ANOTHER 968 failure...
02-22-2002, 06:37 PM
#16
Instructor
Join Date: Oct 2001
Location: Branford CT
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Broken cam teeth?
I know about them
check out my post from a few weeks ago <a href="http://forums.rennlist.com/scripts/rennforums/ultimatebb.cgi?ubb=get_topic&f=15&t=003565" target="_blank">sad day for my 968 motor</a>
My wear pads were in real good shape, looks as if they had been replaced recently. But what I think caused it was wear in the chain, it was stretched therefore putting excessive pressure on the teeth of the sprocket.
Anyway my cams are going out in a few days to Partswerks to have new sprockets "cold welded" on. They will have them for a week or two but when I get them back I will post again about how they look, then again after I run them for a while.
Man.... those dual cam sprockets look real neat, really got my wheels spinning.
I know about them
check out my post from a few weeks ago <a href="http://forums.rennlist.com/scripts/rennforums/ultimatebb.cgi?ubb=get_topic&f=15&t=003565" target="_blank">sad day for my 968 motor</a>
My wear pads were in real good shape, looks as if they had been replaced recently. But what I think caused it was wear in the chain, it was stretched therefore putting excessive pressure on the teeth of the sprocket.
Anyway my cams are going out in a few days to Partswerks to have new sprockets "cold welded" on. They will have them for a week or two but when I get them back I will post again about how they look, then again after I run them for a while.
Man.... those dual cam sprockets look real neat, really got my wheels spinning.
02-22-2002, 06:48 PM
#17
Three Wheelin'
Join Date: May 2001
Location: Asheville,NC (Don't move here!!!)
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Bob... (cue Jim playing Devil's Advocate here):
I dunno... starting with a Yugo, average age ~15 years, w. 100K+ on the clock (both typical for our cars). I'd expect to find a _lot_ more wrong with the Yugo, if it had started out as mecanically complex as a 944... I'd also expect that the Yugo stuff I fixed would not stay fixed because of the POS parts quality, which doesn't generally apply to the 944's.
Worse, the Yugo would lose a fender or rocker panel or floor to the tinworm right after I fixed the clutch.... not likely with our zinc-dipped wundercars.
Now, shifting out of Devil's Advocate mode, I'll agree that they are expensive forms of madness. Most of our cars are worth about as much as the repair cost of any one or two major mechanical failures... _not_ real encouraging when it's time to pony up major repair bucks. "What if the engine blows right after I fix the clutch? I'll have spent more than the dam' car's worth!" This is a good ad for DIY'ing, but we do still have to account for our time...
And yes, Porsche has tended to run & hide when long-term responsibility for faulty design / manufacturing issues comes up..."Pinion bearink? Vot pinion bearink? Ve don't know nussing about no pinion bearink! Iss _your_ car now, vun hunret percent! Yah!"
More gripes? Non-functional raingutters... GM-level plastics (sunvisor hooks).. poor ergonomics (entry-exit, location of high-beam indicator, etc.)... and that f&%$in' thermostat snap-ring!
But I'm dumb enough to have bought another one after six years with the first one.... and I still love it, "vorts & all"!
Jim, apologizing in advance for a Southern boy trying to do a German accent...hey, you oughtta hear me do 'Valley girl'...
I dunno... starting with a Yugo, average age ~15 years, w. 100K+ on the clock (both typical for our cars). I'd expect to find a _lot_ more wrong with the Yugo, if it had started out as mecanically complex as a 944... I'd also expect that the Yugo stuff I fixed would not stay fixed because of the POS parts quality, which doesn't generally apply to the 944's.
Worse, the Yugo would lose a fender or rocker panel or floor to the tinworm right after I fixed the clutch.... not likely with our zinc-dipped wundercars.
Now, shifting out of Devil's Advocate mode, I'll agree that they are expensive forms of madness. Most of our cars are worth about as much as the repair cost of any one or two major mechanical failures... _not_ real encouraging when it's time to pony up major repair bucks. "What if the engine blows right after I fix the clutch? I'll have spent more than the dam' car's worth!" This is a good ad for DIY'ing, but we do still have to account for our time...
And yes, Porsche has tended to run & hide when long-term responsibility for faulty design / manufacturing issues comes up..."Pinion bearink? Vot pinion bearink? Ve don't know nussing about no pinion bearink! Iss _your_ car now, vun hunret percent! Yah!"
More gripes? Non-functional raingutters... GM-level plastics (sunvisor hooks).. poor ergonomics (entry-exit, location of high-beam indicator, etc.)... and that f&%$in' thermostat snap-ring!
But I'm dumb enough to have bought another one after six years with the first one.... and I still love it, "vorts & all"!
Jim, apologizing in advance for a Southern boy trying to do a German accent...hey, you oughtta hear me do 'Valley girl'...
