928 Achilles Heel T-Belt Cure?
12-22-2005, 11:27 AM
#47
Per haps a dumb suggestion from a extreme 928 novice. Why couldn't they have designed this with two belts, one of which would be sufficient to keep things in order. One breaks, the warning light comes on but the remaining belt keeps things running. No disaster and now you just replace the broken belt or better yet both of them. Just my simplistic mind at work.
12-22-2005, 11:43 AM
#48
Captain Obvious
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Originally Posted by PatrickP
Per haps a dumb suggestion from a extreme 928 novice. Why couldn't they have designed this with two belts, one of which would be sufficient to keep things in order. One breaks, the warning light comes on but the remaining belt keeps things running. No disaster and now you just replace the broken belt or better yet both of them. Just my simplistic mind at work.
12-22-2005, 12:06 PM
#49
928 Collector
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I've never heard of a failsafe belt and that would not help us. Failsafe would be two of everything including rollers and water pump because the most common failure in that system is the water pump. As for chains, look at 964 chain tensioners ... and 911 tensioner failures .... catastrophic engine failure built right into the engine. Same with 944 cam chains.
12-22-2005, 12:35 PM
#50
Captain Obvious
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Originally Posted by heinrich
I've never heard of a failsafe belt and that would not help us. Failsafe would be two of everything including rollers and water pump because the most common failure in that system is the water pump. As for chains, look at 964 chain tensioners ... and 911 tensioner failures .... catastrophic engine failure built right into the engine. Same with 944 cam chains.
I was thinking of other DOHC engine manufacturers with chain driven cams. Such as Nissan, Ford, even some Toyota products that have chain driven cams that last the life of the engine. I know, all theses engines need the guide to be replaced at least once (~200K km) but other than that they are very robust and safe.
12-22-2005, 12:39 PM
#51
928 Collector
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True Imo. Audi have a very interesting chain that doesn't look like a chain. It can be done and it can be retrofitted. But the amount of work methinks no-one is up for. It would need an oiltight housing attached to the front of the engine and oiling system, and move all the crap out of the way, radiator forward or tilted ....
Personally I strongly believe the best answer for us is replacing the water pump with an external electric one, and replacing some wear items on the tbelt system with high tensile parts. The second part of this is easy, first part actually not too hard.
Personally I strongly believe the best answer for us is replacing the water pump with an external electric one, and replacing some wear items on the tbelt system with high tensile parts. The second part of this is easy, first part actually not too hard.
12-22-2005, 12:57 PM
#52
Rennlist Member
Originally Posted by Garth S
Regards the pivot bolt (< '87) or the pivot stud (> '87), the diameter of the bearing surface is 12mm. The hole diameter in the carrier arm is 15mm, leaving the max. wall thickness of the bushing @ 1.5mm.
Firstly, the perhaps simplest approach to the bushing is to press in one turned from phosphor bronze: this material has served in small electric motors, alternators and starters over the years . If it can survive the side thrust of a starter's Bendix drive, possibly it could handle the small arc of rotation and 'twist' seen by the carrier arm.
Who is the materials expert?
My expressed concern over a bearing is shared by Dave - but I don't know enough about their service specs to say otherwise. The carrier arm pivot bolt hole has as little as 3.5mm wall thickness ( up to 4.5mm) ... leaving little to machine for a bearing in any event.
Secondly, the 12mm pivot is not the problem ... it is the reduction to the thread diameter at the shoulder: the pivot bolt has 8mm threads, and this was increased to 10mm for the S4 pivot stud. Why live with this reduction?
If one really wanted to stiffen this unit up, we need a pump boss that is capable of accepting a 12mm thread ..... try to bend that !!!!!
If that is not enough, and a suitable bushing material could survive at 1mm wall thickness .... then a 13mm stud could be fitted
.
So, any votes for a M12 thread on the pivot bolt/stud that slides in a bronze bushing?? Edit ... on reflection, in this service, the Nylatron is probably superior
The pics are self explanatory - the bushings shown were the first 'interference fit' ones turned from delrin.<-- wrong! Edit .... they are of black Nylatron, a molybdenum disulfide filled nylon .... I used the delrin on a BMW project ..
Firstly, the perhaps simplest approach to the bushing is to press in one turned from phosphor bronze: this material has served in small electric motors, alternators and starters over the years . If it can survive the side thrust of a starter's Bendix drive, possibly it could handle the small arc of rotation and 'twist' seen by the carrier arm.
