I'm rebuilding a 1999 3.4 996, she popped a core plug and had intermix.
I'm rebuilding the bottom end with all new bearings and cylinder liners.
My cylinder heads are ok once the core plug is replaced but there are some signs of wear, the exhaust cam is scored on the end journal near the scavenge pump .
The shiny coating on the cam bearing has gone and there is some slight marking on this one cam bearing , the others seem ok.
Anyone had this or know if it can be re-used as it is not replaceable , it's part of the head, thanks Porsche !!
Thanks Lee

 

Are you sure the intermix wasn't due to the heads? I would expect you to have the heads removed and magnafluxed to check for cracks.

 

I do not have pics of the 996 cams. I have pics of the cams from the passenger bank of my 02 Boxster. From the pics I do *not* see any signs of a coating. That is the shiny coating you speak of is just the fine polished finish the cast iron cam bearing journal has is what it looks like to me.

If this is gone this would suggest this item is not reusable as the wear has worn the diameter and destroyed the proper finish.

What is the dimension topology of the bearing? How much taper? What is its size compared to other journals which should (I assume) have the same diameter but not the same amount of wear.

And the same dimension info from the aluminum bearing in which the camshaft spins is needed.

You might be looking at replacing the cam if not the head to ensure the bearing diameters are in good shape and do not have too much clearance.

Now it might be possible to resurrect the cam bearing. One might be able to build this surface up with say a hard chrome plating and grind this down to the proper diameter. But the bearing diameter in the head is worn this presents a tougher problem.

(I note that the subject of hard chroming the crankshaft bearings came up to bring the crank journals up to their factory size and Jake Raby said his experience is hard chroming doesn't work well. Whether this applies to the camshaft bearings I can't say.)

Anyhow, the aluminum bearing diameter can't be plated.

One possible fix is to bolt the camshaft cover down properly and bore (or possibly hone) this diameter larger just large enough to clean it up and create the proper surface finish and then plate and grind the bearing diameter on the cam to have the proper clearance.

A ballpark dimension, one good enough (for an experienced engine rebuilder) can possibly be obtained from measuring some other cam shaft bearing and using the clearance there as what the clearance should be at the repaired bearing. Of course the camshaft bearing diameter will be slightly bigger while the aluminum bearing diameter will be bigger too from the honing. But not by much. Maybe just a few thousandths of an inch if done right. It all depends upon how worn the aluminum bearing is.

Added: *not*; and this... I looked at the front bearing journal on my Boxster cam in the pictures and that journal doesn't look to have quite the polished finish the other journals have. It could be this journal for whatever reason is just a duller finish and if so maybe not as worn as I at first feared. A precision micrometer and bore gage are your friends in determining the extent of the wear. And very close inspection to determine if you can if the finish is from wear or the way the cam is supposed to look.

 

Ok looks like I might get the wear checked at my local machine shop, I will definitely be replacing the cam with a good used one to keep cost down, maybe that pert ocular journal is subject to some wear normally due to loads and stresses from the cam chain/scavenge pump, after all it is the point where the cam assembly picks up drive from the crank via the chain which is under quite a bit of tension , then on top of this , it also drives the scavenge pump??
Thanks for your detailed reply !

 

I'm pretty sure it was the core plug as it was loose in the cam housing when I removed the cam cover!

 

While that bearing journal is subject to the possibility of wear (they all are) it can't experience any in the normal course of events, or the bearing clearance would become very large in no time and oil pressure (at least that bank's valve hardware) would suffer as this bearing's increased clearance due to wear would represent a large internal oil leak.

When my 02 Boxster had a VarioCam solenoid/actuator repair a while back -- which gave me an opportunity to photograph the cams among other things -- the engine had over 220K miles on it.

Yet none of the hardware other than the bad solenoid and the bad actuator were replaced. The tech didn't even feel the wear on the actuator cam chain rails warranted taking the other bank's camshaft cover off to replace the actuator as a preventative measure.