The SAAB V4 Web Pages - V4 Inlet Manifold Gaskets
V4 INLET MANIFOLD GASKETS
This is a potentially important matter, though it could be familiar territory to some readers.
The V4's cylinder heads are not 'handed'; consequently there's a superfluous waterway connection on each head that is blanked off internally by the inlet manifold gasket. If you examine a new manifold gasket, you'll see that, at one place on each side, the gasket features a pin-hole. These are the blanking points (the holes, I imagine, are there to facilitate bleeding and prevent air pockets from building up inside the heads).
Genuine Saab manifold gaskets were reinforced at these points by rectangles of thin copper sheet (each also with a small hole), However, proprietary gaskets, as contained in most currently available decoke sets, don't have this reinforcement and, eventually, the gasket can blow through. When this happens, the coolant takes a shortcut and the rear of the affected cylinder head will end up much hotter than intended.
I first came across this problem when I rebuilt an engine a couple of years ago. A notebook supplied with the car indicated that the previous owner had taken the heads off about 60,000 miles earlier. When I stripped the top end, it was immediately apparent that the (non-reinforced) manifold gasket had blown through on the left-hand bank - and was bulging ominously on the other side. The combustion chamber to the left-rear cylinder had clearly been running very hot indeed - it was covered in hard white deposits and its exhaust valve had started to burn. By contrast, the other three combustion chambers were normal coloured and their valves were fine.
So, the moral is: whenever you carry out a decoke, save those little copper plates for re-use. If they're absent, you'll have to make your own (thin brass shim stock should do the job if you can't get copper). They measure 18mm x 32mm and appear to be around 0.1mm thick (about 4-thou) - I'm guestimating the thickness as I haven't miked one. The hole, which is 3mm diameter, should be centred 9mm in from one corner. Using gasket cement, stick the plates on to the head side of the manifold gasket (holes aligned to holes) in such a way that they will just overlap the relevant openings in the heads. Alternatively, you could stick them straight on to the heads themselves. Either way, take great care not to dislodge the plates when fitting the gasket.
Incidentally, apropos gasket fitting - as with any wedge-shaped inlet manifold on a vee-engine, it's very important to seat the gasket correctly (which is the reason for the interlocking tab on the head/manifold gaskets. The engine mentioned above had felt 'flat' and down on power, despite decent compression readings and spot-on timing. Turned out that the manifold gasket had been masking the ports. The improvement, once properly reassembled, was quite remarkable.