New chains without new sprockets? Read this!

[Gunter]

For all the people who just feed a new chain because they don't want to split the case:
I would not recommend new chains on old chain sprockets: The pitch is the same but the sprockets are worn on the bottom of the tooth valley to a point were the new chain links do not completely sit in. Just think about it and add up the tiny difference between the worn tooth structure on the sprockets and the new links in the chain going around the sprockets.
Personal experience:
Rebuild at 160k miles, new chains, ramps, etc. Runs great! After about 15k on a routine oil change with sump off, a lot of teeth are in the screen! Wow! On closer inspection, they are the tops of the chain sprocket-teeth. No other choice than to split the case, yep, both chain sprockets have most of the tops of the teeth missing. The ALU gear was good, the ramps were still good, idler sprockets good, alignment checked out, no other wear.
I thought about it long and hard and it makes sense to an old Mech. Engineer. In high-speed industrial applications, we always replace both sprockets and chains because mating the new chain to a worn sprocket is not acceptable.
I can hear the replies: I never had any problems with fitting a new chain to old sprockets, o.k. good luck to you! But, have you ever checked the condition of the sprocket teeth after you have used a new chain on old sprockets? Run for about 15k and have a look. Sadly, the sump plate disappeared after '83? so, there is no more screen. For anyone who just feeds a new chain on to old sprockets: I would be very concerned about the teeth wearing to a point of jumping with disastrous results. In fact, I will take pictures of the worn sprockets with my new digital as a warning.
You can see most of the teeth missing and some bent and about to go. Makes me wonder how many engines are running with the chains just about to jump.

[juanbenae]

back in my dirtbike days, and more recently go-kart racing i would never used old/new combinations when it came to chain & sprockets. infact, karting i had to change drive ratios for particular tracks by sprocket size, i kept the chains and sprockets together as pair even then.

thats a shame though, hope it did not hurt other engine components.

[zotman72]

Thanks as I am about to do a tensioner upgrade this month on my SC and was going to replace only chains. I have only replaced the chains in the past on two other motors. Very insightful.

If you want to see something really ugly, imagine a chain sprocket with no teeth at all. This was a result of the left cam being frozen at 6K rpm due to valve adjustment screw backing out/off and wedging itself between a lode and the cam tower. That was on a brand new 2.4S motor too!

[creaturecat]

new chain on an old sprocket - doesn't even work on a bicycle

[ficke]

Thanks I will add that to my list, Fritz
I am starting to really understand this "free" site

[exhaustfumes]

i might be missing something here, but isn't the picture of the timing gear lock-down sprocket, which does not have any direct contact with chains??

in this case, who cares what the teeth look like, all it has to do is take the load from the pin.

joe, who just timed his cams

[dtw]

Joe - The pin is only loaded during cam timing procedures. After that the nut (or bolt on later engines) with the massive torque spec takes the load. The sprocket (teeth) is what maintains time. Teeth go away, time goes away, engine makes cool grinding and banging noises and turns into a Bridgeport piston machining mill.

Healthy teeth are a good thing.

Gunter, I assume that sprocket is from the engine in your other thread. I'm glad you went the distance and tore it down, looks like that was a ticking time bomb.

[Superman]

Almost no matter how worn the chain or sprocket is, the chain should virtually never touch the teeth. I don't believe the problem here was the use of a new chain on old sprockets. I believe something else happened.

[exhaustfumes]

yep, my fingernail is now black about 50% of the way up, thnx to a slip while tightening those nuts!! ya gotta have the special tools, and 2 people help.

[911pcars]

The photo shown looks like the intermediate shaft drive sprocket. The only sprocket wheel on the cam is the timing sprocket with indexing holes. The indexing "flange" wheel is keyed to the camshaft and only has indexing holes in it, no sprocket teeth.

The pin engages in both the cam sprocket and the "flange" wheel and allows both to rotate as one unit. Without this pin, I would not trust heavy torque, even 100 ft.lbs., to keep the two from rotating independently.

Pelican carries this replacement part (~$30)

Sherwood
(no affiliation)

[2.7RACER]

Sherwood is correct. The sprocket shown is one of the intermediate shaft sprockets. Beyond Sherwoods spot on description, the sprocket shown is 24 teeth. The cam sprockets are 27 teeth, with holes as Sherwood described.

