Running in cams

[Evodelta]

Hello Guy,

As per title; Running in cams, is it necessary? If so, why? and what is the best way?

I have seen instructions for using special lube, some say engine oil, some say 'Keep at a high tickover (above 3000Rpm) for 20 minutes, I cannot think why this is necessary.
Any info gratefully received!

Thanks,
Martin.

[Guy Croft]

This definitely is necessary, the contact stress is much higher at lower rpm on most cams, that's why they need to be bedded-in that higher speed, 10min is the bare minumum. Cam lube also essential, esp with cast iron cams.

GC

[Evodelta]

Ok, so it is emphatically a 'Must Do'.

But I don't understand why, lets just say you didn't do this, what would happen?
This isn't a leading question (as in: I've just done it wrong, etc) But I am genuinely interested into why this process must be followed, does something happen to the metal in the cams during this initial period or is it to relieve any possible failure of the cambelt due to seizure? Or something else?

Martin.

[Guy Croft]

When you examine the stress generated by a polynomial cam (which most are), in other words the rate of acceleration of lift vs valve train mass the contact stress over the general surface contact area betwen cam flank and follower can be excessive and lead to microscopic galling/pairing on both parts and extremely quick destruction of one or the other.
The rubbing action at lower stress (higher cam speed) allows more progressive bedding-in (bit like lapping). After they are bedded in, they are pretty well good for all time.

GC

[Uros Piperski]

What sort of cam lube is used and how is it used? Is it an oil, grease or something third?

[Evodelta]

Ok I get it now - thanks.

[Guy Croft]

Uros, hi

I use Kent Cams 'cam lube'. In its appearance it's like a very sticky quite thick oil. Contains a high concentration of zinc dithiophosphate - and anti-oxidant and powerful friction reducing chemical that forms microscopic layers on the surfaces.
I know some people just use Graphogen (a graphite paste) and some use Molybdenum grease. The cam lube should be applied right before cranking/start up and it's poured liberally over the cam/followers.
I ceased to use graphite at all - as a engine build lubricant when I discovered that aluminium - in the presence of graphite - has an affinity for cast iron. I was investigating wear mechanisms at Napier Turbochargers. I never had a problem with it on the early engines I did it's quite true but now I no longer use it, except on exhaust threads where seizure might be a problem.

I do use moly grease mixed with oil as a priming lubricant. You need to be careful with any grease like that because if it dries out it can block oilways.

GC

[Uros Piperski]

Thanks for the thorough answer, Guy!

I do use moly grease mixed with oil as a priming lubricant. You need to be careful with any grease like that because if it dries out it can block oilways.

Do you use this just as a cam lube? How quick does it get washed off by oil? When do you change the oil after that? And how much oil and grease is in the mixture?
Thanks!

[Guy Croft]

Uros, hi

no more than a tiny bit on the tip of finger lightly smeared on the brgs and thrust washers, cam journal housings, bucket housings - before oiling too. It won't dry out if it's oil wetted at build and it helps to keep the oil in place if the engine is left unstarted for months. Using it to lube the bucket housings without oil (they can be oiled after head build) stops the buckets falling out all the time..

I use it on oil pump gears too, to stop the priming oil draining away.

GC