can add all these guides under one main post that I only can add more guides to it.
 

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A very helpful and needed thread, I especially like the last photo of post #3 with those neat arrows. You might want to link that other youtube clip of somebody showing a hydraulic de-tensioner with the sequence "before startup", "engine running", and "engine turned off"—a fun clip for the uninitiated.

A couple of tips on sourcing the hydraulic unit. There was as most oldtimers know a national dealer-wide recall of the hydraulic unit. That means that many Alfa workshops have or had a barrel of the removed (but still good) de-tensioners. Pity that this happened at all but that is another story. Anyway, taken-out-of-service hydraulic de-tensioners are out there, it just takes a little time and perseverance to find them. On this very forum "Jim G" had more than 6 of them that he offered in the last year for very reasonable prices. I'm in the book business and they say the best friend of the book collector is patience. Same is true with the hydraulic de-tensioner, just wait, one will pop up, grab it. Clean it and rebuild it for less than $25 (there is no reason to buy that silly "repair kit").

The other thing is the bearing. The reason it is so expensive (you might add this in the guide) is that what's on the market is the 24v bearing equipped with the wrong center piece (which needs to be pressed out; Steve Alfisto has posted tons of information on this on the BB). The original 633254 SKF bearing for the 12v cars, without any center piece, is available from time to time at a good price. I recently bought one from ebay.it for $52 including shipping. It was packaged under a brand called "Autokit" pn K054 (or 03.054) and is described, usually, "cuscinetto tenditore cinghia alfa romeo", applicable to the Alfa 155 (2.5 V6) as well. If in doubt the dimensions are 30 x 58.5 x 35mm (H OD ID). Again, patience, patience, stock up when you happen to see one and save it for a rainy day (sage advice of Steve Alfisto).

Finally, in my opinion, you will do yourself a great favor not installing the new bearing "as is" but instead, carefully removing the seals, flushing out the old grease and repacking with Krytox, with the hope of making it a "lifetime" bearing. dennism has a nice thread on this. The hydraulic de-tensioner with a Krytox-fortified bearing is a winning team.
 

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"Finally, in my opinion, you will do yourself a great favor not installing the new bearing "as is" but instead, carefully removing the seals, flushing out the old grease and repacking with Krytox, with the hope of making it a "lifetime" bearing. dennism has a nice thread on this. The hydraulic de-tensioner with a Krytox-fortified bearing is a winning team"

Jason does this as well.
 

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Very nice write up.

Here you can see the conversation plus using hydraulic tensioner.

http://www.alfabb.com/bb/forums/164-168-1991-1995/441465-facelift-pulley-upgrade.html





As far as hydraulic bearings and Krytox, just depends on condition. I have lots of new bearings in stock and access to 12FL pulleys. I have 2 crank cogs in stock, the hard one is Oil pump but I can probably source them still…used. I do Krytox if needed.
 
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