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Discussion Starter #1
During the restoration of my 164 (see the “And Now for Something Completely Different” thread) I reconverted from the mechanical tensioner back to the hydraulic tensioner. I had acquired several used hydraulic tensioners, and carefully rebuilt one using the available rebuild kit. I have put around 7K miles on the car since the engine restoration. Initially the tensioner did not leak oil at all. However after about 6K miles the tensioner started leaking like crazy. Closer inspection revealed that all of the oil leak in the tensioner area was coming from the plunger shaft (as I was expecting). This was totally unacceptable life. So the rubber cup seal is just not doing a good job.
I decided that I would try to figure out some modifications I could do to improve the plunger shaft sealing. And if that was unsuccessful, I would go back to either the mechanical tensioner or a fixed tensioner.
These cup seals are widely used for sealing in master cylinders and slave cylinders. In brake systems, these seals are expected to seal up to one or two thousands PSI of pressure. So they ought to easily handle the less than 80 PSI of oil pressure. But in all of the brake and clutch applications that I have seen, the cup seal works in a different way than in the hydraulic tensioner. In the typical application, the relative motion at the seal occurs at the outer diameter of the seal. This is shown in the first attached drawing. High pressure on the right side of the cup seal tends to force the outer diameter of the seal into the bore of the master or slave cylinder, thus forming a tighter seal. There is typically no relative motion at the inner diameter of the cup seal.
1612501

In the standard hydraulic tensioner application (see the second drawing), the relative motion is at the ID of the cup seal. Also, since the undercut groove that holds the seal is wider than the height of the seal, the seal is free to move back and forth in the groove by about .5 mm, and the ID of the seal is under no compression in the direction of the plunger motion. It is not possible to know for sure if the cup seal is leaking at its ID or OD. So I decided to come up with a modification that would improve the plunger mechanism sealing for both possible leakage paths at the ID and OD of the cup seal. It turns out that this change requires no modification of any existing tensioner parts, and is totally reversible, which is nice.

1612503

I acquired some 8 mm ID, 11 mm OD bronze bushings from Amazon and some -011 (5/16” x 7/16” x 1/16”) O-rings. First I installed one O-ring into the cup seal groove at the outer (left in drawing) end of the plunger bushing. Then I installed 2 O-rings into the inner end of the plunger bushing. The middle O-ring functions primarily as a spacer. Then I cut my bronze bushing to a length of 3 mm with a Dremel tool. Inserted it into the inner end of the plunger bushing and tapped on it with a hammer until it was flush with the inner face of the plunger bushing (it is a light press fit). I inserted an 8 mm shaft to make sure all of the O-rings were properly lined up. See the last drawing for this setup.
1612504

The 3 O-rings and the cup seal the plunger shaft at their IDs. The outer O-ring forces the cup seal against the inner (right) wall of the cup seal groove. It also makes a seal at the outer (left) end of the cup seal groove. The rightmost O-ring forms a seal at the 11 mm counterbore of the plunger bushing with its OD. And the bronze bushing captivates and puts some axial compression on the 3 O-rings and the cup seal, along with greatly reducing the volume and pressure of the oil impinging on the seals.
I decided NOT to replace the leaking cup seal. If I put in a new cup seal I would probably have to wait for thousands of miles to see if my modifications actually did anything, and I would never know for sure that they were an actual fix. By working with a leaky cup seal, I would know for sure right away if the modifications stopped the leak. I would only have to wait to see if the fix would last for many thousands of miles. I will give updates as time and miles accumulate.
 

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I think you simply had bad luck and shouldn't throw in the towel nor try to invent something new. Don't rule out the possibility that maybe the cup-seal you installed either had a flaw, or while installing it, got damaged. Installation is ticklish because you must install it into the cavity sideways, and then with a pick flip it on its side (the extra size of the cavity which you remark on is on purpose to allow this maneuver). The beauty of the design is simplicity, one sees this abundantly on the seals of the 50s and 60s Italian lever espresso machines as well. This hydraulic system is not perfect by any means but I don't think it is easy to outdo Busso. I personally have not had leaking problems, but I don't go more than a couple of years before renewing the o-rings of the detensioiner. Others have mentioned using a dab of RTV silicon with this cup seal with good results.
 

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I rebuilt dozens of these before Zat introduced his mechanical tensioner and then the factory version with the spring. I can't replay the picture of all the parts in my mind... I do remember that Fred Dimattio(?) had sourced better seals that were interchangeable and made a significant improvement in longevity.
 

