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b930 06-04-2017 01:40 AM

Ni-resist rings for 100mm, how do they work?
I'll be pulling the motor on my 930 to fix a sloppy gearbox, and also address some head leaks. I recently did a build with 100mm cylinders, light 8.0:1 pistons, 3.2 heads twin plug heads, big valves, light rods, light pressure plate, etc. I run to 1.5bar on a GT3582R.

While the engine makes phenomenal power and revs like a NA car, I am having some cylinder head weeping. I never used cylinder or head seal rings but now I plan to while its out.

I have been asking a few vendors about Ni-Resist rings for 100mm cylinders. A vendor sent me details of the ring dimensions and groove dimensions. They need a groove in both the heads and cylinders.

When I look at the dimension of the depth of the rings (3.3mm / 130 thou) and compare with the total depth of the grooves at the minimum tolerance (tightest fit) in the head/cylinder, it runs a minimum clearance of 0.1mm meaning the rings don't get compressed. The diameters are the same; at tightest groove diameters, there is clearance between the ring dimensions and the groove.

I was wondering then how these work. The coefficient of expansion for inconel (or a similar hi nickel alloy of which I think they are made, or are the SS?) is much less than aluminium, so even when things heat up, I am not sure there will be any preload on the seal does anyone know how do they work?.

Can anyone comment on the stock 100mm rings they use of the 3.6 motors, i.e. how do the groove dimensions stack up against the rings, and are the rings compressed on assembly etc. Also, could I use these rings in my set up?

I am also trying to find information on nitrogen filled Willis, or cooper fire rings. They are made by a company called Trelleborg. The can be made in high temperature inconel, and designed such that when hot the nitogen expands increasing the ring preload. They have featured in some very trick engines, but information and availability is scarce.


Tippy 06-04-2017 07:07 AM

Watching. I'm in the same boat. Some feel you can seal by a lot of preload torque and freshly machined surfaces, others feel you need flame rings or use a gasket system.

I too had been running 1.4 and spiked to 1.6.

m42racer 06-04-2017 11:11 AM

There is certainly a more modern approach to this issue available. The old Ni resist rings are from the 80's.

Look at – Engine Design. under their tech corner. They have what you are looking for.

m42racer 06-04-2017 11:50 AM

Also, take a look at Garlock ring. They are the gas filled type you mention. Not sure who sells them.

b930 06-04-2017 04:50 PM

Interesting insights from performance developments site:

Some of the early 911 Turbo engines used a stainless fire ring between the cylinder and the head. This ring was not very good at sealing the two parts as it was primarily there to stop the flame travel from burning its way out. This primitive system also did not stop the head from banging against the cylinder top. Our system uses some interference between the sealing gaskets and the head and cylinder. This interference compensates for any head lift due to pressure and temperature.

Speedy Squirrel 06-04-2017 08:52 PM

What's wrong with the 993 CE rings?

chris_seven 06-05-2017 03:55 AM

Any idea of the peak cylinder pressures?

m42racer 06-05-2017 04:42 PM

Without knowing the spec of the engine that would be hard to estimate. If his engine is making torque in the region of 450+ Ft/lbs with a comp ratio of 9 @ 22psi MAP, he could be seeing upwards of 120b without a leak.

So many factors could lower this, cams, head flow etc. Best guess would be somewhere between 90 - 120 maybe. That's a lot to expect the heads to seal without any sort of gasket.

With a leak who knows.

b930 06-06-2017 04:45 AM


Originally Posted by Tippy (Post 9612861)
Watching. I'm in the same boat. Some feel you can seal by a lot of preload torque and freshly machined surfaces, others feel you need flame rings or use a gasket system.

I too had been running 1.4 and spiked to 1.6.

Tippy, yours leaking also? I think I am going to send my parts to performance developments. I quite like their solution.

Tippy 06-06-2017 05:30 AM


Originally Posted by b930 (Post 9615120)
Tippy, yours leaking also? I think I am going to send my parts to performance developments. I quite like their solution.

Mine weren't leaking, they were blowing out!! I had oil/combustion byproducts blowing out from underneath going straight up.

Looked like an oil leak going up instead of down.

Mine was from using 3.2 head torque instead of 993TT, which are what my studs are.

Speedy Squirrel 06-06-2017 10:20 AM

For design purposes such as this we can just use a couple of estimation methods.

First, the BMEP in kPa is (6.28 x 2 x torque)/Displacement, with torque in Nm and displacement in liters.

For a turbo motor making 500 HP at 6800 RPM, the torque is 386 ft lbs or 523 Nm.

Plugging that in the BMEP formula for a 3.4L engine:
(6.28 x 2 x 523)/3.4 = 1932 kPa or 280 psi.

