Why is my deck height 2mm and what can I do to reduce it?

[ohecht]

I'm in the middle of rebuilding my 3.0 from my '82 SC and ran into a problem with the deck height...I'm not sure how it got to be 2mm or what I can do to get it closer to the 1.3mm that I think is ideal.

Here are the relevant specs of the motor: new 10.5:1 98mm JE pistons and Mahle Cylinders; stock crank and rebushed stock rods with ARP bolts, stock heads twin-plugged, ported, and rebuilt by Xtreme to fit the Mahle cylinders and with larger valves. Besides the head work, the only change was to enlarge the valve pocket on the cylinder for the intake valve to provide adequate clearance for the 50.98mm intake valve.

I've been measuring the deck height using the crushed solder method with a piece of solder on each side of the piston aligned with the wrist pin. I've taken several measurements and consistently get 2mm deck height using a .5mm cylinder base gasket. I'm getting the same measurement on each side of the engine. Here are a few other measurements I've taken: I removed the cylinder base gasket once to see the impact on the deck height, and it reduced it to 1.72mm...not sure why I didn't get a .5mm reduction from removing the .5mm base gasket. This measurement was also different on either side of the piston (1.68 and 1.76mm) while all the other 2mm measurements were almost identical on either side of the piston. I also measured the distance from the top edge of the cylinder to the outer "lip" on the piston while at TDC...this is 1mm.

Besides losing a lot of my CR (down to 9.7:1 by my calculations and possibly having issues with the cam and chain dimensions, I'm also worried that too much deck height could actually cause detonation in the "pocket" around the edge of the piston...I've read a few threads on that topic.

I'm also confused about how this could have happened...there was no machine work done on the cylinder openings on the case, and I think the work on the heads to remove the sealing ring and match them to the cylinders would actually reduce deck height. I will call him to confirm, but I don't think Bill at Xtreme would have added material to the head/cylinder mating surface as part of the process to remove that groove and fit the heads to the cylinders.

Finally, I have no idea how to fix the issue...I assume I could find .25mm cylinder base gaskets to get me a little closer, but do they come any thinner than that or could I skip the gaskets all together? I'd be surprised if that was an option. If I need to get some machining done to address it, what's the best way to do that and keep the other critical dimensions related to the heads and cams right?

Thanks in advance,

Olivier

[cornernfool]

Sounds to me like you got the wrong pistons.

[snbush67]

You can eliminate the base gaskets, they serve simply as spacers as the cylinder is sleeved into the block.

Still your measurements sound less than exact. Did you actually measure your base gasket with a caliper? If you remove the 0.5 mm base gasket than that should change the reading by 0.5 mm.

Perhaps you are not tightening down the head completely?

Either way 2mm on the squish height is not going to be an issue, and 9.7 is a great CR which will only benefit in the long run for longevity and less chance for bad gas to ruin your day.

Other than that your option is to shave the heads 0.5mm. Simple job for any machine shop.

[Walt Fricke]

Oliver

You are confusing me when you speak of removing a CE seal ring groove from the heads. I think some turbos, perhaps just hot rod ones, used a "flame ring" which was cylindrical and fit into grooves in both the head and the cylinder.

All the heads I have seen have no groove for any kind of seal ring. The groove, which all the SCs used, is in the cylinder. Perhaps when you bore a 95mm cylinder out to 98mm you have to do away with this ring groove?

No matter, all this would lead to increasing CR and reducing clearance, not the other way around.

You could have your rods rebushed with offset bushes, making them effectively a little longer, though that may be more than is worthwhile.

When you had your intake pockets widened, did you factor that into your CR calculations?

[ohecht]

Thanks all for the responses so far. Here are some additional questions and information:

I think the JE pistons are right...the part numbers on the pistons themselves match everything I've seen for this piston application, but I will check again. Of course, they have been modified as described with the intake valve pocket.

