Cam chain and sprocket
 

Hi All!
I'm into a rebuild of my -71T engine. It will be a stock rebuild without any big upgrades. I have never seen this engine running and have no history of it at all.

Is it default to put in new cam chains and sprockets? If not, how do I determine if I can reuse mine.

Thanks for all input,
Jörgen Forssling
Gothenburg
SWEDEN

You probably should be more concerned with rust inside the cast iron cylinders
The cam,chain gears should be good if the alignment was kept in tact from any previous opening.
Bruce

Thanks Bruce,
what should I look for regarding wear on chain and sprocket? I guess the chain gets stretched over time, is it possible to measure if it's within proper margins?

//Jörgen

The sprocket wear isn't too hard to check, if you have wear and alignment issues you can visually see it on the sides of the teeth. The root of the teeth should be the same thickness as close to the peaks.

Normally you keep the gears and replace the chains. They are a wear item and inexpensive.

Thanks all!

//Jörgen

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.

http://forums.pelicanparts.com/uploa...1561043295.JPG

Hi Henry, unfortunately I'm in Europe and cannot use your services. From what I can see, (will get second opinions as well) sprockets and chain looks good. The tensioners firm. Even so, do you recommend me to get new chains? And if so, is it Porsche original chains or any other brands you recommend?

KR
Jörgen

Absolutely use new chains. IWIS is a branding you will see on the factory chains.

If you are going to split the case, it is a no-brainer to replace the chains. You can visually check the sprockets.

Generally, with the motor still together one can estimate the chain wear by judging the distance the tensioner is extended or how much space is left before the idle sprocket tops out or hits the guide.

http://forums.pelicanparts.com/uploa...1561076315.jpg

The standard wisdom is to replace chain and sprockets together. That way you can't go wrong. But you can spend more money. And bite off more work, because replacing the intermediate shaft chain gears isn't a snap.

Of course, having a chain actually fail would lead to a minimum of six bent valves immediately, and if lucky up to six broken rockers, with worse to follow otherwise if you kept the engine running (it will run on three cylinders).

But as you can see, that's not the only way guys have done things, and I haven't run across a post where someone said "I wish I had replaced the chain - or the sprockets - or both" when I did the rebuild. Long ago I replaced just the chain when I installed Carrera tensioners. You can replace the chain without removing the engine from the car. I didn't replace any of the gears. When it came time to take that engine apart, all looked fine in the cam drive department.

Misaligned chain gears wear the sides noticeably.

Chain stretch is due to wear on the insides of the links, where you can't see it. The tip about seeing if you have adequate room for the tensioners to work is a good one, which you can also eyeball by how far the tip of the tensioner is extended. But I've not seen any of this quantified - so many mms here or there.

That motor was made rather a long time ago. Do you think that equates to many miles (kilometers) each year, all adding up? Or has it been rebuilt several times, or sat unused for a decade, or something?

Anyway, look at and measure if you still can, those dimensions affected by chain stretch and sprocket wear.

Oh, and trust Henry to come up with a jig to quantify the length/condition/stretch of a chain. If a guy knew what the dimensions were, it shouldn't be too hard or expensive to make one. The articulating part might be one of the old style, bad idler gear holders we all got rid of to replace with the newer, better ones (though Henry can convert the old to the new).

Thanks all! I have not seen any indications yet of damage or extensive wear. But we're just started dismantling everything. I'm borrowing the tools to get the cams out soon, so that's what's up next. I will not save money to the extent where i sacrifice peace of mind driving it.

I'll let you know when we're continuing the tear down. The engine is not at home but I have an image of the chain.

All the best,
Jörgen

http://forums.pelicanparts.com/uploa...1561107396.JPG

That chain looks like it should be replaced.

Post up some detailed pictures of the gear teeth, then we can ya or neh them;)

Will get back with images when we have the sprockets out.

//Jörgen

The image you showed makes the gear teeth look not so very good - lots of silver showing on the sides = wear.

Excellent thread. Thanks for sharing your wisdom guys. Answered several of my nagging questions.

Henry

Look at the antique second generation chain rails...
Bruce

Could they be original to a 71 engine?

//Jörgen

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.

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

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

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

//Jörgen

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

http://forums.pelicanparts.com/uploa...1564421739.jpg

My original IMS sprockets:

http://forums.pelicanparts.com/uploa...1564421749.jpg

My cam sprocket :

http://forums.pelicanparts.com/uploa...1564421749.jpg

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."

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.

http://forums.pelicanparts.com/uploa...1565394414.jpg
http://forums.pelicanparts.com/uploa...1565394536.jpg

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.

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.

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

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

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.

http://forums.pelicanparts.com/uploa...1565657591.jpg

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?

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

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:
http://forums.pelicanparts.com/uploa...1565663063.jpg

http://forums.pelicanparts.com/uploa...1565663063.jpg

http://forums.pelicanparts.com/uploa...1565663063.jpg

http://forums.pelicanparts.com/uploa...1565663063.jpg

http://forums.pelicanparts.com/uploa...1565663063.jpg

http://forums.pelicanparts.com/uploa...1565663063.jpg

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.

http://forums.pelicanparts.com/uploa...1565663203.jpg

http://forums.pelicanparts.com/uploa...1565663203.jpg

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?

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.

Relax....(?)
 

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?

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