I agree fully with Terry's post above! Based on what I read in the above posts, I can see that there are many guesses and assumptions around how the lobes that do fail. Yes, inadequate heat-treating is the root cause, but not in the way that most, or all of you are thinking! I should have provided more details, but quite frankly, I thought that it would only be boring, plus I didn't feel like getting into the lecture mode!
So, let me explain by giving you the steps in the manufacturing process foe the camshaft, and dwell on the steps that are the root-cause of this defect:
1. The camshaft starts out as a piece of forged steel.
2. The forging is machined to shape, but some materials are left on the cam lobe surfaces for final machining (rough and finish grind).
3. The camshaft undergoes,what we old-timers calls "flame-hardening", which is a surface-hardening
process, and NOT a through hardening as many of you are thinking! The process is applied to just the cam lobes
. Many decades ago, a flame would have been played over the surfaces, and then quenched with water. Nowadays, induction coils are used to provide the heating. Induction heating occurs only on the surface layer, just like the flame, and the inner material will be heated via thermal conduction, and so TIME
is an important factor as to how deep the temperature gradient extends
. Keep this last part in mind! Water quenching, in today's process, is done with sudden spray of water onto the heated surfaces. Hardening of the steel occurs ONLY where the steel had reached, or exceed certain temperature and then quenched, otherwise the material will stay "soft". What I am trying to say is that, after the process, there will be a hardened shell (case) of some small thickness over the "softer" steel forging. The thickness of this hardened shell will depend on how long the heating had been applied to the surface of the steel.
Keep this in mind as well!
4. Rough and finish grind to bring the dimensions of the cam lobes to specifications. Certain amount of material is ground away in this process, and this is where one can have issues IF
the hardening process had been inadequate to provide sufficient thickness to the hard shell! If the shell is too thin, the grinding processes would grind away all of the hard surface material, and leaving the softer inner material exposed to wear! The defect in NOT visually detectable.
5. The lobe surfaces are polished to provide smooth surfaces.
So, with this defect, you will find taht the cam lobes will wear very quickly right away, as reported. It's my opinion that if you had checked the condition of your cam lobes at the 12k miles servicing, when you do the valve clearance check, you are probably quite safe. Just my guess!