Can anybody explain why early 3.8 engines had a chamfer on the combustion chambers and later ones didn’t. The gaskets appeared to be the same, but I believe later models had a chamfer on the pistons instead.
Early 3.8’s had more hand fettling (“blue printing”) to achieve better performance for the “Road Test” Magazines. That’s why the early E’s did 150mph., which the later production cars never seemed able to achieve.
The chamfer on the head was to reduce the chance of preignition when operating at full load as when doing the high speed runs.
Never seen any OE pistons with a chamfer, but have seen it on some after market ones.
Here are a couple of piston photographs. The second shows a MK9, (C14806) in the block. You will notice it has a very small chamfer .
The first picture shows a 1966 S Type piston,(C23509/1). The top edge has, not so much of a chamfer but an undercut which I have highlighted in green.
All these chamfers seem to be similar to all other pistons I’ve seen.
The cylinder head itself should have it’s sharp edge radiused a bit after being resurfaced.
The only reason I can see for the undercut piston is if the block has been decked and if the OE steel shim gasket was used, it would stop the piston hitting the head.
Does the dia. of the undercut match the combustion chamber dia. and is the depth 1mm or more.
Wonder if it has something to do with promoting combustion gases getting behind the top ring to increase ring pressure on the walls?
Hello Terry,
definite not. These camfers have influence on emissions and knocking.
Camfers to increase gas pressure have to be on the top side of the first ring groove plus an adapted clearance of the top land.
Thomas (working for Mahle)
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I’d better answer my own question as it was me who
posted it originally back in April!
When the 3.4 engine came out it had a bore of 83mm and the cylinder head was machined so that the combustion chamber matched the block where the gasket interface is.
When the 3.8 engine was introduced it was bored out to 87mm. At this time a new cylinder head was made which had a chamfer at the bottom of the combustion chamber so that it matched up with the new bore size. It had a different part number to the old 3.4 one, and was painted metallic blue. The pistons at this point were flat apart from the dome. Towards the end of Mk9 production, Jaguar decided to make things simpler in order to do away with the cylinder head chamfer on the 3.8 head, and just fit the 3.4 head to all cars. Therefore they changed the pistons and introduced a 3mm X 1mm deep undercut on the outside diameter of the new piston.
This would ensure that the outside edge of the piston wouldn’t contact the head or be virtually in contact with it, especially if the block was subsequently decked.
So if anybody wants to fit, say a head from a 3.4 or a later 3.8 to an early MK9 or 3.8 XK150, they need to check the pistons. If the early pistons are fitted they can either fit the later ones, chamfer the head, or machine the pistons to suit.
There was apparently a service bulletin issued when the MK9 was about to go out of production, saying that replacement heads would only be of the later, non chamfered type.
I fitted a 3.4 head from a 1967 S type, to my early MK9 a couple of months ago and chose to machine the early pistons, on the basis that I own a lathe, but not a milling machine!
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Thanks Thomas I’m curious as to how a small chamfer on the piston would help detonation.
Sorry Wardell I’m completely confused. Are you saying that the top of the piston rose above the deck height so they had to chamfer/undercut it? It could only touch the head if it did that - no? It’s confusing because there is a head gasket that raises the head above the deck giving extra clearance.
PS Let me clarify this - the dome rises above the deck but fits into the combustion chamber in the head, but the squish area of the piston ordinarily does not.
No, the piston doesn’t rise above the top of the face of the block, (the domed part does, obviously). it is flush with it. But, if the block was decked more that the thickness of the head gasket it would.
As someone days from fitting the head to a 3.4 engine bored/linered to 3.8 and using 3.8 solid skirt AE pistons, this thread is very timely. I regard the bit of squish as a nice bonus and on such a long-stroke engine I will aim for circa 40 thou/1mm minimum on the newly-linered and lightly decked block. This is my first time using a steel gasket, on the basis that if its toot tight I can always switch to a thicker composite.
Nice to know we have a Mahle expert here