The 928 Timing Belt - a Rite of Passage

[dr bob]

Mine has been creeping up on the radar screen for a couple years now. The miles on the belt way low, but at 9 years old the tribal knowledge point to needing a new belt. I usually plan for a casual twelve hours or so, with all the cleaning and WYAIT tasks that seem to show up. I think this is the third or fourth belt replacement that I've done on this car, but have close to a dozen other clinic-car efforts to point to. At our previous home near Los Angeles, I'd invite owners to come and do tasks like this, with all the tools and some helping hands to make an otherwise- daunting task a little less daunting. I think we did three in one marathon day, but don't recommend that.

This time the project stretched out over four days, and a bit over sixteen hours of tools-time. The belt I took off could have easily remained on the engine for another 9+ years, truth be told. It was running on all new gears and rollers, and except for some very slight scuffing on the back it's indistinguishable from new. Even the little cast-in pattern on the teeth looks perfect. The technology of the belts has changed a lot since the mid 1970's when the engine was designed. Steel in the belt has been replaced with aramid/kevlar reinforcing, so no issues with corrosion or the steel cords suffering fatigue failure. After this experience, I'm ready to agree with the factory's original 60k change interval with no time limit.

The project was completed with no excitement, and except for some age-related lower back muscle aches, both the car and I seem to be no worse for the experience.

I did decide that I need to get busy fabricating a new front-of-engine wiring harness in the next few years. The wire insulation is getting a little stiff, and I'm sure there are some cracks inside the harness sleeve. Greg Brown used to make them but seems to have given up as it takes some serious care and time to make a perfect harness. I've made a few for other cars before, and except for sourcing some of the particular insulation colors there's not a lot of magic involved. In a recent life I spec'd a lot of kynar-insulated wiring for large-frame gas turbines, and may go hunt down some sections of that cable to make a "lifetime-plus" harness. It all lives inside smooth black plastic sleeve, so shouldn't be too tough to make better but with full original appearance.

Meanwhile, new parts in the front of the engine will be good for the near future. Need to top up with treated fuel, change the oil, and get it up on tall stands for hibernation season. Seats will be out for some conditioning during the off-season. We'll see how well Leatherique works at 62º in the workbay.

[Tom]

Sounds like a great future DIY entry dr bob. I assume you are a factory 9201 tension tool guy? I'll give a shameless plug for my "Tensions Run High" article in the May '18 Panorama (issue 734) if you feel like reading about belt tensioning tools.

I've done quite a bit of wiring repair and actually kind of enjoy it, but if you had any interest in out-sourcing that job, look no further than @Kroonwireharness.

https://www.kroonwireharnesses.com/pors ... e-928-1989

I looked over one of their pre-production 951 harness and was wildly impressed with their work, including in particular the care they take to make accurate replicas -- wires/color codes/connectors/sheathing/grommets, etc. It would take a super eagle-eyed concourse judge to realize it wasn't a factory part.

[dr bob]

The timing belt effort is already well documented, so I'm not sure it needs another "list of extra stuff that I do" added to the pile. I thought I took more pictures of the WIP, but it turns out I have a few that show the order that brackets are stacked, plus one that shows the patient dressed out padded and gowned for the surgery.

On the Kroon harnesses, I'm very aware of the quality and care they put into their work. Meanwhile, I may go on an independent hunt for some kynar-insulated industrial multi-conductor cable, add some fine-strand marine battery/power cable to it, and find some respectable black sleeve that will pass for factory after I add an original-appearing factory part number label to it. The high-temp kynar cable comes in several handy conductor counts and size mixes. There's one shorter (approx 2.5 ft) wire for the AC clutch trigger, else all the other signal wires go all the way across the front of the motor, and terminate on the opposite side behind the front accessory plane. The primary power feed is a separate conductor all the way to the starter, plus there are a couple stubs coming back forward to the alternator and to the ABS terminal behind the left headlight. I'm guessing that the industrial cables are using standard solid-colors insulation rather than what Porsche used, but since it's all inside the outer plastic sleeve their shouldn't be any conflicts. In my experience, the kynar is enough better and long-lasting vs. the common PVC insulation that I'm willing to stretch my originality standards some to use it.

The "current" replacement harness options include Sean Ratts via Roger, Greg if he ever decides to do this kind of things again, and I think they they are still available from Porsche. And I could send mine to Kroon and have them do the work too. Lots of options besides Bob sitting with some cable, signal wires, a crimping tool and a soldering gun. I've made more than a few for vintage British and Italian cars over the decades and am quite comfortable doing the work when needed.