[Gunter]

Originally posted by exhaustfumes
i might be missing something here, but isn't the picture of the timing gear lock-down sprocket, which does not have any direct contact with chains??

in this case, who cares what the teeth look like, all it has to do is take the load from the pin.

joe, who just timed his cams

Joe:
You are on the wrong track; the picture is showing the chain sprockets. (The ones on the intermediate shaft)

[Gunter]

Originally posted by Superman
Almost no matter how worn the chain or sprocket is, the chain should virtually never touch the teeth. I don't believe the problem here was the use of a new chain on old sprockets. I believe something else happened.

???????????????
The sprocket shown is one of the chain sprockets fitted to the intermediate shaft! The other one had the same damage.
There is no question in my mind that feeding a new chain onto worn chain sprockets goes against mechanical principles.
I should have know better.
The chain sprockets are the "Drivers" meaning: They work harder than the cam sprockets.

[2.7RACER]

Gunter,
What you are saying regarding new chain on "worn" sprockets is classic engineering.
However, let's refine the discussion.
Chain stretch and the resulting loads on the sprocket teeth increase throughout the life of the application.
So we are left with wear and the resulting tolerances.
A chain that has run say 50,000 miles in the average Porsche has worn somewhat, stretched in the vernacular.
A chain that has run 15,000 miles has stretched/worn.
So the question becomes at what point does a worn chain introduce sufficient wear on a sprocket to warrant replacement?
How can we inspect or measure a sprocket to determine it's condition?
Actually like gears, the loads sprockets see depends of course upon the "load".
If a drive sprocket and the driven sprocket are the same tooth count, the forces acting upon the sprocket teeth are equal.
Since the typical 911 motor uses two 24 tooth sprockets driving driving two 27 tooth sprockets the load is actually slightly lower on the drive sprockets than the driven sprockets.
But, back to my original question, what is the specification for sprocket wear? And for that matter, what is the specification for chain wear?

[Gunter]

Good point.
In my case, it was a rebuild at 160k miles on a 3.0 liter; heads, new rings...............sprockets looked good and the guy doing it had much more experience on engines than myself. I was somewhat uneasy but went along with bad results as described.
I would say that anytime over 100k, new chain sprockets are in order if a new chain is used.
If someone does a top-end rebuild without splitting the case, I would recommend that the old chains are reused rather than feeding a new chain onto old sprockets.
You can always reach the cam sprockets but, if the chain sprockets fail, a complete tear-down follows.
Is it worth the gamble?

[mb911]

gunter. Your point is exactly why I reused my old chains on my top end. I really wanted to replace the chains but stopped myself do to wear etc. This is a very valid point. Much like changing gear sets not just one gear

[ficke]

Gunter, This thread prompted me to look at and see the wear on my sprockets. Since I had split the case I changed all sprockets and the aluminum intermediate shaft gear. This is an old thread and since, I have put over 11,000 miles on the engine, I treat it like a new car, track it go on road trips etc. It is because of gems of info. like this thread that gave me confidence in the rebuild, Thanks

[88911coupe]

Is there some standard wear limit that can be checked or measured for this?

[Gunter]

One way is to take a worn sprocket and wrap a new chain around it.
Use a magnifying glass and you'll see that the new chain will not seat into the bottom valley of the worn teeth.
It's easier to go by mileage.
If you have ~150k plus miles, and do a complete rebuild, get new chain sprockets on the I-shaft.
The cam- and idler sprockets should still be alright.
The reason for more wear on the chain sprockets is that they are the "drivers" while the cam- and idler sprockets are driven.
If you don't split the case and just do a top end, I would keep the old chains.
I really don't know where the practise of feeding new chains onto old sprockets or the term "chain-stretch" came from.
These dual chains don't really stretch; the rollers wear and the sprocket-teeth wear.
Under normal use and regular oil changes, there is no reason why these chains shouldn't last 300K plus miles.
It's more importan to keep the chain tight with good tensioners that press on the idler sprocket assembly.
I still regret that I went along with the engine builder at the time, contrary to my experience in industrial applications. New chains on old sprockets makes no sense.

Are you doing a complete rebuild? Details? What type engine?

[billybek]

Excuse my ignorance in this matter but I am going to ask this question anyway.
Is that sprocket a driven or a drive sprocket? Is the "chipping" damage at the top of the sprocket on the left side of the tooth from contact entering or leaving the chain?
Just trying to understand how this could happen..