I did measure the gaskets with my digital calipers, and they were coming out right at .5mm, so I don't think that is the problem. Good to know eliminating them is an option, since that gets me a little closer. I have thought about the additional safety factor of the slightly reduced compression, but I would like to be in the 10:1 range, and I am worried about causing detonation in the pocket at the edge of the cylinder with the excessive deck height.

I think I am tightening down the heads properly...using Supertec studs and hardware and tightening to 30 ft/lbs with a digital torque wrench, and the readings have been consistent over the 4-5 times I've made the measurements.

Walt, I think you're right that the ring grooves were in the original SC cylinders...those are no longer in the picture. I can't remember if the heads had a groove on that side for the ring, but one of the heads had some minor damage where the ring was pulverized since one head stud was broken for a while on that cylinder. Bill at Xtreme was able to fix that easily, but I'm not sure if he just cleaned up the surface or if he had to add material and then refinish. It was very minor damage...just tiny coils of the little spring smashed into the head sealing surface, so I would be surprised if it warranted building up the material on all the heads and then resurfacing. I will try the measurements with another head when I get home today to make sure they are all the same here, and I will call Bill now to confirm how he fixed that.

[jpnovak]

Were the heads chamfered on the edge for the 98mm pistons? If so, your deck height will be slightly larger depending on the depth of chamfer.

JE pistons at 98mm usually have a flat edge around the perimeter. What is the height from this flat to the cylinder top edge when at TDC? This may be a better method to measure deckheight.

When I built my 3.2SS using Mahle RSR pistons I did not use base gaskets to achieve deck height.

[ohecht]

Hi Jamie. I believe the heads were champhered to match the cylinders and that might explain it. I'm still trying to confirm with Xtreme.

I did measure from the top edge of the cylinder to the edge around the piston and it was exactly 1mm at TDC. Is that the more relevant measurement here and should I optimize that instead?

Or should I place the pieces of solder higher up in the piston?

[3literpwr]

I did this last week and thought my deck was 2.20mm... Turns out the solder was creeping up from the piston ridge and not getting the measurement I wanted. With the caliper method, my deck was 1.15mm... Much better...

[ohecht]

3literpwr, when you say the "caliper method" got you 1.15mm, do you mean the more tradition DH measurement of measuring from the top of the cylinder down to the shoulder of the piston at TDC? If so, have you gotten further enough in your build to see how that translated into your expected CR, cam chain distances, and other critical measurements?

I will take some more measurements and make sure I have solder running from the shoulder quite a ways up the piston to make sure I'm getting measurements from across that range to see what the tightest one is, in case I was having a similar issue with the solder creeping up the piston that I didn't notice. I did use Curil T to stick the solder to the piston, but the solder ended up stuck to the top of the combustion chamber in the head each time when I removed the head to take the measurements. The pistons are ceramic-coated, so I assumed that was keeping the solder from sticking to the piston and didn't think it mattered overall.

[3literpwr]

Yes, my measurement was from the top of the jug to the flat edge on my pistons. That said, I have yet to move forward with the build or verify that it's correct.

[Eagledriver]

I don't understand why there is so much confusion on this thread. You aren't measuring deck height when you use the solder. You are measuring the piston to head clearance. The deck height is what you measure with your calipers when you measure down from the top of the cylinder to the edge of the piston. You state that the caliper method is about 1mm. That is all you need to do. The other measurement with the solder is just to make sure you don't have interference issues due to the shape of the pistons and chamfer of the heads.

Shoot for 1mm deck height and do your compression calculations based on that.

-Andy

[ohecht]

Thanks, Andy...that's what I'm beginning to realize based on the help in this thread and speaking to a few experienced builders. I'm getting 1 mm between the top of the cylinder walls and the flat edge around the pistons, and 2 mm of space between the curved area of the piston and the top of the combustion chamber.

Here are some pictures of what I've been working with so others can see the flat edge around the piston and the heads. The last picture shows the pieces from the last solder measurement I took where I used pieces that extended all the way from the flat edge around the piston onto the domes part of the piston...I'm getting 1mm at the thin edge that confirms the direct caliper measurement there and 2mm between the domed part of the piston and the head surface. On one of the solder pieces, you can see the smaller flattened end from the 1mm space at the edge of the piston.

