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Thanks Mark for look out. But I just pointed them to make sure bolts matched holes and to see how it was going along. I did remove already since I've begun to put all parts back on.
Hey guys thanks for the support and thumbs up. Actually engine was pretty dirty, needed a new crank seal but took extra parts off to clean up and now is almost like new. Here are a few pics.
Good job on putting it all back together but I have to ask, why did you keep obviously worn cam gears and oil pump gears on there? Granted they are not "bad" but they are worn past the coating and now you will be wearing down the actual metal. That oil pump gear will be the first one to start wearing the fastest.
So I'd like to put my sense into your question and those of others concerning gears wearing out. Now, I don't want to come across as an arrogant guy who is new to these cars but I also do have an engineering background for mechanical/metallurgical engineering.
What I think some of you guys are considering as worn out might in reality not affect the function of the gear with respect to the belt grabbing the gear as resistance is concerned. I've attached a pic with a few arrows drawn on it. The black arrow shows the wear on gear. But the red arrows show where the push or pull is coming on to the gear and where that force is coming onto it the wear is significantly less. The wear of the "U" potion of gear is non existent and that the portion that matters most.
Now if the top of the notch is worn down by a millimeter or two then that when we have an issue, because now the belt has a smaller face to push against and thus can skip over. BUT, you'd also have to consider how much less resistance from piston pushing on pulley for tension would have to be. So in reality there's two things that would have to happen before you have gear failure.
Even then consider how much wear is put on belt vs gear! Belt being rubber vs gear being aluminum. The belt that I took off is like new and no showing of wear. Like Mark mentioned, metal would take many many more miles of wear to have an issue with gears.
RVII
This is an interesting subject matter and personal experience is very valid. Ricardo has the same dilemma I had two years ago and if we can develop practical experience in this regard so much the better.
As I understand the general consensus is that when the coating has worn through that is the performance indicator that the sprockets must be changed and no responsible professional can recommend anything else.
Whether this performance indicator comes from the factory or anywhere else for that matter I know not but whether or not this criteria is spot on and the evidence that supports or refutes this should be the main point of the discussion. Opinions are two a penny and where differences exist such should be fodder for constructive discussion [not aggression please].
That being said I reiterate a point I have made on several occasions and that is why do the flats wear and [generally] not the valleys? I note from Sean's photos a couple of sprockets that show some wear in the valleys- the first I have actually seen but that is in addition to the wear on the flats. The teeth are there to take the shear load so one might reasonably expect the leading edge of the cam sprocket [left side of the tooth viewed from the front] to show the most wear but this does not seem to be the case. If I were to hazard a guess I might think that excessive wear on the flats might be due to too much tension as might be water pump bearing failure [that I have not experienced- plastic impellers falling off done that twice] .
Too much tension would obviously cause some issues besides parasitic power losses. When I changed my timing belt two years ago I posted some pics about sprocket wear and again got a mixed bag of responses from "totally shot" to "lots of life left in them". I secured some re-coated sprockets from Colin ready to go in if and when needed and decided to run with the worn ones a while longer and monitor them from time to time. There was plenty of discussion about whether re-coated sprockets actually work- I have no position on this but an interesting point nonetheless.
Now this is where it gets a bit interesting. I did a List search for timing belt failure and only secured 11 hits. Of these most seemed to be early model square teeth, with only one failure recorded on an S4 and that was down to a seized water pump. I cannot remember ever having read of an HTD sprocket having jumped a tooth and for that matter have not read of a HTD belt failure due to tooth wear or degradation. I inspected my last belt closely under a magnifying glass and could not see any obvious signs of degradation irrespective of whether anything may be brewing but of course changed the belt out.
The object here is to ensure failure does not occur so premature retirement is inevitable as there is no such thing as "if only I changed it a week earlier". Owners can risk such things - the professionals cannot realistically do this as one failure could wreck a good reputation.
Seems to me the sprockets should be good for at least 120k miles, doubtless many have achieved more. Mine have covered some 100k miles but then I live in a dusty climate- maybe a service life reducing factor?
Now if the flats wear, does that subsequently bring the belt into contact with the leading edge of the sprocket? This is what I am currently looking for signs of. If it does, might this suggest the sprocket design is incorrect [i.e. too high?].
I noted Roger's comment about the sprocket wear and timing being affected by it- I would very much like to understand how that works- must say I cannot visualise how that happens at the moment . I can understand that different parts of the belt are under different tensions in the cycle depending on the variable spring resistance of the cams as they spin and I can understand how cam timing might be effected by belt tension and higher engine speeds [natural retardation of cam timing with enhanced peak power].
Fred, the simple of it is when the coatings is worn off, the gear wears faster. When the gear gets worn to the point the belt is eating in to the valley, the tension is way less than it was when set. If you are still using the factory tensioner you will get a warning. The worst oil gear you see there had the tension checked and set on 3 different occasions over a 4 week period. By me. Then we decided it was time to dig in and find out why the tension had kept changing and then we found the oi pump gear was well worn. And apparently it kept wearing down over that 4 week period rather quickly.
Now if the PKT had been used, the tensioner would have kept constant tension on the belt until it couldn't and then the failure would have been major. When you fit a PKT, put on perfect parts and you won't have this issue.
Taking Kiborts advice is akin to playing Russian Roulette but it seems that a good majority already know not to listen to him anymore. You can see dust build up on the OP's gear in one of the above pictures with a finger swipe on it. That's due to the mismash of gear and belt. I can take apart a car with good gears and 5 years of constant use and will not see any belt dust.
I'm too lazy to look up in the archives what material makes the coating. However, the thought did cross my mind about how durable a parkerized coating would be if anyone has tried it.
Now that Porsche has changed the material makeup of the cam and oil pump gear I believe wear is a thing of the past. So once you change to the new gears (OUCH!!) they could be good for a very very long time. I hope I am alive to be able to confirm this.
I have the new gears on my GTS so time will tell. Also assuming that the belt is at the correct tension.
As for very bad wear of the cam gear effecting timing - the wear was very bad (I will try and find pictures) and the belt had worn into the surface of the gear by 0.060" thus reducing the diameter. So my simple mind says that would speed up the rotation of the cam - maybe. As I said others came up with that conclusion.
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Does it have the "Do It Yourself" manual transmission, or the superior "Fully Equipped by Porsche" Automatic Transmission? George Layton March 2014
928 Owners are ".....a secret sect of quietly assured Porsche pragmatists who in near anonymity appreciate the prodigious, easy going prowess of the 928."
01-04-2016, 09:18 PM
George Layton March 2014