Deck Height and Head Chamfer - Chamfer Done
 

I just got back my heads and p&c from the machine shop. The P&C were reconditioned and the heads rebuilt. The heads were fly-cut as were the tops of the cylinders as the mating surface was rough.

My rebuild thread is here -> http://forums.pelicanparts.com/911-engine-rebuilding-forum/298777-my-3-0-3-2ss-rebuild.html

In measuring my deck height it looks like I need about 1.25mm of base gaskets - and perhaps more. The pistons are max moritz 98mm, the cylinder as max moritz as well - the case is a 3.0SC as are the heads.

I'm starting to wonder if the heads should have been chamfered to accept the larger diameter pistons. If not, does 1.25 cylinder base gaskets sound reasonable???

When my engine was built to 2.7 from 2.4 the machine shop chamfered the heads to work with the bigger cylinders. It sounds like you need to do the same. I'd call a couple of shops and ask them. If you don't know who to call I'd call Ted Robinson and German Precision.

-Andy

The heads should always be chamfered when increasing the bore to eliminate any potential "hot spots" on these conversions. Any machine shop or engine builder worth his salt will tell you the same.... :)

I am very surprised the company that sold you the pistons/cylinders or the machine shop didn't advise you of this.

Ralph

Yes, I was shocked at the machine shop who performed the work as the owner is highly regarded and very well known to this forum.

I have found a local machinist to complete the work for me. It means a delay and extra expense as the heads might have to be dismantled to fit on the machine.

I'm not surprised.
I'd never heard of it before I read about your rebuild Ralph. I think the places selling the parts generally assume that the engine builder knows what they are doing - which in my case is was not always true. ;-)
-Chris

Somehow, I don't quite believe this statement Chris, maybe a few years ago when you were just getting started but I think you have enough engines under your belt now (that are still running) and satisfied friends/clients to make you one of the guys "worth his salt".:)

Ralph

When I started there was Bruce Anderson's book and the factory manual. I know more now but I made a lot of mistakes to get to this point. Like I tell people, "It's not so much experience as it is accumulated remorse." :D
-Chris

Amen

Chamfer Done
 

Well I got my heads back from the 2nd machine shop. They performed a chamfer on the heads.

The chamfer seems to have helped however I'm still unsure about the deck height.

To begin with, the case was spigot machined 6 thou, the heads were flycut and the tops of the max moritz cylinders were cut as well. I'm thinking that's about .75mm in total. Add on the standard .25mm base gasket and I would think a 1mm gasket size is required.

I measured the deck height using the crush solder and it looks like 1mm is required. It looks like its possible to take a little more chamfer and I might get down to .75mm gaskets.

I'm at a crossroads here and I can't proceed until I get this sorted out. Any help would be GREATLY appreciated.

My understanding of the squish is that it should be the tightest right at the cylinder wall. This forces the mixture to the center of the chamber. If you don't need more chamfer to achieve this than the 1mm gasket should be fine. It sounds like you should have a 1mm gasket from the machining anyway. If you need gaskets, EBS sells the 1mm ones. FYI, I used K&W copper coat on my base gaskets per recommendations on this forum.

I've done a lot more testing and research.

The right side case was machined at the spigots, the left was not.

It looks like I can run 1mm on the right and 75mm on the left, this leaves me with a 1.1mm deck height which I gather is good.

I didn't know ebs had the 1mm gaskets, I was going to use 2 50mm on the right side and a .5 + .25 on the left.

I don't imagine its worth getting the 1mm over the 2 .5 at this stage as it would mean another 1 week delay and i'm already forced to use 2 gaskets on the left.

Thanks for the input!

-Rob

This was an interesting discussion on the need to chamfer the heads when increasing the cylinder bore size.

What about going the other way?

Specifically....Can 2.7 heads be used with 2.2 or 2.4 cylinders and pistons. Are there specific mods required to allow a slightly larger cylinder head (90mm) to be used with the smaller cylinders (84mm)??

There was an earlier thread that talked about heads that were chamfered for 100mm cylinders but going onto 95 or 98mm pistons. One option was to weld the heads but many people don't like that approach - i.e. welding the heads.

H'mm - I have had stock 964 heads machined to accept 102mm cylinders, but there was no chamfer done. Should I be concerned?

If the head is flat and the piston is curved at the outer diameter then yes. The concern is if the mixture can get trapped and not pushed out to the center. If the edge of the piston is flat where the head is flat, it's not as much of a concern.