Worn 2.1 cam - what are the options?

[itchyfeet]

https://club8090.co.uk/forum/viewtopic.p ... 0#p8158748" onclick="window.open(this.href);return false;


all crank gears same size but old ones may be worn smaller

[tencentlife]

If you regrind a cam then you will change the profile and you'll struggle to get it properly hardened again I think, tbh the forging they are made from was just good enough for 100k miles

No, none of those things are true. Here's why I won't use a regrind. When it's reground the lift profile is the same, profile meaning the lobe height relative to the base circle radius at any given degree position is reproduced (within machining tolerances). If you measure and plot the action of the reground cam it will produce the exact same valve lift curve. The problem is a simple geometric one: to reproduce the lift profile, the reground lobe's cross-sectional shape is not a simple shrinking of the original, with all dimensions reduced proportionally. The entire lobe is reground, resulting in a smaller base circle, but the same peak height relative to that base radius, and a shorter circumferential lifter travel distance in which to produce the same lift curve. So the primary geometric problem is that the attack angle of the lifter face on the lobe flanks is steepened. The main concern then is that lobe/lifter interface stress is increased during the transition from base circle to initial lift phase on a hydraulic grind, or from the take-up ramp to initial lift phase on a normal grind. The same goes for the other side of the lobe when it has to slow the valve down when it comes to a close, where the stress can be just as high. Initial lift event and last closing phase is already the most stressful part of the cycle and on a reground lobe it is made even worse. Sideforce on the lifter is also increased during those phases, for the same reason, the steeper attack angle.

So, even ground and finished competently, the reground cam will cause a higher wear rate at the lobe/lifter interface and in the lifter guide bores. That's why, to me, a reground cam is false economy. If it were an overhead cam engine where the cams are easily changed, maybe one could argue for the economy of a regrind vs. new, but in a VW boxer or other pushrod type engines where the cam is buried deep in the block and a complete overhaul is required to repair it, I think it's unwise.

Cam's are made of chilled cast iron.

There is only one crank gear size.

Lifter faces can be reground very easily, to a domed profile. Here a machinist will charge $3 apiece.

[itchyfeet]

Interesting, thanks, I wondered how on earth you reground a cam to make a different profile when there was no extra metal added.

[bustrucket]

Thanks itchy, your link is an inspiration!

Since NOS cams with +3 gears don't seem available, I think I'll go for one of the cheaper sizes and swap the gears. By which I mean I'll ask a machine shop to do it, not sure I'm confident enough to try it myself...

I've got a few scrap heads here, I think I'll try your method of swapping guides on them and with a bit of luck get a pair of useable heads out of it at the end.

Cheers

David

[Mr Bean]

I don't see the linear scuffing which often results from poor lubrication so:
I am assuming that the face imaged is the lifting face. If so then then is the offset wear pattern commensurate with the design objective of rotating the cam followers as I believe is the case in some engines? Out of interest I would like to have seen the other side of the lobes.
CS