So I had a long phone conversation with Jim Rowe at Metric Mechanic yesterday. We ended up on the subject of M42 cam gears for a little while, and I explained this situation. His feedback was interesting. I am sure that this will cause some controversy, but based on my experience as a customer of MM, I trust that they know their stuff. Zero of us here have built thousands of engines over 35+ years. Here is a general summary of his thoughts on M42 cam gear wear.
1) There are a few versions of the M42 cam gears out there. It sounds like early ones (or ones from a specific OE supplier) came from the factory with "pointy" teeth, so those are by-design in many cases, not due to excessive wear. Other versions had square teeth. As far as materials, there are two: one seems to be cast iron and you can see some of the rough sand casting texture on the back surface where it was not machined, and the other is some sort of powdered metal creation which is found in later parts and is a LOT harder/more durable.
(thinking about it, the pointy tooth theory we all know of makes no sense...why would the non-contact side of a square tooth tip be worn down at all?)
2) He has almost never seen these cam gears wear out in the way that we commonly seem to think. There are lots of stories in here about the timing chain skipping a tooth, and then looking inside to find pointy teeth, and assuming that the pointy teeth are the reason. He was fairly sure that the pointy teeth were unrelated, and that issues with the tensioner rails/piston/deflector sprocket were more likely to be the real culprit, or even just loosening of the sprocket bolts which allow it to get a few degrees out of alignment.
3) If one is concerned about wear, then pointy teeth are not what to look for as they are not what will result from the chain rollers meshing with the teeth. You should be looking for checking at the edges of the teeth, asymmetrical teeth (the contact side having a different curvature than the non-contact side) and galling marks.
4) MM has worked with mostly 6 cylinder cars, many with 24V engines. Those camshafts put >50% more torque on the sprockets, with single and double row chains, and he has never seen the cam sprockets wear out there either. The sprocket materials are largely the same as the M42's.
5) Extreme neglect of the lubrication system (shitty oil, not keeping it at the right level, not changing it) and extreme neglect of an original timing system are about the only ways that you are going to start eating into the sprockets at all, and even then the engine is going to fail due to something else before that.
Overall, his opinion is that the cam sprockets should be good for upwards of a million miles if oil changes are done at a reasonable interval with decent oil. The engine will crap out well before that for other reasons. So much of the worry and concern over the sprockets that many of us here experience is completely unwarranted, and largely based off of internet anecdotes. Jim kindly called me out on believing a lot of the "conventional wisdom" expressed in forums about the sprockets, and he had a point: I cannot actually think of a realistic mechanism by which the pointy sprockets that we see photos of would ever happen (if one started with square teeth).
Speaking of oil, MM has done a ton of lubrication tests and basically the only oils worth a damn are Castrol 10W-60 TWS (best) and Mobil1 15W-50 (second best). All of the other stuff they tested (Rotal Purple, Amsoil, non-botique stuff) all came in waaaaay below these two. I used to run the Castrol stuff, but it is stupid expensive, requires more frequent changes and leaks through paper gaskets like a motherfucker. So, I am sticking with M1 15W-50 which has always been my go-to for the M42. M1 publishes basic data on this oil, and it has some of the highest ZDDP levels you can find in commercially available oil. This is essential to our flat tappet lifter system.
Topic: Un-Conventional Wisdom on M42 Cam Sprockets (Read 5055 times)