Tensioner Sprocket Angst

[Ernesto 911]

I’ve been sourcing new engine parts for my 2.4 to 2.7 build and have been having a problem picking a pair of tensioner sprockets.
Two different manufacture OEA sprockets are available to me.

“Type one” is a dead perfect match for the originals
but nearly all have small dings in the top of the teeth from being shipped together protected only by plastic bags.

“Type two” are labeled “Made in Germany” and travel much better, being packaged both in paper AND plastic bags. They have more robust bronze bushings.
Their issue is that they are machined extremely sharp on all edges of the sprocket…A lot sharper than the other type.

I’m not too eager to use either dinged sprockets or hydraulically tensioned knives gnawing on my new chains.

I am concerned about wear either way, but I'm thinking dinged sprocket tips are less of a problem than sharp metal edges where they tend to wear.

Am I overthinking this?

[2.7RACER]

Ernesto,
It is difficult to comment on the sprocket's condition without a picture or two.
Keep in mind the chain does not contact the tips of the sprocket teeth.
Dings and raised edges in the valleys of the sprockets of course would be reason for concern.
Are you getting new sprockets for the cams and intermediate shafts?
The condition of these sprockets is more important than the idle sprockets.
Chains wear from load, lack of lubrication and misalignment. As do sprockets.
Perfectly engineered and installed chain drives begin the process of wear from the moment they are put into service.
I would be concerned about burrs in the valley or sides of the sprockets.
Sharp edges would not concern me.

[Ernesto 911]

Thanks Doug,

No pictures at this point. I'll try some macro work tomorrow.

Indeed, I will be replacing the cam and intermediate shaft sprockets along with the updated idler arms and hydraulic tensioners. I want a clean slate on the build.

I was thinking along your lines that the tips of the teeth on sprockets weren't as important as the valleys.
Even though the edges on the "type two" sprockets aren't burred, the old sprockets and ones I've seen in pictures seem to "wear in" on the outside edges.
I was thinking the extra sharpness would leave more metal behind as they learn to play with the chains.

-Ernie

[Ernesto 911]

I took some shots of the two sprockets in question. Sorry for the glare I haven't took the time to inform myself of the subtlties of macro flash.

The sprocket on the left is identified as "type one" and is most like the AG part and has the dinged up points (see red line).

The sprocket on the right is "type two" and has the sharp edges and what the flash revealed as rougher finish in the valleys (see yellow lines).



The type two sprocket has a tighter bushing ID by .05mm, is also thicker by .11mm, and has a narrower center channel by .17mm than type one...

I think I've talked myself into using type one and living with a few dinged tips.

-Ernie

[2.7RACER]

Ernie,
The pictures show exactly what you were talking about.
I would be comfortable using either sprocket.
As I mentioned the small dings at the tips of the teeth don't contact the chain at all.
No issue.
I suspect the type two sprockets with the slightly rougher finished valley's would wear smooth quickly.
Timing chains wear internally from load and the constant working back and forth as they go around a sprocket. This is where lube is important.
Additionally sprocket alignment is extremely important. This is where the sprockets get the wear marks on one side or the other as the chain link rubs the side of the sprocket if not aligned properly. Makes extra noise too.
Some designers always use sprockets with an odd tooth count to spread the wear on sprockets. They can't alway do it because of specific ratios.
The Engineers at Porsche did this with a 27 tooth driven sprocket at the cams.
The sprockets at the intermediate shaft however are even at 24 teeth.
They will show a wear pattern at the side since the chain link passes over the same teeth.
The idle sprockets have some latitude to self align as they can move side to side. This is designed in.
The trick is to align the driven sprockets on the cams with the drive sprockets on the intermediate shaft.
This procedure is described in the manuals.
Careful attention here will give you quieter chains and longer life.
I'm sure most of this is info you already knew.
Good luck,

[hcoles]

a thing I noticed was that on my 3.2 the idler sprocket arm can slide fore and aft a lot. I'm not convinced that this will always center itself using the chain. So I did some measurements and made spacers that go on the shaft to hold the idler arm inline.