If they've been overtightened, I understand that they can warp quite easily.  I remember when I did mine I tightened them to the recommended torque values and it didn't seem very tight, so it's quite tempting to 'nip' them up abit more, and this probably happens quite often, leading to warpage and leakage. HTH.

I've never seen a warped set, I've seen them out of shape due to overtightening but they straighten out once loosened. If you're tempted to go down the ally veccy b cam cover route, DON'T! The oil filler is in the wrong place and if your breathers get clogged up again it'll blow the front crank seal, which is an ar$e of a job.

Lets clarify the wording... Warped means it's shape has altered and will not return to it's original shape. When the bolts are over tightened it forces the top of the cam cover down spreading the sealing edges outwards slightly. This creates a weakness in the seal and any excess pressure in the breather system will cause premature failure at that point. Once the bolts are removed the cam covers return to their original shape. Please bear in mind though that we are talking less that a couple of mm outward spread but it is enough to cause a problem.

IMHO what's really happening here is that the o-rings and main seal harden and degrade with age and thermal cycling, thereby losing compliance/flexibility.
Pattern parts are almost certainly made of cheaper, less durable rubber compounds, so their failure is accelerated relative to genuine parts.

The plastic covers will have a much greater coefficient of thermal expansion than the alloy parts to which they are bolted; so every thermal cycle the engine goes through will require tiny flexure of the rubber seals in order to accommodate the relative movement of the parts they are attempting to form a seal between. 

At a certain point of degradation, the rubber compound of the o-rings and main seal no longer have enough springiness to meet this requirement, and they start to leak (especially if breather blockage is creating positive pressure inside the cover).

Here's by tuppence worth based on an experience I had last year.

DLK did my breathers and cam cover gaskets in January. Soon after I had a leak. nsr outside edge, running down onto the exhaust manifold.

After a few cycles of cleaning the oil away/it reappearing I contacted Daz and he sent me another cover to try.

Before I fitted it I compared the outer lip of each against a straight edge. My old one, under no tension of course, had several large areas where it lifted from the straight edge. The replacement, whilst not a perfect flat edge, was nowhere near as bad. Also, in plan view, the outer edge of the old one had a distinct wave to it.

I fitted the replacement and have had no leaks since, so either I am a better mechanic than one of the forums finest or the original cover was u/s, which may explain the copious amounts of bathroom sealant some muppet had tried to seal it with previously.

Sadly the evidence went to the tip soon after.

Steve

Elite Pete wrote on 24. Jan 2011 at 17:09:

And they are still not leaking 

Right, ive got mine off and given them a clean up, noticed that the sides wernt the straightest but i think they are ok, what do you think? (this is the waviest of the 4 sides!)