15 years of 944 work and have *never* seen a sprocket stamped with a "U".

every single one has a "0" on both sprockets but there are 2 ways to install them.

I highly doubt the B-shafts are affecting the motor mounts, JMO.

 

Only 15 years, Ha! You haven’t been working on them long enough lol. The early cars (iirc 82 to 84) had them but as they were changed out they disappeared and then you could only buy ones with the “O”. Process is the same though, they fit two ways and can be fitted 180 out, especially easy on the lower sprocket as you are aware. Clark’s describes the confusion perfectly.

http://www.clarks-garage.com/shop-manual/eng-08.htm

 

I still see the "U" marked sprockets come through the doors of my shop, though we don't work on as many 944s as we used to. The balance sprockets tend to last a long time because the specified balance belt tension is relatively loose . . .

As for the 180 off belt causing this issue, I find it surprising as well, but vibration-induced harmonics can be incredibly destructive, and perhaps more so on a 3.0-liter 968 engine than with a 2.5-liter 944.

Andy, since you are changing motor mounts (again), I would strongly recommend dropping the oil pan and checking the oil pickup tube. I believe these are a bit beefier on the 3.0 engines, but since the engine has apparently been running like this for a while, it's not worth taking a chance . . . . A nut-and-bolt re-check of any and all engine fasteners would also be prudent (you'll be touching most of this stuff during the belt change anyway).

 

I went back to your posts and did not see if you had recently re-sealed the front of the engine BUT... If you ever do, There is a pitfall that one could fall into with these Balance shafts...
I just happen to be building another engine and was at the part last night where i was installing the BS sprockets... So here goes.. The pulleys are the same top and bottom, Porsche probably did that so there is only one part number for them. However to accomplish this they had to put the Keyway slot in different locations. They originally marked them for "U" and "O" Indicating what SLOT to put the keyway in depending on if you're working on the Bottom - In German "Unten" or the Top - "Oben" The Manual says to put the lower gear keyway in the U and the Top in the O which makes sense, (never really gave it much thought...) But you can see now how even if you aligned the Timing marks on the back of the pulleys correctly that it can still be off if the actual keyway is in the wrong slot. and vibrate if someone before you messed with it. Double Check. The vibration in hardly noticeable till the upper RPM range.. The pulleys for the engine I am putting together only have the O … But my other car has bot U and O... It is conceivable that if a Previous owner had them wrong, and only drove it on the street that the problem would be almost transparent because of the lower RPM's but once on the track it becomes MUCH MUCH more pronounced...

 

The front cover has a tab that slides into the keyway that’s not being used.
On a side note from everything I found that RS BARN made a new harmonic balancer for 3.0L specific crankshafts. Good stuff. I’m sure there are posts about them already but another wouldn’t hurt.
http://www.rsbarn.com/catalog/index....roducts_id=103

 

Been away for a few days, pursuing the first stage of a lifelong dream - I took the first of three two-day racing classes at Sonoma Raceway, in their KTM X-Bows. The most incredible beast I've ever driven - 300 hp, 1800 lb; felt like being strapped to a Saturn V rocket the first time I floored it, with brakes and cornering grip to match. My plan is to participate in their arrive-and-drive race series when I finish the three classes in May. I'm afraid this experience will spoil me for driving any other car, including the 968.

Having said that, I still want to put the 968 back together, although my enthusiasm for the truly miserable job of replacing the engine mounts has waned considerably - I will take my time this time, as opposed to devoting an entire weekend in the nicest part of the year slaving away at this and the belt change. One thing I want to clarify - my balance shafts WERE NOT 180 degrees off. What I found when I removed the covers still baffles me - the belt was VERY loose, even though the nut securing the adjusting cam was as tight as could be, as were all the other nuts securing the components of the belt system. The outer teeth of the belt was clearly "beat up" from slapping against the pulley on the water pump. I'm 1000% certain I set the tightness of the belt correctly (I've done this job many times, and understand how to set its tightness). Maybe the belt itself was defective, and stretched somehow, although this seems unlikely - I'll need to compare its overall length to the new belt. But I think the extreme looseness of the belt caused the lower balance shaft to "freewheel", setting up all sorts of bizarre vibrations and harmonics. I attributed the vibration to the stiffer-than-stock engine mounts, but I now realize this was a mistake. The lower balance shaft was about 40 degrees out of alignment when I looked at it, but who knows how many times it may have spun around before that. I'm convinced that this is what caused the bolt holding the oil lines to the block (and several other nuts and bolts I've found so far) to back oil to back out, spilling several quarts of oil onto the track. I didn't remove the sprockets, or replace the rollers, this time around, and the engine was extremely smooth (I have Pauter "lightweight" forged connecting rods, lighter-than-stock true 11:1 compression Wossner pistons, and I had the entire rotating assembly, including the flywheel, balanced at an excellent machine shop when I rebuilt the engine five years ago), so I'm as sure as I can be I had the alignment set correctly before this belt change. The upper shaft was still aligned correctly. When I replace the balance shaft belt, I'm going to leave the covers off, and check its tightness, and BS alignment, after running it many times, before I put the covers back on, and venture back to the track (if they'll even let me back on, after leaving two giant oil slicks). I did check the oil pick-up tube when I repaced my rod bearings (which looked almost brand new) after my last incident in November, and it was in perfect condition - I had it reinforced at the same machine shop during the rebuild, and my oil pressure light never came on during my most recent incident, so I'm sure it's fine. I will definitely be checking the tightness of every bolt on the engine, though. If anyone has any idea how a belt that was initially set correctly could have gotten loose enough for the lower BS to spin uncontrollably, without any of the nuts securing the components of the belt assembly having come loose, I'm all ears.

 

So I just seen this same issue with the 944 I gave my brother. 86 block, but still the same BS setup. Tensioned the balance shaft belt with that lower adjuster pulley turning from top center to the left via counter clockwise. This allows for that not so immediate turn off the lower balance shaft gear to the BS idler adjuster. However since the tightness was addressed with the counter clockwise for the idler tension and the locknut loosens in the opposite direction one could loosen that nut with a fast enough rev of the engine. Wait no it would tighten it more. Getting old sucks. The balance shaft belt spins four times faster. Think about that torque that BS belt sees on a quick rev. Maybe tighten down that locknut for the balance shaft belt idle adjuster a little bit more. I’ll do the same. And you may want to check out that 3.0 harmonic dampener if your running a 3.0l crankshaft with super lightweight internals that revs much faster than stock and not using a dual mass flywheel setup, and address what I believe is a transaxle mount issue causing the transaxle to “swing” to much which will take out engine mounts. Might want to check you CV joints while your at it.

 

The B-shaft spins twice crank speed, 4 times cam speed. FYI

 

I knew that. Just needed you to point that out Spencer. Keeping us all on our toes.