Chris White

We might be getting into semantics – but the end result of “quenching” a flame is that combustion is no longer supported. This happens in areas that are in close proximity to the cylinder walls unless there is a lot of turbulence. The recessed area protects the gasket from direct exposure to the turbulence in the combustion chamber and as a result the combustion process is not well supported in that area - and therefore the direct exposure to heat is minimized.
The quenching of the flame front by cooler surfaces is why we can have combustion (up to 2000f) take place in an aluminum cylinder head (and block for Alusil) that melts at approx 1400f. For reference steel (depending on alloy) melts at about 2500f.
I understand what you are saying (I think!) – but I don’t see any difference in the exposure of the two type of gaskets – both are exposed to the same pressure and heat for the same amount of time. If the point is that the Cometic will have a greater surface area exposed I will agree to that but counter with the fact that the structural strength of the Cometic gasket is way better than the stock type.
Maybe you have some concern that the piston might come in to contact with the gasket…if that part of the pistons ever hits the wall (or gasket) you are in trouble!
That would be a nice fix for our cars…although it would make resurfacing the block / head a real pain in the butt!
So, just to further the discussion – what would you think is the best way to work with an engine that has had the block cut more than stock?

Chris Prack

Either order pistons to spec or if the pistons have already been ordered then you can either send them back to the manufacture to be cut down to the correct height or have a machine shop cut them down. I have a jig to mount pistons on a lathe so that they can be cut accurately. Of course there is a limit to this as I am sure you know. IMO a turbo piston needs a minimum thickness of .250" on top and a minimum of .060" or so in the valve reliefs if any. Most aftermarket turbo pistons that I have measured are ususally thicker than this so you have room to modify them if necessary.

If you tried to replicate the 993T design on the 951 it is possible but time consuming and expensive. Think about it.

m42racer

I disagree. Go figure that.

Surely it depends upon how much the Block has been cut. We are not talking about alot here. There is absolutely nothing wrong with using a thicker Gasket. In fact, when using the Garlock seals the minimum Gasket thickness is 0.051". This is thicker than stock. Is this a problem. I think not. Stock thickness is 0.040". A repair thickness could be as much as 0.060". In most cases the Block is decked only a few thousands. The amount usually does not change the CR by much. Less than 0.1 of a point.

Quite often the reason is two fold. You need to lower the CR and gain clearance.

As long as the Gasket does not go into the chamber, the issue if heat and pressure is not really an issue. Not in these engines. I believe a Cometic Steel Gasket will withstand more abuse than the old style Fibre types will.

I have seen engines built where bthe Piston is stood proud of the deck and the Gasket sets the clearance. No issues seen.

If you have all the money and time in the world, sure get the correct Pistons. In most cases this is not possible. You can as stated cut the Piston down. This can have other factors attached though. A 944 Turbo Piston is easy to cut down, but you will increase the CR. Some Pistons are not so easy. So if the thicker Gasket works on these, then its an opition here for most.

Chris White

My main complaint about ordering custom pin height pistons is that the engine is then stuck with the spec, any future work will have to have the same custom spec used – and if you are building a motor that is intended for hard use (racing) it would be nice to build it with the customers ease of maintenance in mind.
Cutting the pistons down is not really the right way – it implies that there is extra unneeded material on the piston – if that is so it would make sense to spec the piston crown thinner to begin with.

jerome951

While we're discussing headgaskets and clearance, what's a reasonable amount to shave the head without risking problems assuming the head is original thickness?

I lost the headgasket in my car a few weeks ago (haven't started disassembly). I don't believe the head has ever been off so should still be stock thickness. Assuming I want to shave the head even if flat, how much can I take off without worrying? I intend to use a stock-thickness gasket.

I beat my car on the track for 8 years (now has 118k on it) (and Prack, no comments on how gently I've driven it) so I figured I was on borrowed time.

Chris Prack

How is shortening the piston in the bore increaseing the CR?? You are removing material.

Chris Prack

Jerome, you head may not even need to be cut. You need to have it pressure tested and checked. If you don't have a good machine shop, drop it off at the shop and I will send it to mine. I have two other heads going this week.

Chris White

You may be removing some piston material - but you are also removing volume from the bowl. The decrease in bowl volume will increase the CR...but not by much

jerome951

Thanks for the offer, but won't have it torn apart for at least a few weeks.