Who is the materials expert?My expressed concern over a bearing is shared by Dave - but I don't know enough about their service specs to say otherwise. The carrier arm pivot bolt hole has as little as 3.5mm wall thickness ( up to 4.5mm) ... leaving little to machine for a bearing in any event.
Secondly, the 12mm pivot is not the problem ... it is the reduction to the thread diameter at the shoulder: the pivot bolt has 8mm threads, and this was increased to 10mm for the S4 pivot stud. Why live with this reduction?
If one really wanted to stiffen this unit up, we need a pump boss that is capable of accepting a 12mm thread ..... try to bend that !!!!!
If that is not enough, and a suitable bushing material could survive at 1mm wall thickness .... then a 13mm stud could be fitted
.So, any votes for a M12 thread on the pivot bolt/stud that slides in a bronze bushing?? Edit ... on reflection, in this service, the Nylatron is probably superior
The pics are self explanatory - the bushings shown were the first 'interference fit' ones turned from delrin.<-- wrong! Edit .... they are of black Nylatron, a molybdenum disulfide filled nylon .... I used the delrin on a BMW project ..
12-22-2005, 01:00 PM
#53
Rennlist Member
Originally Posted by heinrich
True Imo. Audi have a very interesting chain that doesn't look like a chain. It can be done and it can be retrofitted. But the amount of work methinks no-one is up for. It would need an oiltight housing attached to the front of the engine and oiling system, and move all the crap out of the way, radiator forward or tilted ....
Personally I strongly believe the best answer for us is replacing the water pump with an external electric one, and replacing some wear items on the tbelt system with high tensile parts. The second part of this is easy, first part actually not too hard.
Personally I strongly believe the best answer for us is replacing the water pump with an external electric one, and replacing some wear items on the tbelt system with high tensile parts. The second part of this is easy, first part actually not too hard.
12-22-2005, 01:20 PM
#55
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Has anyone considered a more robust or modified water pump? Perhaps larger bearing or maybe even an additional bearing on the outside of the pulley with its own bracing back to the block to reduce load on the original water pump bearing and to equalize the load on the water pump itself? This could also serve as backup in the event of a failure of the pump bearing, possibly allowing enough movement to trip the warning light but keep things together long enough to save the engine.
What is the real world life expectancy of our water pumps anyway? Clearly there needs to be a replacement interval for the pump before failure.
What is the real world life expectancy of our water pumps anyway? Clearly there needs to be a replacement interval for the pump before failure.
12-22-2005, 01:21 PM
#56
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Originally Posted by heinrich
Where would the zerk fit in Dave?
12-22-2005, 01:35 PM
#59
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I love the technical depth of this Forum.
Garth's heavier-duty lubricated pivot bolts are an example of a simple upgrade aimed towards robustness. I like it.
~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~
Last night I was on the Jennit archives and read this one:
http://home.planet.nl/~Jennit/Techni...4/MyTip444.htm
If high-mileage cars (which are discussed another post) that have been through several belts have worn cam gears from miles, and maybe from over-tightening...
How can we upgrade the wear strength of these important sprockets?
In Big Dave's write-up on his rebuilding post T-belt failure,
http://www.928oc.org/928oc_michigan/tbelt.html
he had a lightly-worn set of used cam, oil-pump, and crank gears re-finished with a "poly-moly" coating which he stated would wear better than the OEM anodizing.
Note that his originals were worn past being re-useable due to the overtightening that probably killed his belt and then valves.
Anybody else have data/suggestions on strengthening these items?
Garth's heavier-duty lubricated pivot bolts are an example of a simple upgrade aimed towards robustness. I like it.
~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~
Last night I was on the Jennit archives and read this one:
http://home.planet.nl/~Jennit/Techni...4/MyTip444.htm
If high-mileage cars (which are discussed another post) that have been through several belts have worn cam gears from miles, and maybe from over-tightening...
How can we upgrade the wear strength of these important sprockets?
In Big Dave's write-up on his rebuilding post T-belt failure,
http://www.928oc.org/928oc_michigan/tbelt.html
he had a lightly-worn set of used cam, oil-pump, and crank gears re-finished with a "poly-moly" coating which he stated would wear better than the OEM anodizing.
Note that his originals were worn past being re-useable due to the overtightening that probably killed his belt and then valves.
Anybody else have data/suggestions on strengthening these items?