02-22-2002, 06:58 PM
#18
Race Director
[quote]The links on a chain should give out long before the sprocket teeth get worn down. <hr></blockquote>This would be the case if you had an overload failure where you stress the chain to the point where it breaks. But we're talking about wear over time, not ultimate tensile strength in a single yank.
Well, the chain & sprocket wear issue is like the chicken & egg problem. They both wear each other out. It's not possible to wear out the chain such that the links are further apart than the sprocket teeth and vice-versa. It's kinda like a feedback loop where one change causes a change in the other component.
This is actually very common with... say bicycle and motorcycle chains & sprockets. When you replace the chain, you replace the sprocket as well. When they're worn, say 5% increase between links, the sprockets will also have cup-shaped half-moon cut-out on one side and the other side of the sprocket tooth will be fine. This causes the sprocket tooth take on the profile of an cresting ocean-wave with one side almost spilling over the other.
When you put a new chain on with a worn sprocket, only one sprocket tooth and one chain link is in contact with each other. This places ALL of the load on a single sprocket tooth and chain link. Rapid wear of both is the result. So you really need to have a matched set from the beginning.
I have no idea why Porsche didn't have a replacable bolt-on cam sprocket. It's such a simple design. Just imagine you cut off ALL of the sprocket teeth on the cam so that it's just a smooth circle. Then cut off another sprocket from second cam, but at the base circle on the cam itself. You'll have a resulting loose sprocket that's like a donut with a hole in the middle for the cam to fit. Drop this down the first cam onto the bare sprocket that you ground the teeth off. Drill some holes and bolt it on. Simple, but you've just wasted a cam. Porsche should've had that from the start.
I'll have more pictures of all this later if it doesn't make sense.
Well, the chain & sprocket wear issue is like the chicken & egg problem. They both wear each other out. It's not possible to wear out the chain such that the links are further apart than the sprocket teeth and vice-versa. It's kinda like a feedback loop where one change causes a change in the other component.
This is actually very common with... say bicycle and motorcycle chains & sprockets. When you replace the chain, you replace the sprocket as well. When they're worn, say 5% increase between links, the sprockets will also have cup-shaped half-moon cut-out on one side and the other side of the sprocket tooth will be fine. This causes the sprocket tooth take on the profile of an cresting ocean-wave with one side almost spilling over the other.
When you put a new chain on with a worn sprocket, only one sprocket tooth and one chain link is in contact with each other. This places ALL of the load on a single sprocket tooth and chain link. Rapid wear of both is the result. So you really need to have a matched set from the beginning.
I have no idea why Porsche didn't have a replacable bolt-on cam sprocket. It's such a simple design. Just imagine you cut off ALL of the sprocket teeth on the cam so that it's just a smooth circle. Then cut off another sprocket from second cam, but at the base circle on the cam itself. You'll have a resulting loose sprocket that's like a donut with a hole in the middle for the cam to fit. Drop this down the first cam onto the bare sprocket that you ground the teeth off. Drill some holes and bolt it on. Simple, but you've just wasted a cam. Porsche should've had that from the start.
I'll have more pictures of all this later if it doesn't make sense.
02-22-2002, 07:20 PM
#19
Instructor
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[quote]Originally posted by Danno:
<strong>
I have no idea why Porsche didn't have a replacable bolt-on cam sprocket. It's such a simple design. Just imagine you cut off ALL of the sprocket teeth on the cam so that it's just a smooth circle. Then cut off another sprocket from second cam, but at the base circle on the cam itself. You'll have a resulting loose sprocket that's like a donut with a hole in the middle for the cam to fit. Drop this down the first cam onto the bare sprocket that you ground the teeth off. Drill some holes and bolt it on. Simple, but you've just wasted a cam. Porsche should've had that from the start.
I'll have more pictures of all this later if it doesn't make sense.</strong><hr></blockquote>
Sounds feasable, but I don't think you would have anywhere to bolt it on. The hole dia. through the sprocket required to pass over the cam lobes is quite large in relation to the bottom of the sprocket teeth, there would not be enough solid material left there for any kind of bolts. I'd still like to see pics though.
<strong>
I have no idea why Porsche didn't have a replacable bolt-on cam sprocket. It's such a simple design. Just imagine you cut off ALL of the sprocket teeth on the cam so that it's just a smooth circle. Then cut off another sprocket from second cam, but at the base circle on the cam itself. You'll have a resulting loose sprocket that's like a donut with a hole in the middle for the cam to fit. Drop this down the first cam onto the bare sprocket that you ground the teeth off. Drill some holes and bolt it on. Simple, but you've just wasted a cam. Porsche should've had that from the start.
I'll have more pictures of all this later if it doesn't make sense.</strong><hr></blockquote>
Sounds feasable, but I don't think you would have anywhere to bolt it on. The hole dia. through the sprocket required to pass over the cam lobes is quite large in relation to the bottom of the sprocket teeth, there would not be enough solid material left there for any kind of bolts. I'd still like to see pics though.