Assuming it's flat, pressure tests good, etc., I'm still considering a little shave to pick up a few hp. I hear the cost isn't much.

Already have the machine shop picked out (the place in Burtonsville/Laurel you're probably sending yours). ;-)

Interestingly, I don't have any mixing of coolant and oil. Both look clean. I'll stop by in the next couple weeks to talk and ask for advice (in exchange for adult beverages, of course).

Headgasket blew during qualifying of the first race in Daytona. 14 hour tow down, 2 practice sessions, 14 hour tow back home. An expensive trip even when not including the costs of the head gasket fix.

Chris Prack

How is ordering the correct pistons for an engine making the "customers ease of maintenance" any more difficult? If you order pistons from JE, CP or (insert favorite here) they come with a build sheet and an order number. If you need pistons you order from the order number and they show up at your door.

If you are truly building a "race" motor, I am betting I can find far more well known engine builders that would order correct custom pistons vs. using a head gasket to get the correct fit. This is where this conversation has turned from what it correct to what is acceptable. How many times has anyone on this board been faced with a problem and the cheaper of the options is the best?? I would argue never unless you are scrapping the project. How many have built endurance engines? Not the chicken **** PCA 1 1/2 hour enduro but 12 or 24 hour races?? I have and dropping an excessivly thick HG on the block is a stone cold guarentee you won't make it. Phyisics are against you. Longevity is the key whether it's 12 hours continuous or 100k street driven.

The best part of this whole thing is we still don't know how bsc's motor turned out. We have really run with this topic.

Chris don't you advertise cutting down pistons on your website? What is wrong with that? If you have extra material (and with aftermarket pistons you often do) cutting them to get a specific CR or deck height is acceptable. Years ago I sent a set back to a large and well known manufacture to have them cut down at their suggestion. That was in a super charged 928 engine that still uses the same pistons almost 10 years later. As long as you have a minimum crown thickness there should be no problem plus you are taking weight off the end of the rod.

special tool

There is absolutely no question that the Cometic will withstand more than the stock gasket.
We must consider that a fact.

Chris White

Hmmmm….it s a little early in the day to be getting too buzzed on a Rennlist thread!
One thing that we never touched on is the ‘real’ application. Even using the phrase ‘race’ motor means different things to different people.
If it is an all out race motor the answer is simple. Toss the block and get one that specs out OK.
Sprint “chicken ****” motors are different than endurance motors – generally they are asked to put out more power for a shorter duration. A full on sprint motor also gets refreshed more often than an endurance motor (high wear). Does that make it any less of a motor….not to me – they each have a job to do.
I have respect for any engine that is purpose built – even dyno queen engines!
My comment about the ease of maintence might need a little better explanation. I supply MID sleeved ‘kits’ (unassembled blocks w/new pistons) to a lot of folks – from car owners to other engine builders. By using a piston that has a stock pin height it will save them some aggravation in the future.
I have been using the Cometics in various thicknesses for quite a while. I only have one failure – and that was on an engine I was testing to the limit. I will take a Cometic gasket that is .010” thicker than stock over a stock gasket with shaved pistons any day…(as long as the head and block are flat!).
All that being said, both answers will work (and I have used both with success), but from a production stand point (since I use Cometic gaskets anyway) I prefer the head gasket thickness as the way to make up for a ‘short’ block.

bscpanther

Well, surely you both have more experience and knowledge than I. As the individual who started the thread allow me to interject a third party perspective. To me it sounds like you both have solutions to the problem. I like the idea of returning the stock specs with the cometic and for my street aplication I feel it would be the best solution. I would also guess that this solution would be more common to a majority with limited resources as myself. I just got the bill for the short block build (+some minor machine work) and to me it was a small fortune. So assuming the pistons would have to come out of the block to get them shaved and paying once again for the labor... sorry to say I just couldn't squeeze it now. Even just a piston exchange would fall under this same catagory for me. I understand there are some like you two who would be able to save this cost by doing the work yourselves but this is a small minority. Thank You.

m42racer

Calm down my friend. Ther are many who have built engines for racing.

Are we discussing purpose built race engines or can I assume that we are discussing 944Turbo engines that may need rebuilding, as we are discussing this on the 944Turbo forum?

No one is disagreeing with you about which is the best way, but you need to relax a little and understand that for 944Turbo engines, a thicker Head gasket will work, for most if not all applications that engines will be used for by those on this forum.