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During the restoration of my 164 (see the “And Now for Something Completely Different” thread) I reconverted from the mechanical tensioner back to the hydraulic tensioner. I had acquired several used hydraulic tensioners, and carefully rebuilt one using the available rebuild kit. I have put around 7K miles on the car since the engine restoration. Initially the tensioner did not leak oil at all. However after about 6K miles the tensioner started leaking like crazy. Closer inspection revealed that all of the oil leak in the tensioner area was coming from the plunger shaft (as I was expecting). This was totally unacceptable life. So the rubber cup seal is just not doing a good job.
I decided that I would try to figure out some modifications I could do to improve the plunger shaft sealing. And if that was unsuccessful, I would go back to either the mechanical tensioner or a fixed tensioner.
These cup seals are widely used for sealing in master cylinders and slave cylinders. In brake systems, these seals are expected to seal up to one or two thousands PSI of pressure. So they ought to easily handle the less than 80 PSI of oil pressure. But in all of the brake and clutch applications that I have seen, the cup seal works in a different way than in the hydraulic tensioner. In the typical application, the relative motion at the seal occurs at the outer diameter of the seal. This is shown in the first attached drawing. High pressure on the right side of the cup seal tends to force the outer diameter of the seal into the bore of the master or slave cylinder, thus forming a tighter seal. There is typically no relative motion at the inner diameter of the cup seal.
View attachment 1612501
In the standard hydraulic tensioner application (see the second drawing), the relative motion is at the ID of the cup seal. Also, since the undercut groove that holds the seal is wider than the height of the seal, the seal is free to move back and forth in the groove by about .5 mm, and the ID of the seal is under no compression in the direction of the plunger motion. It is not possible to know for sure if the cup seal is leaking at its ID or OD. So I decided to come up with a modification that would improve the plunger mechanism sealing for both possible leakage paths at the ID and OD of the cup seal. It turns out that this change requires no modification of any existing tensioner parts, and is totally reversible, which is nice.

View attachment 1612503
I acquired some 8 mm ID, 11 mm OD bronze bushings from Amazon and some -011 (5/16” x 7/16” x 1/16”) O-rings. First I installed one O-ring into the cup seal groove at the outer (left in drawing) end of the plunger bushing. Then I installed 2 O-rings into the inner end of the plunger bushing. The middle O-ring functions primarily as a spacer. Then I cut my bronze bushing to a length of 3 mm with a Dremel tool. Inserted it into the inner end of the plunger bushing and tapped on it with a hammer until it was flush with the inner face of the plunger bushing (it is a light press fit). I inserted an 8 mm shaft to make sure all of the O-rings were properly lined up. See the last drawing for this setup.
View attachment 1612504
The 3 O-rings and the cup seal the plunger shaft at their IDs. The outer O-ring forces the cup seal against the inner (right) wall of the cup seal groove. It also makes a seal at the outer (left) end of the cup seal groove. The rightmost O-ring forms a seal at the 11 mm counterbore of the plunger bushing with its OD. And the bronze bushing captivates and puts some axial compression on the 3 O-rings and the cup seal, along with greatly reducing the volume and pressure of the oil impinging on the seals.
I decided NOT to replace the leaking cup seal. If I put in a new cup seal I would probably have to wait for thousands of miles to see if my modifications actually did anything, and I would never know for sure that they were an actual fix. By working with a leaky cup seal, I would know for sure right away if the modifications stopped the leak. I would only have to wait to see if the fix would last for many thousands of miles. I will give updates as time and miles accumulate.
Tom,

This is absolutely brilliant!! Please let us know how it works out for you!

Mike
 

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I will be re-rebuilding my hydraulic detensioner in the next few weeks, and am tempted to give this a try.

Just so I'm clear, is the current recommendation (ie. not involving Tom's method) to apply some RTV silicone to the outer lip of the cup seal? Or on the ID against the plunger? I've been reading a thread on another forum on RTV, and I feel like using it in this application is sub-optimal.
 

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Fred DiMatteo was "The Godfather" of the Alfa Romeo Owners Club-USA. I first met him and Pat Braden (both now deceased) at 1996 Santa Fe, NM AROC convention. Their pet project at that convention was a 2.5L V6 block bored out to accept 3.0L pistons and liners to make it a 2.8L. I next saw Fred at the 2001 Nashville, TN AROC convention. He was setting in his white 1991 164L at NASCAR track where we had our track event. He just stayed in the 164 with his O2 generator nose piece in place. Fred DiMatteo, an American Alfa Romeo legend, passed away on April 19, 2003. I ended up with his white 91 164L in 2016 and rehabbed it and resold it. It ended up in Pick-N-Pull in Virginia Beach in 2019 and I harvested enough parts off it to save my 91 164L after my engine fire in June 2019.
 

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Discussion Starter #8
So winter driving, with its high oil pressure when the engine is cold, is over.
The engine is still as dry as ... well, you know.
So the hydraulic tensioner mod is still hanging in there.
 

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Interesting. Any info on which rebuild kit? A general search has not found anything (internet search)
 

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Great news Tom. I will be doing mine whilst in isolation over the coming weeks. I hope Amazon will still deliver me some bronze bushing!
 

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Interesting. Any info on which rebuild kit? A general search has not found anything (internet search)
You have a 24v there isn't a rebuild kit for 24v belt tensioner.
 

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I just couldn't envision one but had to inquire. Crazy expensive those hydraulic buggers. Audi and VW use similar designs... somewhere, besides over-priced Ferrari stuff, is a more common compatible one.
 

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You did replace the oring that seals the housing that the cup seal goes in?
 

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Discussion Starter #15
Since I had recently done a standard rebuild of the tensioner, when I did the modifications I did not remove the "bushing" that holds the cup seal from the tensioner casting.
 

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I got mine done this weekend, I really like the bronze bushing material and am almost tempted to think of a better arrangement for the press-fit plug that holds the leaky seal. Got it all zipped up today and hit the key only to have the belt slip....guess I'll find out how it went next weekend!

1630343
 

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DSCN5793.JPG
I don't mean to hijack your thread, but I thought that perhaps one of the folks following this thread might be interested in this. Removed from a 164 in the 90's.
$25 plus shipping from PA.
 
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