Understanding 964 Head Gasket Modification

[Porshaah]

Hi All

I'm not sure I understand why and what is involved in modifying pre-1990 3.6L heads/cylinders to accept a gasket. Can the existing gasketless cylinders have a groove machined into the top surface to accept a gasket or does one have to source post-1990 cylinders which have this gasket groove from the factory? If so, why? What other modifications are done that a machine shop can't perform? For the heads, I believe the machining process called "flycut" involves increasing the head surface area at the cylinder interface. If this is true, I'm not sure why this is necessary as the surfaces of the gasketless head/cylinder mate to one another already?

All responses appreciated
Bill

[Henry Schmidt]

The issue with the early head to cylinder seal is one of functional design.
The early head were designed with a flaw that causes the head to bend making cylinder sealing impossible.
The update is designed to support the head around the head studs so that the head can not bend when heated.
Machining a groove in the cylinder will help but ultimately does not address the problem.
When you flycut the head and install new cylinders the updated cylinders provide a stable platform for the head to rest on supporting the entire head and preventing distortion/ bending.

[Porshaah]

Henry thank you for your response

If I understand you correctly there is more to the cylinder surface than just machining a groove in the pre 1990 gasketless cylinders to accept a gasket in order for them to match a new post 1990 cylinder from the factory. Is it, that there's a step up or increasing slope to the outer edge so that more contact is established at the outer edge or in other words "designed to support the head around the head studs"?

TIA
Bill

[Henry Schmidt]

Here are the two different cylinders.
As you can see the late cylinder add material around the stud hole to reinforce the head.

[Porshaah]

Thanks Henry

Pictures tell a thousand words unfortunately I'm still a bit confused. The top photo of the early gasketless cylinder shows a step down at the outer edge. I'm assuming the corresponding profile on the head being the same but opposite (i.e mirror image) and slightly less (i.e. I'm guessing a few thousands of an inch) on the outer step down, so that pressure is placed on the inner raised edge of the cylinder - correct?
On the bottom photo of the later cylinder.
"As you can see the late cylinder add material around the stud hole to reinforce the head." It's hard to see, but I'm assuming the small surface on the inside of the gasket groove is lower (i.e. I'm guessing a few thousands of an inch) than on the outside edge of the gasket groove. I'm assuming the corresponding head surface would be flat from the edge of combustion chamber out to the stud.

Your knowledge is greatly appreciated
Bill

[Porshaah]

Hi All

The photos in this thread: "http://forums.pelicanparts.com/911-engine-rebuilding-forum/515540-964-head-machining-chain-box-compensation.html" displays photos of an unmolested head. Can I assume the head is of the later version which requires a head gasket? If so, what does a gasketless head look like or a description of the head surface profile?

TIA
Bill

[175K911]

Steve Weiner has a process to machine the early 964 heads and barrels to accept the 993 sealing ring. He recently completed mine but haven't gotten them back yet to take pics. This mod allows me to reuse my current barrels which were in great condition, so I really didn't want to have to purchase a set of later used barrels.

[Porshaah]

175K911

I didn't know Steve or anyone did this, good to know. Right now I'm just trying to understand the 2 versions so that when the time comes I'll know what my options are.

Bill