Cam cover cut-aways

[Hugo]

I bought myself pair of early cam covers to replace my ribbed ones. Polished them up nicely, but on fitting them I discovered there is a discrepancy in the size of the cut-aways between the cam covers and the head. Not much, maybe a millimetre or two, but enough to stop it sealing I'm sure. With the cam cover on the head, you have a 'hole' at the back shaped like a pair of capital 'D's back to back, but of slightly different sizes. A bit of work with some chemical metal and a file & I think I've got them right. A bit tricky since rubber doesn't compress, so if those plugs in the back are too tight, the gasket won't seat.
I sawed the back extension off the right bearing cap so the R.H. plug would fit straight in the hole that was left.
I think I've sorted it now, but I'm curious as to how such a weird problem should arise in the first place?

[PeterCrespin]

It's not a problem, it's a change of design following a change of equipment (i.e. a change from AC tach, with consequent deletion of the bolted-on generator.

Jaguar's initial fix was two neat alloy plugs/blanking plates with an o-ring that sealed the back of the head at the back of each cam cover. The exhaust side is easy as you just fit the plug with two screws. The right side would have been the same with a plug held by the same three fasteners that previously secured the tach generator.

Now that you've hacked the extended bearing cap off, you presumably have an asymmetrical hole that is larger above than below, unless your substitute covers were earlier or later ones from models with no tach generator.

If you don't have the blanking plates you will need to get creative. Which on the left side means two half-moon rubbers cemented back to back to fill that hole. I know you keep citing the incompressibility of rubber but in gasket or sealing contexts no true compression (i.e. reduction in volume), but merely a flexibility and distortion to seal adjoining faces whilst bulging slightly elsewhere.

On the inlet side you could do the same if the cover cutaway is the same above and below the gasket faces. If the upper cutaway is larger than you shouldn't have cut the rear cam cap. Instead you will now have to. turn up and slice an alloy semicircle to fill the large upper cut-out and give you an even and continuous gasket face like S3 XJ covers.

[Hugo]

I wish it were that simple. The good news is that I have the plugs that go in the back. But, taking the exhaust side, as that is simplest, the recess in my new/old polished cam covers is very slightly larger than the recess in the head. Only about a tenth or a sixteenth of an inch, but enough to stop it sealing.
The recess on my inlet-side cam cover is exactly the same. The arrangement with the ribbed cam covers was that the inlet side had a much bigger recess which fitted over the rear bearing cap extension, and the plug then fitted inside this bearing extension. The plug OUGHT to fit in the gap I now have in the cam cover, after chopping off the bearing cap extension. Indeed it does fit, but it will never seal, as the recess in the cam cover, just like the other one, is fractionally too big, or at any rate fractionally too wide.
What I don't understand is how they can be so close but not quite right. Not that it matters now, as I put a thin layer of chemical metal on both cam cover recesses and carefully shaped them to something resembling a semi-circle.
The tricky bit was making the apertures tight enough to seal on the 'O' rings, without being so tight that they would prevent the cam cover from seating on its gasket.
The lack of compressibility of rubber is crucial with 'O' rings, since the rubber has nowhere to go. Once it is fully squished into the groove, that's yer lot. If I was using half-moon seals they would, as you say, just find somewhere else to bulge out.

[PeterCrespin]

The lack of compressibility of a groove-housed o-ring is not a problem at all....unless mis-matched parts are used and there is too large a ring for the groove. Possibly the two most critical joints in the engine are groove housed o-rings: (oil pump connections).

Because they aren’t dowel located like, say, the rope seal holder is, the cam cover back seal is a different scale of precision, and the two rear blanking plugs use quite chunky o-rings in an external groove to seal adequately the split bore of the head and cam cover.

Either way, very many smooth cover retro-fitted engines are bone-dry at the back of their cam covers, so there’s nothing fundamentally wrong and you should win. Your issue appears due to mix & match parts and/or indifferent assembly. Your hard filler may well seal this low pressure area but may be a nuisance if it needs removal?

[Hugo]

The other side of the coin from having too large an 'O' ring, is having too small a hole, and that is what I'm up against. The hole started off being too big (for the plug to seal), and I filled the cam covers a bit to get it correct. Getting it correct was the tricky part - too big a hole & it will leak, too small and it will stop the gasket seating.
I think I've got it right - I'll know soon enough when I start the engine - but I'm intrigued as to how such an anomaly can have arisen. How can the dimensions be so close and yet be wrong?

[rossbraithwaite]

Hi Hugo, if you are worried about the o-ring material 'compressing' perhaps you could widen the groove in which it sits? Ensure that the o-ring is stretched onto the plug (aim for 6% stretch on the ID) and remember that the resultant reduction is x-section is about 50% of the stretch. So you could possibly go up a size in x-section, down a size in ID and compensate by widening the seal groove? Ross

[Hugo]

My main concern is getting the apertures the right shape and size - too tight & they will stop the gasket from sealing; too loose & they will leak. I can see that the apertures in the cam covers are too wide, and I can see the mark left by the 'O' ring when I press the plug into the cam cover - what is harder to judge is whether the depth is correct.
With the application of some chemical metal and some judicious filing, I think I have got it about right. The engine hasn't been run yet, but we'll find out soon enough.
The puzzle is how they come to be like this in the first place! The width of the cut-out is noticeably wider - though not by much - than the cut-away in the head.

[PeterCrespin]

The puzzle is how they come to be like this in the first place! The width of the cut-out is noticeably wider - though not by much - than the cut-away in the head.

Why is it a puzzle when your first sentence in this thread states the used cam covers you bought were never made for your engine?

[Hugo]

Are you telling me that they changed the diameter of the cut-away by a millimetre or so along the way somewhere? I doubt it. That's the size of the discrepancy we're talking about here - just enough that the resulting aperture, instead of being a beautiful circle, has an ugly step in it where the cam cover joins the head.