cam gears
#1
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Thread Starter
cam gears
I just started in to my TB job after finishing the MM and gasket and cleaning gunk from everywhere. I have had the car 4 years, postponed the work because I was having way too much fun. No history when I bought it but runs perfectly and has 110,000 miles on it now. Thought I should get at it before my luck runs out.
Looking at the belt it clearly has been changed at least once, belt is perfect, no signs of wear, no rubber powder residue or evidence of wear anywhere. Belt tracked very straight, tensioner system is in good shape and working fine. I am replacing it with the porkensioner however.
My cam gears are showing wear however and are a bit shiny in the track line. The belt however shows no sign of distress from the gears. I am having trouble uploading the pictures but I will keep trying.
Are the cam gears still hard to get and expensive if I decide to replace them or can they be re-coated...
Looking at the belt it clearly has been changed at least once, belt is perfect, no signs of wear, no rubber powder residue or evidence of wear anywhere. Belt tracked very straight, tensioner system is in good shape and working fine. I am replacing it with the porkensioner however.
My cam gears are showing wear however and are a bit shiny in the track line. The belt however shows no sign of distress from the gears. I am having trouble uploading the pictures but I will keep trying.
Are the cam gears still hard to get and expensive if I decide to replace them or can they be re-coated...
#3
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The belt one my '86.5 looked like brand new also except the part that spun off the crank. I saw a set of new old stock (not the new version) on ebay awhile ago for $300 or so.
#6
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Thread Starter
pics I hope??
Thanks, I am still trying to upload pics. I have a damaged connector near the left side cam gear, I am wondering what it is. Looks like it rubbed something.
no pictures arghhhh.
no pictures arghhhh.
Last edited by Dads928; 08-31-2012 at 08:53 AM. Reason: no pictures again
#7
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I think you need to be a member to upload to this site, but if you can upload them somewhere else you can link to them or embed them here...
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#8
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Thread Starter
pics I hope??
i hope I have a picture. I need to know what the connector to the left does, it is damaged.
Last edited by Dads928; 04-26-2020 at 10:31 AM.
#10
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New gears are in your future unless Colin thinks they can be recoated.
__________________
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."
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."
#12
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Thread Starter
Roger, how do I know if the knock sensors need replacing and do I do that when I do the intake stuff. I was planning to do fuel lines and intake stuff next spring..
How much is the oil pump gear, and do I need a new one and a crank gear???
How much is the oil pump gear, and do I need a new one and a crank gear???
#13
Rennlist Member
OK, I found this in another thread:
Ian, it is hard to tell from the pic's how worn the gears are-- it is hard to photograph shiny surfaces.
The cam sprockets/gears are aluminum, coated with a relatively hard coating. They wear primarily at the top of the teeth, where it contacts the flat part of the belt. Once the coating wears through then the aluminum starts wearing pretty quickly.
Wear on the top of the teeth does three things, all bad: It reduces the diameter & circumference of the sprocket, which changes the pitch of the teeth and the belt no longer fits properly, which wears the teeth; More of the load is taken by the teeth rather than the flat part of the belt, which accelerates wear; and, as the teeth wear, the edges get sharp which helps the teeth chew up the belt.
The definitive test is to lay a straightedge (e.g. a steel ruler) along the teeth and look for daylight between the sprocket tooth and the straightedge. (It helps to back-light it with a flashlight). The teeth wear in the middle where the belt contacts them, and not at the ends. If the gear is worn then you will see light in the middle, and not at the edges.
From the pic's, and from your description of a belt in good condition, I suspect that you haven't gotten to that point, yet. And if they are not yet worn (except for the coating), then they are candidates for recoating. Once the aluminum starts to wear then they are junk.
But if you are seeing shiny areas now then they will not make another 60K miles. So you either need to replace (or recoat) them now, or commit to checking on them every year or so.
If the oil-pump sprocket is coated aluminum then it should be changed also. I thought '89's were steel but that my be wrong-- check it. Steel sprockets (cam and later oil-pumps) last longer and should be fine at 110K miles, but do check them.
The cam sprockets/gears are aluminum, coated with a relatively hard coating. They wear primarily at the top of the teeth, where it contacts the flat part of the belt. Once the coating wears through then the aluminum starts wearing pretty quickly.
Wear on the top of the teeth does three things, all bad: It reduces the diameter & circumference of the sprocket, which changes the pitch of the teeth and the belt no longer fits properly, which wears the teeth; More of the load is taken by the teeth rather than the flat part of the belt, which accelerates wear; and, as the teeth wear, the edges get sharp which helps the teeth chew up the belt.
The definitive test is to lay a straightedge (e.g. a steel ruler) along the teeth and look for daylight between the sprocket tooth and the straightedge. (It helps to back-light it with a flashlight). The teeth wear in the middle where the belt contacts them, and not at the ends. If the gear is worn then you will see light in the middle, and not at the edges.
From the pic's, and from your description of a belt in good condition, I suspect that you haven't gotten to that point, yet. And if they are not yet worn (except for the coating), then they are candidates for recoating. Once the aluminum starts to wear then they are junk.
But if you are seeing shiny areas now then they will not make another 60K miles. So you either need to replace (or recoat) them now, or commit to checking on them every year or so.
If the oil-pump sprocket is coated aluminum then it should be changed also. I thought '89's were steel but that my be wrong-- check it. Steel sprockets (cam and later oil-pumps) last longer and should be fine at 110K miles, but do check them.
#14
Rennlist Member
Thread Starter
oil pump gear
It looks aluminum, same as the cam gears and is shiny on the tooth surface as well. I will check it when I go back out to the garage but I am pretty sure its aluminum. Thanks for your input, I think the general consensus is that I am in need of replacements.
Roger will know if the gear is aluminum..
Roger will know if the gear is aluminum..
#15
Rennlist Member
Ian, here's some pic's for reference. (EDIT: These are not Dads928's gears-- they are from my junkbox, for illustration purposes. They are from a '87 motor).
First, a cam gear, clearly "toast" :
And here is the straightedge check, same gear:
Here's an aluminum oil-pump gear, from an '87. No shiny bits, just polished coating:
Here's the straightedge check, no light visible:
And here is a steel crankshaft gear, which is making a liar out of me-- it is clearly worn and should be replaced:
And a failed straightedge check:
These are all off of an '87 engine, unknown history but stated to be 57K miles. If that is correct then the belt spent its life severely over-tightened.
First, a cam gear, clearly "toast" :
And here is the straightedge check, same gear:
Here's an aluminum oil-pump gear, from an '87. No shiny bits, just polished coating:
Here's the straightedge check, no light visible:
And here is a steel crankshaft gear, which is making a liar out of me-- it is clearly worn and should be replaced:
And a failed straightedge check:
These are all off of an '87 engine, unknown history but stated to be 57K miles. If that is correct then the belt spent its life severely over-tightened.
Last edited by jcorenman; 08-31-2012 at 01:07 PM. Reason: Clarification