BMEP is the constant pressure that, if applied during the power stroke would produce the brake torque. We are not really interested in the BMEP though. We are interested in the peak firing pressure. That is what lifts the heads and gives the head studs a workout.

That takes some figuring with the Weibe function, or some cylinder pressure data. I have done it for a 3.2 Turbo motor, and it came out to be about 4 times the BMEP, so I would hazard a guesstimate of about 280 x 4.0= 1120 psi for the peak firing pressure.

chris_seven 06-07-2017 07:19 AM

I am not sure how the interlocking Ni-Resist Ring still seals if the heads lift.

I can't find any cross sections of the grooves and how the seals locate.

I understand the Wills Ring - originally known as a Coopers Rings and have used the MOT style for many years.

If the head lifts sufficiently for this ring to lose its preload then it will still leak so the extent of the lift is quite significant to using these parts.

If we assume 120 bar then the force trying to lift the head will be around 5500lbs per stud.

I would estimate the preload in a stud torqued to 24lbsft to be around 5000lbs so if heads were torque to this level then leaks seem to be inevitable.

If we torque the studs to 36lbsft then the preload should be around 7000lbs so if the heads still leak then the preload needs to be greater. or a better seal is required.

Using an MOT type ring to gain a pressure seal at around 5000psi would ideally need the ring to be seated with a preload of about 300lbs per inch of circumference.

I think using a 1.59mm dia ring then if you can preload at the higher level these will still be enough preload to seal but if the head lifts completely they will still leak.

I have always wondered why the CE rings are used in the wrong direction as normally the 'C' would be open to pressure and this would help with the sealing even if the heads did lift slightly.

I guess that they must collect carbon deposits and cause pre-ignition.

Tippy 06-07-2017 08:38 AM

Awesome info, again, Chris!

Why not use dead soft copper rings? Easy to punch out and seal really well.

chris_seven 06-07-2017 09:01 AM

I think that the Wills Rings is a better bet as the internal gas pressure helps with maintaining the seal.

I don't think that the Nitrogen helps with expansion but it helps the ring to maintain its resilience when hot and compensates for the reduction in Young's Modulus.

We use a 321 Stainless Steel Ring rather than Nimonic and they will easily withstand the temperatures.

I think using a copper ring would still need a high contact force to maintain a seal and it seems as if the reduction in preload that occurs with the high peak cylinder pressure is the problem but I am sure that there are others with more knowledge in this area.

The Wills Ring will tolerate this reduction in load as its has more capacity to deform elastically which must help maintain the seal.

There's lots of information here:

Tippy 06-07-2017 10:00 AM

^^^ Good stuff

m42racer 06-07-2017 01:23 PM

Regardless of what type of sealing ring type is used, you must have control of the assembly procedures. Often these are taken for granted.

Once the parts are mated together and zero gap, you cannot change the thickness of the parts unless some collapse under the compression pressure. This should not be happening and the forces should be going into the stud stretch, to act as tensile force clamping and holding the parts together resisting all of the pressure forces trying to push them apart.

You have to know the stretch of the stud being used at whatever torque you use. You should be using an angle method to remove any friction changing the clamping load. Grease type and or oil type can change this as well. If the stud does not stretch, then either the stud starts to pull out of the block or the head collapses under the head washer. Resulting in different clamping loads on each stud.

Whatever stud you are using, you need to know the stretch from the supplier or at least test before installing. The cylinder pressures in these engines are very high and usually unknown to most. When there are leaks and head lifting, its always blamed upon the stud, when in fact the stud may not be made for this application and the basic assembly procedures are not applied.

b930 08-11-2017 05:41 AM

So after much research I deduced to go with Performance Development head sealing system. Generally building proper race motors with years of experience, all I can say is these guys know their stuff. Not cheap but you pay for what you get. Their solution is like nothing else on the market.

They measure torque / stretch of the studs and machine both cylinders and heads. The gaskets are EDM machined to very fine tolerances.

racing97 08-11-2017 08:25 AM

Metal Seals | Technetics Group

willtel 08-11-2017 08:35 AM


Originally Posted by b930 (Post 9696089)
Not cheap but you pay for what you get.

How much is "not cheap" in dollars?

chris_seven 08-12-2017 12:27 AM


Originally Posted by m42racer (Post 9613128)
There is certainly a more modern approach to this issue available. The old Ni resist rings are from the 80's.

Look at Engine Design. under their tech corner. They have what you are looking for.

I can't find a description of how these parts work.

There is a 997 part described as a Garlock Ring and another hint at what sounds like an MLS system.

I am very interested to learn more

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