Piston-Head clearance "issue" and questions
 

George @ Ollie's did the machine work on my heads, as well as inspecting my P's & C's. The P's and C's were good, but he did see a bit of taper to the cylinders, so he machined the step on the cylinders and the pocket on the heads as shown in the pics. He also provided a "shim" that is .020" thick.

Today, I was checking clearance using the solder method. I could only find .080" acid core solder in the 3 places I looked, but since 1 to 1.5mm seems to be acceptable clearance for "street" motors, I figured it would be fine. I put the shim between the head and the cylinder and ran thru TDC. The solder was .078" on one side, and .070" on the other, which I thought seemed high as this is 2mm, roughly.

I do have the DR cams DC19 grind, which is a little hotter than a 964 cam, so I probably need to be closer to the 1.25mm clearance. I'm not planning on running over 6800rpms, and I do have Aasco racing springs and Ti retainers, so hopefully float won't be an issue.

At any rate, I pulled the .020 shims out and rechecked, and I got measurements of .056" and .050" without it, so I guess that would be expected.

The issue? Has anyone seen or done this type of mod to the cylinders and heads? It looks like there is .002" of difference to the good in the pocket in the head and the cylinder ridge, so I guess I could run without the shim. Having said that, using a vernier caliper to measure these heights is not scientific at all. Perhaps I should just call George on Monday, and have him send me some .010"/.25mm shims and be done with it, and safe as far as a good seal between the head and the cylinder? This would give me about a 1.5mm clearance which would maybe be a little excessive, but it would be safe....

Thoughts or opinions?

http://forums.pelicanparts.com/uploa...1191699160.jpg

http://forums.pelicanparts.com/uploa...1191699180.jpg

http://forums.pelicanparts.com/uploa...1191699202.jpg

One last comment....George did suggest a 2 mm clearance when talking to him. I guess I have that, but at what performance cost?

Ok, a bit of an update. I re-measured with the shim/seal in place. This time, I put the solder as close to the edge of the piston as possible without it dragging on the cylinder. This made a difference. The first time, I'd say the solder was close to the edge, but not right on it. This time, I got .067" on the LH side, and .063" on the RH side. Just over 1.5mm.

So now, my original question still stands-do I go for a .010" thick sealing ring? That would bump my compression a bit, getting it down to about a 1.25mm clearance. Or, should I just leave it as is, and be happy (and safe) with what I have? I don't know what kind of power difference (if any) I'd feel with the higher compression?

I have never seen the "shim" though I am sure others have. Based on your numbers I would go without the shim and assemble one side of the motor and measure the valve clearance. If that looks good I would also consider CC'ing the assembly to determine the CR.

Cheers

Thanks Jeff. I guess I should call it what it is, a sealing ring. While it looks like the head and cylinder have .003 of positive sealing, I guess ol' George had a reason for providing the sealing rings.

How much power do I realistically leave on the table with the 1.65mm piston-head clearance? Is that a really big issue? I know that the consensus is Wayne's recommendation of 1.5mm is conservative, but by virtue of the fact that number is put forth, it has merit in some circles. Also, my notes show me that that is what George @ Ollie's recommended.

What did Porsche provide as stock, I wonder? Off to research...

I am not sure how much HP is on the table, I guess I could figure out the CR difference and guestimate from there. But, the higher deckheight is more prone to detonation. That is why I liked the measurement without the shim....

What is the shim or sealing ring made out of?

Cheers

I think I'd keep what you have with the shim. There are compression ratio to HP calculators on the internet. The one I used showed 4 HP difference for 9.3 vs 9.8 to 1 compression ratio. Unless you are racing I don't see it being worth the risk.

-Andy

The sealing ring is made out of S/S, I believe. It's not alum, and it's not magnetic.

I was really hoping to get the P's & C's as well as heads on this weekend, but I'll hold off and talk to George on Monday. I'm still leaning toward going with a .010" sealing ring, and splitting the difference, basically. That would get me to about 1.4mm of clearance to the heads.

With the higher lift cams (.015" intake & .020" exhaust greater than 964 cams), this may be the safe bet, which I'll double check when I get to valve clearance.

One of the things I was worried about was whether my .25mm base gaskets would be too thin with the machining that was done. That is definitely not the case, so I suppose I can get my pistons loaded in the jugs, and get them onto the rods tomorrow, at least that is some progress...

Thanks Andy, I just checked one and went from 9.8-9.5, it was only 1% or a little over 2HP.

I would not worry about the HP difference, but do a search on deck height here, there is plenty of info stating why you want it lower, provided you have the valve clearance....

cheers

Jeff: I've researched, and see the potential for detonation. I'm going to talk to Ollie's about making me some thin seals, unless he's OK with the head to cylinder seal au natural...

I thought about using some plastigauge as well to check the seal between the mating surfaces.

I had missed the part where he machined a step into the cylinder top.....

Cheers

Ed,

If George cut the head then the spacer is there to compensate. When Bob at Anchor did mine he took off three thousandths (0.0762mm). Removing TWENTY thousandths (0.508mm) is a pretty big cut.

Distinguish between deck height, which begins at the point where the dome starts and is used to determine compression ration and squish, and piston-to-head clearance which MUST be maintained. Those terms are often used interchangably on this forum but they aren't the same thing. I know you know this but it's something to be aware of when browsing the archives.

Thanks John. I'm strictly dealing with head clearance on this. I did a lot of searching last night, and I agree, deck height is plugged in erroneously too many times.

I figured out that these seals/shims were provided to make up for the material he took off of the cylinders, but either he is REALLY conservative, or supplied too thick of a shim, IMO.

I rechecked today using solder on the edge of the pistons. I got .057" on the C/L of the piston on the LH side as you look at the top of the piston (front of the engine) and .051" on the RH side at approximately the 2:00 position. With the DME pistons, there really isn't much dome to check at the 3:00 or C/L position. I got the same measurements twice today, so I'm confident in them.

The average of .054" clearance would seem to be fine. I also put some plastigauge on the top surface of the cylinder, and it was absolutely squished, so I'm confident in the fact there is a seal between the head and the cylinder.

I'll call George tomorrow to see why he sent me .020" shims. Either it is a goof up, or he's pretty conservative in his thought process as to what I needed. I'll report back.

Ed, come'on, it has been 2.54 days, give us an update!!!! :)

I am interested to know what George has to say......

(like I should talk:) )

Cheers

I'll take whatever hp you leave on the table:)

Sorry guys, I'm in Atlanta til Sunday, but I did talk to George. He insists that I'm fine with this clearance, and that I need the ring as a seal.....

I suggested that there are some of the opinion that when getting over 1.5mm of clearance, that the potential for another form of detonation issue appears. He said to ask for proof, he's never seen this.

??

Ed. Take a look if you haven't seen it at my issue. Something else to worry about. http://forums.pelicanparts.com/911-engine-rebuilding-forum/371883-head-gaskets-destroyed-but-why-74-2-7-911-a.html