bscpanther

have to be decked off before you need a thicker head gasket. So as I've stated before my 2.8 is at the shop being assembled and the block has been decked previously and so has the head... twice now I believe. The builder tells me a total of 12-15 thousandths of an inch have been decked off between the two and he doesn't believe a thicker head gasket is required. Is there some kind of rule on this??? I have already bought a custom made cometic head gasket and I've never run one before...so I got to thinking perhaps the cometic head gasket will compress that much less than a stock head gasket and all the better for me but on the other hand this could just be wishful thinking.

gt37vgt

you will be able to find the 2 gasket compressed heights but you have have about .4 mm off at his stage your still pretty safe .

Chris White

Depends on how much came off the block. If a lot has been shaved off the block the pistons may hit the head. You will need to measure how far the pistons protrude out of the block at TDC.

Chris Prack

If at all. You certainly need to measure the deck height clearance of the pistons. If you have a negative deck (the piston out of the bore) you are not sunk but it will get expensive to continue with the same block.

The other thing you need to determine is, what is the new compression ratio of the engine?

How thick is the cometic gasket? How thick was the old gasket?

bscpanther

I don't know the gasket thickness measurement but I know both the old gasket and the new cometic gasket are stock in terms of thickness. I will be sure to note/measure if the pistons protrude outside the block when I get it tomorrow...in the mean time I will pray they don't.

Chris White

Cometics can be ordered in many different thickness - not a problem there.

bscpanther

I can be slow so work with me here. On a stock block everything is setup so the pistons come perfectly level with the top of the block.?? so if you deck the block at all you need to...??? shave the pistons?? thicker head gasket??
On my drysleeved 2.8L block I have the stock crank, stock rods but JE pistons which must mean the height of the JE piston is identical to the height of the stock piston to get the same stock height out of the stroke at TDC. Is this line of thought way off??? I guess I need to be hoping that whatever was shaved off the block was little enough that my pistons will not extrude.. then again if the pistons originally came up perfectly to the deck height then any amount of decking will expose the top of my pistons.. no??

bscpanther

on the other hand it would make sense if the pistons could extrude slightly as long as they cleared the valves. I could confirm this with clay...right??

Chris Prack

Usually on a stock engine the pistons are slightly below the deck of the block. If you deck the block you then need to set up atleast one cyliner and see how much deck height you have. If the piston is protruding from the deck then you need to make a desicion. The right way to fix it is to shorten the rod to get atleast a zero deck or having another set of pistons made with more offset to get the correct deck.

You could use a thicker head gasket but that is a hack fix in my opnion. It's like putting a band-aid on the problem. There are signifigant down sides to using a thicker headgasket. Hopfully you will not have any problem. I am sure someone else will chime in with other options/opinions.

m42racer

Stock Block Height is 230.25mm. Do the math here. Measure the block height now, or after it is re decked if required.

Add 1/2 the stroke, + the Rod length, (150.00mm if stock length) and the Piston Compression height(Pin Centerline to top of Top Ring band). Subtract this from the Block height. This will give you the calculated Piston Deck Height. You should measure this also, in case the stroke is not correct, or something else maybe be wrong. Then CC the Chamber and the Piston. Then calculate the CR you have. At this point with this information, you can them make the correct call. A thicker H/Gasket just may be the answer. No band-aid required. The thickness of the head will only affect the Chamber volume, (CR) and the Piston/ Valve Clearance.

Whatever the Gasket thickness is, this is what it will stay at. They do not compress. Only the embossments compress.

Chris Prack

Let me preface this by saying that I am not trying to be a dick.

Let's say that the deck height is negative and the piston protrudes from the top of the cylinder. How is just throwing a thicker head gasket on the motor not a band aid repair?

You have increased the exposure of the firering in the gasket to high cylinder pressure. Now a question. At what point does the cylinder see the highest pressure and where is that pressure located?

Can anyone tell me at what point this became a problem with Porsche and what did they do to fix the issue?

Chris White

No. You are not worried about valve clearence (Unless you have swapped out your cam to a very high lift version). You need to be concered about the pistons hitting the head.

Chris White

The thought never entered my mind….
Good questions – using a thicker than stock head gasket will return all other aspects of the engine back to stock – compression ratio, piston to deck clearance, cam timing (all good things). You can also fix the issue by shaving the pistons or ordering them with a different pin height, this will fix the piston to head clearance and the CR can be stock if you change the piston design – but the slight change in cam timing will not be addressed.
I am not worried about the pressure (as long as we are talking about Cometic gaskets – keep in mind that Porsche does offer thicker than stock standard gaskets). I am more concerned about temperature and exposure to the combustion process. If you spec out your head gasket correctly the edge of the gasket will be recessed slightly from the edge of the cylinder wall and it will create a quench area where little or no combustion will take place. This offers a significant amount of protection from the heat issue.
I am not sure I follow the question…Porsche does offer a 1.3mm head gasket to accommodate heads / blocks that are ‘under spec’ from cutting.
What I have found with the sleeving work I do is that almost all of the blocks I get are not flat or square anymore (can you say ‘headgasket problem’?). I have to true up the block with a deck cut before I start the CNC process. Then I have to skim it again with the sleeves installed to finish the installation. This results in a negative piston height fitment. There are several ways to fix this but my preferred way is to use a Cometic gasket that is speced to fit the reduced deck height. I have found problems with the other methods so I will stick to this one!
One last thought – I would not want to use a Cometic unless I was sure the both surfaces (head and block) were flat.

bscpanther

lively conversation, I like it. The block is ready for pick up today but I won't be able to do so until tomorrow. I will let you guys know the findings. Appreciate the helpful feedback.

Chris Prack

I do not agree with this. What do you mean "quench". My understanding of "quench area" is there must be two opposing flat surfaces to qualify as a quench area. I am not sure how a recessed area perpendicular to piston travel satisfies this requirement.

In your example, the head gasket would be no less susceptible to flame travel or thermal expansion. It would see very high cylinder pressure and heat. The answer to my question about cylinder pressure and when it is highest and where it would occur is approx. 10 degree before TDC and it would be most significate at the top of the piston. If the piston has a negative deck then that puts it right on the gasket. I don't see a Cometic gasket being less susceptible to thermal load then a Victor gasket by that much.

To answer another question I posted, Porsche found this to be a problem during the develpment of the 993 turbo engine. The solution was to machine a recess in the cyliner head to allow a matching machined surface at the top of the cylinder to fit into the combustion chamber and overlap to protect the head gasket. I have seen my fair share of turbocharged 993 N/A motors with broken heads and split cylinders but that is another story.