Interesting that the three-pin oil pressure sender connector is not available as a separate part. Common workaround is to install an earlier sender that has screw posts on it. Is it possible to 3D print a connector with the pin receivers in place? Else I'll build a fixture and mold it in silicone.

[Tom]

With the Form 3 printer, it's certainly possible to print connectors. Formlabs claims its x-y resolution is 25 microns (25 thousandths of an inch), and its Z resolution is user-set between 25 and 100 microns depending on the material used. So that's plenty precise enough for a connector I'd say. I'd likely need a spare connector and terminal pins to create the CAD model (unless you already have one?), and we'd need to be mindful of heat if the sender is in a hot part of the engine bay. My go-to resin is rated to 171.5F, but I've torture tested it under the hood of my 951 and it seems 'ok' at higher temps (hanging over exhaust headers for example), but just gets more and more rubbery the hotter it gets. There is a resin rated up to 450F, which may be the better choice if it's going to see sustained temps over 200F or so, but I have not tried it (yet)...

[dr bob]

I have a separate "donor" connector, but it's different from the one that's on the car. The donor is a little hard plastic knob, while the one on my car is a bigger (~~1.5" diameter) softer round fitting with the female connector pins molded in. In the factory piece, the wires are captive but the outer sleeve is not, so there's 1/2" of wire that not sleeved there. I can pull that connector off for dimensions and locations for the pins on the sender, and do a CAD of it in AutoCad. Making a mold for it to cast it in silicone or urethane would be pretty easy all things considered. I suspect that the outer profile would be simple with a molding bit in the router, making it a lot better and more authentic-looking than a plain cylinder. I'll snap a couple pics of the connector, and send them when I get the film back from the drugstore...

[Tom]

Sounds good. Let me know if I can help. I received a sample part printed in the very high temp material (good up to 450F) and it looks great. It is translucent as it comes off the printer so you'd probably want to paint it though. The material I normally use is rated up to 175F, but functional well into the 200's. Could also print a mold for you if you wanted to do it in urethane or whatever... Carpokes is quickly becoming the home of parts not available elsewhere. See today's part de jure here:

viewtopic.php?f=21&t=316

[dr bob]

Well, in winter hibernation I'm finding a spot of gear oil under the front of the engine. It has to be enough to puddle and pool on the belly trays before falling to the floor, so it's something that will need attention for sure. The only place in the front where gear oil is used is in the timing belt tensioner, so the mounting gasket, the adjustment bolt or the boot on the piston top the lists of Likely Suspects. Or I overfilled it last fall when I replaced the belt and fitted the tensioner again.

We'll know in a month or so when the roads are open again.

[Tom]

Well, in winter hibernation I'm finding a spot of gear oil under the front of the engine. It has to be enough to puddle and pool on the belly trays before falling to the floor, so it's something that will need attention for sure. The only place in the front where gear oil is used is in the timing belt tensioner, so the mounting gasket, the adjustment bolt or the boot on the piston top the lists of Likely Suspects. Or I overfilled it last fall when I replaced the belt and fitted the tensioner again.

We'll know in a month or so when the roads are open again.

Sorry about that dr. bob. Curious they used gear old and not oil pressure like so many other cars.

[dr bob]

The tensioner uses the oil to transfer heat from the engine, allegedly to cause some bi-metal Belleville washers to change shape as the bock heats up. I'm not convinced that those washers change much, as one of the metals isn't much more than a flash plating. I suspect there's some blind speculation on how that really helps, but it's a commonly-held belief. On the later cars, the oil is also used to dampen flutter, and there's a tiny check-valve that depends on that oil to let the tensioner take up slack faster than it releases and moves back.

Over the life of the tensioner design in the 928, the oil used has varied from 30wt engine oil to 90wt hypoid gear oil. Those are really about the same viscosity at running temps. The common view now has STP Oil Treatment in there, mostly to help slow the leaks that happen when less-than-perfect care is used during assembly and fitment. The adjusting bolt is originally fitted with a dab of hard blue sealant goop in the thread, but that doesn't sometimes work well when reused. So there are a couple places that I might have screwed up the installation. I'll know better when I can get it over to the other side and up on the lift for inspection. That workbay has a more winter-capable car in it right now, plus a snowblower and some smaller projects. I did put a little plastic tray under the front to capture the seepage.

[dr bob]

.....

Then this morning there's a drop on the pristine tiled floor under the gearbox in the back... Oh well, something else to solve in April. It's not like there's ever a shortage.