there are a few reasons why the cylinder to head surface needs sealing help. look at the history of the engine's development. to use an extreme example, compare the surface area of a 40HP cylinder top surface to that of a 2.0l type 4 - i'll bet the ratio of sealing surface to bore size is something close to 2x on the 40HP cylinder over the 2.0 t4.
Air pressure uses hydraulic principles. Sealing surface to bore size is irrelevant. Sealing surface to psi is relevant.
as the bore of the engine got larger and larger, it became more and more difficult to seal the cylinders. the 1.7 seals fine, but my guess is that is about the limit. the 1.8 has very thick cylinders, one reason why it is a good engine combo. it seals well also. then the bore grew a lot more with the 2.0 and VW introduced the head sealing rings.
VW introduced the sealing rings with the 1.7 in the Porsche and 411's on up through the 2.0's.
the sealing rings were there for a reason, the bore was getting too large to be able to seal well without other intervention, but the seal rings didn't work as intended, they introduced other issues. VW knew back then that the 94mm bore was not sealing properly and tried to fix the issue with a steel ring between the head and cylinder.
The issue never was about sealing. The issue was about changes in engine architecture after serious heat events. Those sealing rings work flawlessly until the head *loses* clamping force due to extreme expansion. In the old magnesium engines, the head studs simply tore out. In the later engines, case spigots would cave or the heads would warp. The sealing rings, being the softest link, would blow only as evidence of loose heads.
this is the other issue particular to the type 4 engine. the cylinder head studs are not symmetrical about the centre of the bore. once the bore gets as big as 94mm, it is very difficult to get the heads to seal due to this fact. again it is geometry. as the bore increased, the amount of off-centre loading due to the uneven head stud placement also increased. you see the results of this all the time when you tear down type 4 engines, especially 2 litre engines. massive head leaks, and many of these engine were assembled at the factory.
If you look at your average 2.0 with 7.3:1 compression, and compare it to any of the performance engines that have been built using those head stud placements, I guarantee you that the stock engines were not flushing out that particular issue.
the fact is that as the bore gets to 94mm, the cylinder head to cylinder surface needs all the help it can get to seal off properly. prep work on the machined surfaces is the key, yes. and anything else you can do to give the joint a chance to seal, do it.
The technical service bulletin that recommended deleting the sealing ring did not have to do with intrinsic weakness in the sealing ring. . .
(the sealing ring is but a canary in the coal mine. Its failure says, "low head torque." Why? crap in the threads during assembly? overheat that pounded in the aluminum spigots or distorted the heads? chronic detonation? lousy state of tune? leaves in the head fins -Porsche 914 owners take note-?)
. . . it had to do with instantaneous heat transfer to the cylinder walls. The 94mm pistons had a rash of seizures due to their greater expansion because of their greater mass. This would occur during transient overheats. The factory bulletin addressed this issue with a three-pronged attack.
1.) demand greater piston-to-cylinder clearance.
2.) cut notches in the rods to cool the pistons more readily
3.) delete the sealing ring to get the heavy load heat in the heads conducting down the cylinder barrels as quickly as possible to keep the barrels ahead of the piston expansion curve.
the reason the coppercoat works is because it is a fine copper paste leaving a fine copper powder on the sealing surface. this acts like a very thin copper sealing ring. it works. it isn't snake oil. just because it isn't in the vw repair manual doesn't mean it doesn't work.
Scott. I did not spout off with anything like "it is not in the manual" nor did I say "because it is used on a waterpumper, it doesn't belong on an aircooled engine". I gave my reasons. I said there is damn close to nothing under the sun that is going to survive an air-cooled engine's combustion temperatures and pressures if that air-cooled engine has any sort of structural issues between the barrels and the head. And that's the truth. If you want to copper spray adhere your sealing rings to the heads to make them stay in place during assembly, knock yourself out. I am not saying the stuff will get in the way, I am saying that it has nothing to do with ensuring
a seal, nothing. Nothing can stop hot combustion gases and pressures that have found a defect in the sealing surface or low clamping force.