GregBBRD

The purpose of ring end gaps is to ensure that the two ends don't touch each other, which would put a tremendous amount of pressure on the cylinder walls, as the engine heats up. The problem with too much ring end gap is that the gasses you are trying to seal can escape past the gap.

I'd go with whatever the ring manufacturers says.....they are going to know best.

There's a "general rule of thumb" of .004" per inch of bore for a naturally aspirated engine, so what they are calling out for is certainly reasonable....although .010" is on the "tight" side of tradition.

On some of the race engines, Porsche tends to run very small ring end gaps, but I'm sure that they have studied the expansion rates of all the different parts, at length.

Do the 968 people use these rings with success? They have to be much cheaper than the stock rings, from Porsche.

Kevin in Atlanta

I looked at the packing and nothing jumped out at me. I'll do more searching.

Kevin in Atlanta

104mm = 4.09449"
According to Hasting Piston rings:
Rings in this range: 3.5425 to 4.3299 Ring should be between 0.012 and 0.022

This is pretty good read from Wiseco: Everything you need to know about ring gap - Wiseco

It confirms your gaps, Greg.

Snippet from the website:
The process begins with inserting the ring into the bore, and making sure it is square using a squaring tool to align it properly. A feeler gauge inserted into the ring gap will show you the existing dimension, and from there you can make your adjustments. For a street engine, multiplying your bore size by 0.004in will give you the top ring gap you are looking for.

• 004 x 4.00in bore = 0.016 inch ring gap

For high performance engines, the multiplier changes to add more clearance, but the math stays the same:

• Modified or Nitrous Oxide - 0.005in x 4.00in bore = 0.020 inch ring gap
• High Performance Racing - .0055in x 4.00in bore = 0.022 inch ring gap
• Racing with Nitrous/Turbo - 0.006in x 4.00in bore = 0.024 inch ring gap
• Racing Blower/Supercharger - 0.007 x 4.00in bore = 0.028 inch ring gap

For the second ring, the process is the same, but with a slightly different gap, based on application:

• Street - 0.005in x bore size
• Modified or Nitrous Oxide - 0.0055in x bore size
• High Performance Racing - 0.0053in x bore size
• Racing with Nitrous/Turbo - 0.0057in x bore size
• Racing Blower/Supercharger - 0.0063in x bore size

GregBBRD

Very typical dimensions. You won't go wrong with that information.

GregBBRD

When these engines were originally assembled, the crankshaft main and rod bearings both came in three different sizes, which were tiny variations of "standard". Both the bearings and the crankshafts were marked with a blue, yellow, or red paint mark. Each "paint mark" on the bearings indicated a .0004" size difference, with blue bearings being the tightest
The assembler simply matched the color on each crank journal with the correct paint mark on each bearing. (The assembler did no measuring, he just matched up colors.)

This worked great for initial assembly, however was problematic when supplying replacement bearings from the parts department, as Porsche never talked about these subtile size differences in the workshop manual.

Consequently, they supply one set of main bearings in a box...almost always "double yellow" bearings. The bearing clearance specification, in the workshop manual, is very "wide", ensuring that any crankshaft original size will work with any main bearing...especially double yellow (or a blue mixed with a red.)

Fairly "idiot proof" for the AVERAGE engine person...make sure the crankshaft is somewhere in the "standard" range, grab your main bearings out of the box, and assemble.

For someone with the ability to measure the main bearing clearance, who has a specific "bearing clearance target" (your machinist), it is more complex.

Is anyone going to notice the difference between .0025" of main bearing clearance and .0030"? Doubtfully, given that a 928 oil pump spends 75% of the time being bypassed. Maybe a slight change could be noticed in the heat of summer, at idle, but with the proper 20/50 oil, it will be a tiny difference.

However, your machinist is doing fine. Just let him do "his thing".

Kevin in Atlanta

This is so confounding. You have 100% failure rate and Gary Grimes has never had a Cometic gasket fail. This includes engines where the block and the heads are aluminum. He is interested in talking to you about your experience with Cometic.

GregBBRD

People are free to do whatever they desire. I give my advice (for free) and people can either use it or not.

Understand that my experiences are several (many) years ago. It is possible that Cometic "improved" their coating between the layers, in order to help sealing (I sincerely hope they did something.)

Also note that you mentioned someone you talked to "sprays" their Cometic gaskets with something to help them seal, even through Cometic does not recommend this proceedure.

I'm not interested in "finding ways" to force things to work....I've got enough to do without "jumping through hoops" to figure out how to make something work.

I've got a tendency to find something that works, for me, and use that method, until something goes wrong and there is a reason to change.....this makes for far less "free redo" work, for me.

Carl Fausett

We have absolutely no problems with the Cometic MLS gaskets. But, I know that we install them a little differently than others.

We use these gaskets on our over-bored engines or our 4.25" sleeved high-performance motors. Our 928 engines range from 400+ to 1060 BHP.

Here are some of the steps we take when using a MLS gasket:

1) We are always doing a valve job during the engine build, so the heads receive a skim cut while we are at it to make double-damn sure the head is dead flat. The gasket surface is inspected for pinholes and corrosion. If we find any, the area is ground out and filled in with fresh metal. Then the heads are skim cut again. These steps are repeated until we get a perfect gasket surface. If we are installing steel cylinder liners (4.25" bore motors) we install the liners just a touch high and then deck the block and liners together.
2) Because these are high-performance builds, we install ARP head studs where the standard 5.0 32v motor uses bolts. The WSM makes it very clear how much more clamping force you can get with a head stud over a bolt.
3) MLS gaskets should be re-torqued after installation, and we go further than that. We heat-cycle the motor 3 times before we re-torque the heads in our engine oven. One heat cycle takes the block to 240 deg F and back to room temp. Obviously, the camshafts go in only after that.
4) I coat both sides of the head gasket with Hylomar before they are installed. Here is a link to it on Amazon.

