[alfa] Fred and Herb on the detensioner (long, part two, 1991)

["John Hertzman" <johnhertzman@xxxxxxxxxxxxx>]

In our original article on rebuilding the detensioner (see the March 1990 issue of Velocissima) Fred and I mentioned a modification to the bushing or sleeve which provides a bearing for the piston shaft of the tensioner. This sleeve is pressed into the bottom of the cylinder of the tensioner and has an internal groove to retain the U-shaped seal which seals the piston shaft against the internal oil pressure. Since the original article we have had numerous inquiries about this modification so here is a description of the procedure.
 
Using a press or a large vise and appropriate socket (I use an old valve guide) push the bushing out of the cylinder. The OD of the bushing is 0.6776 and it's an interference fit in the cylinder. Note that there is an O ring which provides a positive seal between the bushing and the cylinder. If you have never removed the bushing, this is the extra O ring which comes with the Alfa rebuild kit that you could never find a place for. The standard U seal for the shaft is 8mm ID. 14mm OD and 4mm thick. It doesn't seem to last very long and has been one of the main sources of oil leaks. Fred found that the seal used in the 105 series cars mechanical tachometer drive is 8mm ID and the sealing lip is spring loaded which is not the case for the original seal. Unfortunately, however, the tach seal is 16mm OD and 7mm thick, and the OD of the bushing is not large enough to bore out for the new seal. A little research at the local bearing companies showed that this is the smallest spring loaded seal available for the 8mm shaft. So the modification we made was to chuck the bushing in a lathe and bore it out to 0.600" diameter and 0.266" deep. Bore from top side. Diagram in inches.
 
The next step is to get one of the 8X1 6X7 seals from your local bearing supply house and make up a mandrel so you can center it up in a lathe to turn down the OD of the seal to 0.599". 1 used a quarter inch shaft and a couple of conical washers to center the seal. You will find that the seal has a steel body onto which the rubber is bonded and you will just hit the steel at the desired diameter. This leaves the bushing with about a 1/32 inch wall thickness The seal should now be a light press fit in the bushing and a little blue locktite ensures that it stays in with no leaks. (Be sure to place the new "O" ring under the bushing)
 
For reference, here are the "O" ring dimensions (ID X cross section in mm) used in the tensioner; Piston 31 X 3.5; oil return rear bracket 9 X 2.5; pivot shaft-rear-9 X 1.7; pivot shaft-front 8 X 1.7; idler bearing shaft (2)14 X 1.7; piston shaft bushing seal 17 X 1.7.
 
So far, we have had good luck with this mod but it is not a step that is easily reversed; Alfa won't sell you a new bushing so it's not easy to go back to the original configuration. The timing belt ought to be replaced by, at the most, 40,000 miles and that is the time to check the tensioner.  Incidentally, we checked the availability of the double row ball bearing used in the tensioner; it is an SKF made in Italy, not exported to the USA. Alfa gets about $150 for this gem! NOTE: Air tests show no leaks at 80O psi!
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