Cam chain sprocket run out .

theiceman · 10-08-2015, 06:34 AM

As I am doing some work on my car I went to my buddies house who is further ahead doing his. To try and learn a few things.

He we went to show me the measurement against the inside of the sprocket on the left side as he had out his tensioners back in. he me measured it on the outside and he was 10 thou off .. WTF !!!

so we took the tensioner off and discovered there was 10 Thou difference between the inner measurement on the sprocket and the outer. The stromski tool is set up so you can measure both sides of the sprocket so it was easy to see without moving anything.
As a further example ( just as an exercise) we saw a greater deflection on the chain housing. although not a recognized measuring spot we were demonstrating the same phenomenon.

So we switched it over to measure the right side .. The opposite phenomenon was observed which made sense and confirmed our findings.

On the drivers the closer side of the sprocket to the centreline on the case was 10 thou closer than the outside.

On the passenger side the closer side of the sprocket to the centreline of the case was about 6 thou further away than the outside.

This caused us to suspect either the tool was running out , or the webbing on the case we were measuring against was out of alignment.

Was really fascinating so i thought i would ask here in case anyone has run into this .

As our options were limited we ended up putting a two tho feeler guage between the tool an the passenger side of the case when we attached the stormski alignment tool. This squared up the tool to the case and everything matched. Then we were able to redo our shimming. Then we called it a night while we thought it over.

The case is pinned and glued so there is no going back now, and doubt there is anything we could do anyway .

Any thoughts on this ? i found the whole exercise very educational on how all the measurements inter relate.

Flat6pac · 10-08-2015, 10:53 AM

You don't load the tensioner, and the idle gear. Have the nut tight so it pulls the cam all the way towards you with shims, backing gear, and cam gear in place.
Once the measurements are confirmed, remove the cam gear and backing gear, load the key way. Seat the backing gear, load the chain on the cam, put tensioner in. Set up for Z1 pin the cams and put the nut on.
Bruce

theiceman · 10-13-2015, 08:56 AM

Thanks Bruce for the process . We were just doing a final check when we noticed the run out , but when we backed everything off and did it as your process states we still had the run out , except we did have the keyway in place, that is the only variable.

Thanks

Flat6pac · 10-13-2015, 09:43 AM

I'm trying to visualize "run out".
The right cam gear gets inverted with the flat side towards you
The left cam gear has the concave side towards you
Bruce

Walt Fricke · 10-26-2015, 06:50 PM

Bruce

They found that the Stomski tool was not holding the straight edge perpendicular to the crank/cam axis. It was tilted, so when measuring on the outside of the chain wheel they got one measurement, and on the inside another. And the same, only reversed, on the other side.

This has to mean that the case half ends on each side of the intermediate shaft were not true to each other. Which they solved by shimming the low side, at which point the straight edge was true and the gear wheel measurements came out as expected. This misalignment could happen if two damaged case halves were machined to make one case without truing these ends of the case.

I have always assumed that Porsche had this area machined as part of the process of mating fresh halves. Because the shop manual relies on these two surfaces being true for making these same measurements.

It would be easy enough to check to verify the Stomski tool was milled true - a granite counter top ought to be flat enough for that.

Before I purchased the Stomski tool I had a heck of a time making these measurements repeatedly. One problem was holding the straight edge so it was up against these two inner lips. There is a second, sort of concentric, lip around the area farther out, but it doesn't stand as proud as the inner one. Easy to rest your straight edge on the outer and inner lips on just one side, in which case it will be at an angle you don't want.

But runout confused me, too, until I realized what he was saying (which he did well).

VFR750 · 11-01-2015, 05:55 AM

That is a tough measurement. I thought I was spot on with 4 shims on both cams. Then I had to pull it apart to reset timing and found in only 38 miles the RHS idler sprocket was showing wear on the front edge.

I looked at the wear and made a decision the cam sprocket had to be too far forward to create that pattern. I reduced the RHS to 3 shims and reassembled.

I did not remeasure, because I trusted the actual engine wear pattern represented fact, whereas my measureing skills must be considred suspect.

900 miles later, again disassembling it, I found virtually no change from 38 miles. So 3 was the right number. All sprockets looked great.

I relate my story to point out it really is important to get that dimension right.

And paying that much attention to detail and concern for the proper use of the tool is comendable.

Walt, interesting comment on the case surfaces.

bpu699 · 08-11-2019, 01:02 PM

Bringing up an old post as I am getting ready to do this...

I know the process for the cam sprocket alignment... but I assume there isn’t one for the idler sprocket, or is there?

My motor has very little to no wear on the cam sprockets, suggesting proper alignment from the factory. But, the idler sprockets do have mild face wear on the sprocket...

How is that? Doesn’t the idler just float and self align???

Flat6pac · 08-11-2019, 03:09 PM

"The idler sprockets are going to run where the chain puts them.
There is several mills of available shifting on the shaft
Bruce

bpu699 · 08-12-2019, 04:32 PM

Quote:

Originally Posted by Flat6pac

"The idler sprockets are going to run where the chain puts them.
There is several mills of available shifting on the shaft
Bruce

That’s what I figured. Thanks for verifying.

Odd that is the only sprocket with lateral tooth wear...

Trackrash · Trackrash's Garage

Not sure what motor you have. But is worth while to check the idler shaft for wear.

bpu699 · 11:42 AM

Just spent a couple hours playing with this...

I gotta say, this is a very imprecise process to get .25mm accuracy...

First, the case halves aren’t exactly flush...

I put the level against the ims shaft casing and measured the difference at 3 o’clock and 9 o’clock.

.07 mm difference right there!!! Shimmed with feeler gauges until it zeroed out...

Measured the right hand cam gear, needs .5 mm more to meet spec. It’s already got 4 shims, that would mean 5 shims...

I really question the reliability of the readings...

Why would the readings change during an engine rebuild from original?

Is there any other way to verify measurements?

For example, hand turn the crank 10 revolutions and see where the chain settles? It’s easy enough to slide a feeler under the chain to see if it’s sitting on the middle of the gear. Just measure the gap on both sides...no?