[snbush67]

Originally Posted by Eagledriver

I don't understand why there is so much confusion on this thread. You aren't measuring deck height when you use the solder. You are measuring the piston to head clearance. The deck height is what you measure with your calipers when you measure down from the top of the cylinder to the edge of the piston. You state that the caliper method is about 1mm. That is all you need to do. The other measurement with the solder is just to make sure you don't have interference issues due to the shape of the pistons and chamfer of the heads.

Shoot for 1mm deck height and do your compression calculations based on that.

-Andy

Andy,

The confusion comes from two sources;

Wayne's rebuild book specifys two methods for determining deck height, the method you describe is stated for use with flat top pistons only, for domed pistons he recommends the solder method (which he states is to measure deck height).

Using the solder method the book recommends a deck height between 1.25 and 1.5mm.

The op is getting 2.2 mm using the solder method, which is much more than recommended. Is that too much clearance? Or is it within normal parameters that you have seen?

Bruce Anderson's book uses a formula based upon the method you described as one of three facrtors to determine domed piston deck height. Page 177.

Thanks, Shane

[Eagledriver]

I can see why that's confusing. It sounds like Waynes book got a little sloppy on that part. Bruce Anderson's book makes it more clear.

-Andy

[ohecht]

Now that I know to optimize the deck height itself and make sure the dome to head clearance is there, is 1mm DH optimal for 10.5:1 98mm JE pistons? I'm getting a CR of ~10.9 with 1mm DH, 10.6 with 1.25 DH, and 10.3 with 1.5 DH. (crank and rods are standard 3.0 SC, head volume is 87.5cc and piston dome volume is 41.8 cc minus whatever material was removed to enlarge the intake valve pockets by 2mm in diameter)

The motor will be twin-plugged with EFI (MS), but it's a daily driver with occasional track use and I don't want to run it on the ragged edge and worry about excessive heat in the summer and/or bad gas, etc. BA's book said 10.3:1 was a sweet spot in the CR for Porsche engines...should I shoot for that?

[snbush67]

I think stock heads are 90cc, and if you only had then drilled for the second plug the area around the plugs should have increased the total volume, not decreased. 87.5 seems lower than normal unless they were milled.

As far as the pistons go you should cc them as well, but a ballpark figure is that they took away about 0.5 cc of material per piston to gain the 2 mm of clearance.

[ohecht]

Thanks Shane. The heads did get new and larger valves...could that explain the smaller volume from stock SC heads? There was no milling around the mating surfaces.

[snbush67]

The larger valves get larger seats that may sit a bit deeper in the head.

I would cc a head and make sure you get all the air out, with the second plug hole there should be more volume not less. If anything you should be at 90. Perhaps the heads had been milled previously?

[ohecht]

You've got me worried now, but I did cc each head very carefully to remove all the air bubbles and got results that were between 87 and 88 cc for each head each time.

I talked to Bill at Xtreme and the head/cylinder mating surface was only cleaned up a little to remove parts of and scratches from the sealing rings that got smashed up from having a broken head stud and leak at the interface for a while...he did not remove any significant amount of material.

Could it be that the significantly larger diameter of the valves themselves is making them sit further towards the middle of the combustion chamber and reducing the head volume? The valves also have 8mm stems and seats instead of 9mm in case that matters...

[snbush67]

I don't think you need to be worried. If you measured each head and came up with 87.5 then that's what it is.

Did you determine how much came off of the piston domes?

If you add .25mm base gaskets then if your measurements are off either way by a cc, that at least gets you at 10.1:1CR or at most 10.6:1. And if your measurements are dead on then your looking at 10.3 ish.

You are in prime band of CR for a twin plugged motor that can run a nice advance curve balanced for street and track action.

My motor is a single plug, 3.2 SS with 98 mm 9.5:1 CR pistons and I ended up with similar measurements as you. I had a tough time getting consistent measurements on the heads and they were stock, untouched. It is a beast.