Cam chain and sprocket

[mepstein]

When I worked at the bike shop, we had a very simple tool that showed chain wear. Chains stretch as they wear so it’s pretty easy to measure that stretch with a ruler or a tool that has markings that corresponds to new, good, fair, poor. I don’t know if a production tool exists in the Porsche world.

[Jonny042]

Originally Posted by Henry Schmidt

We are seeing issues with "new" cam and intermediate sprockets so if yours are in deed in need of replacement (every engine is different) try to source good used sprockets.
Chain are generally replaced but we see so many rebuilds with new parts and poor assembly that we built a tool to check chain wear.

What issues are you seeing with the new parts? I purchased new "OEM" sprockets (all 6)..... is there something to look out for?

[cmcfaul]

That chain is done. Tensioner looks to be at full extension. Chains can be replaced without splitting the case. I would replace the chains and sprocket. Worn chain = worn engine.

Chris

[Forssling]

Thanks Chris. Plan is to do a full rebuild and to replace or restore what needs to be...

//Jörgen

[75 911s]

So I just purchased the Sebro sprockets for my 87 3.2. Now I'm worried. Henry can you clarify what you meant?



My original IMS sprockets:



My cam sprocket :

[75 911s]

Just a followup I reached out to Sebro support regarding the IMS gear shown in my photo above here is what they said:

"Dear Mr. ,

thank you for your E-Mail enquiry.

We are not aware of any issues with the part you are mentioning below.

Best regards from Germany and have a great day."

[Tyson Schmidt]

Like Henry metioned, you are better off running your original sprockets if they are serviceable.

I just took a fresh build back apart to remove the OE Porsche sprockets which a re made by Sebro. I installed a good used set of sprockets after talking to a good number of fellow Porsche shop owners.

Found out in the nick of time. Motor was about to go in the car.

After removing the new Porsche sprockets, I could see a very rough surface in the saddle of the teeth where the chain rides. Does not look at all like they used to.
There may also be an issue with hardening, or possibly the alloy they are using now.

It's sad to see that you just can't trust new parts these days, even from Porsche. These were even made in Germany.

[Tyson Schmidt]

[Jonny042]

I have a complete set of new "oem" sprockets on the bench, they appear to made by Sebro (The /S/ logo looks familiar).

I'll have another look Monday and photograph what I see.

[Tyson Schmidt]

Yep, Sebro makes them for Porsche. These were from Porsche.
I’ve seen pictures of the failed sprockets. They mushroom-out in the saddle area, and you’ll find a lot of metal on the drain plug magnet.

[Jonny042]

Just like you said..... sad. It's not like these are cheap parts.

[Jonny042]

Originally Posted by Tyson Schmidt

Found out in the nick of time. Motor was about to go in the car.

Just confirming - the pictured sprockets had zero time on them, and the rough surface on the scallops between the teeth was as-manufactured?

[Jonny042]

This ^*&%#%@!!! build is going to drive me to drink.

Here's mine..... of course I bought a full set of sprockets in January, thinking I'd need them soon. No doubt that's $500+ dollars down the drain.

F^%$^%! Not to mention having to split the case to replace the intermediate shaft gears. Re-clean everything, replace gaskets, etc.

[Jonny042]

Wait a sec...... are we SURE these parts aren't OK? I mean, maybe not pretty, but is there a chance that they are perfectly fine and I'm over-reacting?

[Jonny042]

I sent an email (with pics) to Sebro. If nothing else hounding them will make me feel better.

[75 911s]

Hmm my plan is to return all four. 2 Porsche OEM cam sprockets and 2 IMS Sebro sprockets. I've just looked a little closer and they appear to be better than Jonny's above, but I can't take a chance.

Check out the Sebro IMS sprockets:












Here's the Porsche OEM Cam Sprocket 3.2 Carrera engine

I actually think the Porsche OEM sprocket for the cam looks worse as far as a non smooth valley. I don't know.

[Jonny042]

Slept on this, and maybe thought about it in the middle of the night. I can't help but feel, given that there are so many examples of these roughly finished sprockets, maybe there's nothing wrong with them?

The roughness of the surface is obviously due to the manufacturing process, in this case cnc cutting. Perhaps they've switched from some more traditional machining process for these, we are seeing the roughness of the finish, and freaking out for no reason?

After all, the chain doesn't slide on the sprocket, the metal on metal movement occurs in the chain itself as it wraps. The outside surface of the chain roller rests in the saddle of the tooth profile and exerts pressure on it, but does not rub on it. This is why chains are so efficient at transferring power.

If sprockets are mushrooming, it's not because of this rough surface, but a hardness/metallurgy issue. Which isn't going to be apparent by looking at it.

Thoughts?

[1980 SC Targa]

Following. You would think Porsche would be very interested to hear when parts associated with their name (or from them) didn't measure up quality wise.

[Jonny042]

One of my oldest and best friends is a mechanical engineer, owns and runs a very successful industrial machine/fab/rebuild shop, and has built his share of race motors for his own cars. Although he teases me quite a bit about being particular, he's basically no different than me when it comes to these things, and I trust his opinion implicitly.

He immediately agreed that you wouldn't be able to tell much by looking at the sprocket, but was sure the surface finish wouldn't be a problem. His explanation of the load distribution on the sprocket by the chain, and they way they wear, matches my thoughts above. There is a lot of load on the chain, but the sprocket, not so much, and not in a way that requires it to be particularly hard or have extreme tensile strength. There is no "rubbing" of chain on sprocket.

That made me feel a lot better.

Although I appreciate, and, like I said, trust, his opinion, I was glad it confirmed my thoughts.....then, we broke out the hardness tester.

He was not surprised that the original cam sprocket tested out to be fairly soft, relatively speaking - on the Brinnel scale an average reading of 94. These appear to be the original 1980 era parts from the factory and really show very little wear (mileage unknown).

The new "OE" sprockets, most likely made by Sebro, measured to an average hardness of 119.

The increase in the reading didn't really surprise him, mostly due to the advances in metallurgy, or rather, the modern availability of lesser quality steels is almost nonexistent, even when compared to relatively recent times (25 years).

We also tested the idler pulley, old vs. new, and the difference was even greater, with 94 (old) and 131 (new).

So, what's going on here? If there have been problems with these sprockets, have they been widespread, or is the same story being repeated? I couldn't find any pictures or any other instances of these sprockets failing. Tyson has seen pictures but from where? Is it possible someone (over)-heated intermediate shaft sprockets to install them rather than pressing them on? A bad chain could cause premature sprocket wear and is more likely than a problem with the sprocket itself (again, examine the forces at work)?

Myself, I am going to rest easy tonight, as confident as I can be that the new parts going in are at least as good as the old, if not better.

Unless of course someone can convince me otherwise?

[eastbay]

My old machinist friend would say those grooves and pits are just for holding more oil. (wink) LOL