Remember that MLS gaskets (any brand) have no compliance whatsoever. They have no tolerance for warped or pitted gaskets surfaces or uneven torque. If you are having MLS gasket failures, take a look at your installation procedures. Do it right, and MLS head gaskets are fantastic and take a lot of abuse.

GregBBRD

Again, it's been years since I tried a set of Cometics on a 928....you were still messing with Ferrari superchargers, when I tried my last set.

And like I said, people can do whatever they want.

I'm doing what works, for me.

When that stops working, I'll try something else.

BTW Carl:
Just as a "point of order", if you re-read your post, you are jumping through a bunch of hoops to get Cometic gaskets to work...a whole bunch. Sealants, special studs, heating the engine through several heat cycles, and then retorquing the heads.

And from a customer's point of view:
I'm guessing none of that "comes for free".
Right?

I set on a pair of modified 968 head gaskets, use the stock head bolts (so the heads can come off in the car), torque the bolts once, and forget it....forever.

Andy's 600hp ITB engine (no "glory run", but tuned for the street that exact same way), now has over 30,000 miles on it.

I've got two clients that now have well over 60,000 miles on their strokers.

Zero issues.

Joseph Fan's engine melted the pistons and fractured the connecting rods from detonation...and the 968 gaskets didn't "blow" (although I wish they had.)

As far as this thread and Kevin is concerned, he is free to install whatever he desires. He is welcome to call Cometic and buy a set of gaskets....or several sets....absolutely makes zero difference, to me.

He asked me....and I told him what I used on my engines.

Carl Fausett

No worries, Greg. When we build a 6.5L stroker, we are far off the page and do not consider these to be extra steps. We consider them to be THE steps. Almost everything about a 6.5L stroker is an "extra step" when compared to a stock build.

Carl Fausett

Help me out with your post here, Greg.

I understand the analogy, as a blown fuse protects the circuit from damage.

Can you tell me what a blown head gasket is supposed to be protecting?

dr bob

Play nice here, please. I have a scalpel at the ready.

This thread is about Kevin's build and the steps he's taking. It's there for others to enjoy vicariously, as Kevin shares what he's doing and why.

It is specifically NOT a pissing ground for a "mine is bigger" discussion. Let's keep everything civil and on-topic, so nobody has to use vacation time unnecessarily.

PM if you have questions. I'm having trouble keeping the tools steady these days, including that scalpel. No telling what collateral damage might be inflicted if I start cutting.

GregBBRD

Discussions like these are important! Forums, like this one, are meant to have discussions/exchanges about different ideas.....I believe that is why they exist, in the first place. Not allowing discussions between individuals that know more about what they are doing that everyone else combined, is counter productive to the entire idea behind a Forum.

Kevin has asked me for help, since the very beginning of his project, which I have freely given. He asked me what I did, regarding head gaskets, to my own engines, and I answered.

It appears he chose to do the same thing.


For anyone building an engine, looking for a reference, in the future:
I build 928 engines. Lots of 928 engines. I'm betting that I've built more 928 stroker engines than everyone else combined has ever built. My clients, as a group, now have 100's of thousands of miles on their stroker engines. I've never had a head gasket fail.....or even suspected that one has failed. As a matter of fact, the only head gasket problems I've had were with two clients who insisted I build their engines with Cometic gaskets. Both of those engines had coolant seep between the block and the head. Both needed to have "extra" things done, which Cometic did not recommend....or even suggest. It's no secret, other people have had the same problems. Mark Anderson had a client who built his own stroker engine with Cometic gaskets, which leaked water into the combustion chambers so badly that he was never able to start the engine.

Maybe Cometic gaskets have been changed and they now work perfectly, right out of the box. Reading what Carl does to use these gaskets would be pure insanity, for me. I don't want to have to do all that "extra" stuff, just to be able to use something that I don't have a need for! And going "backwards" to 1985, putting in studs so the engine must be removed from the car to pull a cylinder head, makes no logical sense, to me.

All that being said, most of us have a "free choice" about things like this...do whatever you want. I don't "market" head gaskets....I don't offer any, for sale. I don't have a single nickel to gain or loose.

I'm going do what works for me.

GregBBRD

My thinking is that I'd much rather have a head gasket blow than damage a piston (or worse) should something go wrong.

Don't forget that I've been working on Porsches for many, many years (with many, many years of racing) and part of that work was very involved with 951 engines (944 Turbos.) Laguna Seca (famous for cold, damp, cloudy mornings) was death to these engines. They would build a bunch of boost and "pop" the engines, on those damp, cold mornings. At many POC events, on a Saturday morning, the pits would have 5-6 of these engines billowing white smoke, after the very first practice session of the morning. The lucky ones just needed a head gasket. The ones with Cometic gaskets, high stretch studs, always needed a rebuild.

The "weak link" saved these engines from more extreme damage.

There's another reason why I love the stock style gaskets:
The "fire ring" is one piece, folded in the middle with a flat portion under the paper portion of the gasket and the other flat portion on top of the paper gasket. This "fire ring" reveals a plethora of information about how that engine was running.

Cometics tell me nothing.

Carl Fausett

I can't agree with your logic, Greg. A weak head gasket is not a safety valve that protects the engine.

A blown head gasket puts water in the oil, damaging the bearings and possibly the crank. It can overheat the motor, warping the heads. If it is a weaper and goes undiscovered for a period of time, the coolant will erode a groove into the head along the path it always follows.

In short, a blown head gasket damages other parts, it does not save them from damage.