To: "John Rees" <jerees@att.net>

Hello John -

I'm doing my due diligence researching VW reliability before considering purchase of a VW with TDI engine. I happened across your website documenting the problems you encountered with premature failure of the intermediate camshaft drive sprocket on your VR-6 engine, and am (to say the least) "disturbed" by what I see.

As an engineer and life-long "gear-head", there's absolutely no question that your worn timing chain sprocket was not properly heat-treated. In fact, the extreme wear indicates that it was probably not heat-treated at all; just left in the "dead-soft" condition that it was initially in to facilitate machining.

In retrospect, at the time of the initial engine tear-down, when the unusual wear pattern was first noticed, all the sprockets should have been sent to a certified metallurgical lab for Rockwell hardness testing.

If (as you claim) all the other sprockets showed little or no visible wear, and all had significantly higher measured hardness values than the "worn" sprocket, you would have had the "smoking gun" needed to win a court case, or perhaps to get VW to acknowledge the defective part.

The normal wear pattern for a properly tensioned and properly heat-treated chain, running on properly heat-treated sprockets, is for the sprocket teeth to gradually assume a "hook-shaped" profile over many thousands of hours of operation This is because as the chain itself wears, it stretches, and the distance between the link pins gradually grows a few thousandths of an inch. This increased "pitch" then causes the chain to ride up higher on the face of the sprocket tooth. Since the torque transmission between chain and sprockets is unidirectional, only one side of the sprocket teeth develop the "hook" profile.

Timing chains and sprockets used in automotive engines normally have an extremely long life span, the original chain and sprockets frequently lasting the life of the engine (150K+ miles). The usual failure mode is for the chain to eventually stretch to the point where valve or ignition timing becomes so "retarded" that the engine runs poorly, the OBD-II diagnostics trigger the Check Engine, or the vehicle fails to pass smog tests. For all the sprocket teeth (on one sprocket only!) to completely wear away to the point of allowing the chain to "slip timing" is unheard of.

It would also be interesting to compare the material used in the worn sprocket (exact composition of the steel alloy) vs. the alloy used in the "unworn" sprockets. If the worn sprocket had different surface hardness, was made from a different alloy, and/or had a different fabrication process, these are all indicators that would support a conclusion that there were physical differences between the sprockets that explain the very different wear characteristics.

I'd also check the "revision history" of the sprocket part number with your VW parts department. If there have been changes in the sprocket P/N since the engine was first released, that indicates that VW experienced some type of "issue" with the sprocket that triggered an engineering change (re-design). No engine manufacturer re-designs an engine part unless there is a very good reason to do so.

Finally, I'd contact as many independent VW repair shops as possible (especially performance shops, or specialty engine rebuilders) to get their input on problems they may have seen with these sprockets. If at all possible, get one or more samples of the worn sprockets for metallurgical analysis. Once you have a "worn" sprocket, I'd compare the P/N on the worn sprocket with the latest VW P/N on a brand new one. If the P/Ns are different, purchase a brand new sprocket from VW and get Rockwell hardness measurements and alloy composition data (spectrographic analysis) on both sprockets. If the "new" sprocket has a significantly different hardness than the worn one, or is now made from a different material, you have yet another "smoking gun" showing that VW recognized that they had a design or fabrication defect in the sprockets, and changed the design (material, heat-treating specification, etc) to correct the problem.

One other observation: Why did Byers Imports replace both Vario-Cam adjusters, both Vario-Cam solenoids, and one camshaft? These are VERY expensive components (total over $1,940) that should (again) have lasted the entire life of the engine. There is no mention whatsoever on the service ticket of ANY problem or defects relating to ANY of these parts, and they constitute 77% of the total parts cost! The worn sprocket is described, but it's cost was only $30. Why were the "defects" in the camshaft, Vario-Cam adjusters, or solenoids not noted?

Since the only real problem was that the worn timing chain sprocket allowed the timing chain to "slip 3 teeth", replacement of the worn sprocket, timing chain, and timing chain tensioner and guide components should have been all that was required. The total parts cost to replace these components would have been only a few hundred dollars. Did you receive any explanation from Byers as to why all the other extremely expensive components required replacement? Did you retain all the "bad" parts that were changed out? Do Byers Imports mechanics receive a percentage of the cost of the parts they use when servicing a vehicle? This is a VERY suspect billing. This is a separate issue from the defective sprocket, but one that I think is just as significant.

Good luck pursuing this with VW and Byers. I find it shocking that, given the photo you have of the grossly worn sprocket, no one at VW acknowledges a material problem with the sprocket. I can only assume that the your sprocket photo was never shown to anyone in VW's Engineering or Failure Analysis groups; any Design Engineer, Failure Analysis Engineer or Metallurgist would instantly recognize and acknowledge a material defect (alloy or hardness) in the sprocket.

Best regards,

Irvine, CA