piston ring gap - forgot to measure
 

I am currently doing some work on my engine. It has 90 000miles on it and the reason to open it is to install DC13 cams and replace cylinder bolts. To get at the cyl bolts I removed pistons and cylinders and got all 12 lower cylinder bolts out using mapp gas and double nuts. Re-assembling I decided to use new piston rings (Mahle nickasil cylinders. Rings are 95.00 mm Goetz something from the Porsche shop). I am now working on bead blasting and painting the engine tin between cylinders.

Question; I forgot to measure ring gap with the new piston rings. I have decided to dismantle again do a measurement but is that really necessay ? Will not the rings get more end gap as engine heats up?


EDIT; went ahead and did the right thing; dismanteled p&c and will measure ring gap tomorrow.

A ring that hasn't enough end gap when cold will end up having no end gap when heated. It will start to develop some resistance to movement. It will begin to wear both the ring and the cylinder bore. Could be a very bad thing.

That being said, I don't know what rings you used or how your cylinders measured out. Others here may be able to tell you if they have ever needed to adjust the end gap on a set of these rings.

Best to go in and check. If it's too small and closes up under heat and load, you can lock the ring in the bore and cause catastrophic damage to the cylinder and piston.

In 30 years I have never seen the ring gap on a Goetz ring undersized in a used cylinder. Generally there is more than enough gap, too much is the norm.
My guess is you will have to decide if you want to run that much gap. Given the cost of new cylinders, most will run what they have.

Gap
 

I agree with Henry, more than you would like
Mike Bruns

Hmmm... well I guess they weren't too big because you would have never have been able to push the piston in.... The last time I did a re ring the Goetz rings needed to be filed quite substantially. This was on a 944 turbo. For me fitting rings is one of the things I look foward to in a rebuild. Take the time to take it back apart, if your gap is too tight when the engine is hot bad things could happen......

Personally never seen ring-gaps that needed to be widened on a stock rebuild with Goetz rings.... If you want 100% piece of mind you should check it, but I'd trust Henry's opinion too.

There's nothing like knowing. Those little checks are what adds up to confidence when you're ready to turn the start key and a proper running engine.

Measure at least twice, be meticulous, be methodical. I know those times I have screwed up, it was because I did not check as I could've, should've.

Many builders order oversize rings and file them for minimum gap when the engine is at operating temp and they wear down slowly from there. In my opinion, if you ordered std size rings for a std size bore, you have plenty of gap cold and this will close a bit at operating temp. Too much gap is better than not enough, as bh912e said above.

The end gap for new rings is 0.006"-0.012" and the wear limit on the top two rings is 0.031" while the oil ring wear limit is 0.039". I would imagine you are close to 0.008" on your rings.

Mark

Trond:

Any word on the ring gap you ended up with when you dismantled the Pistons and Cylinders?
Just wondering what the value is now?

apologize for not posting any measurements but have been too busy. I dismantled all but just have not done the measurement. I inserted one ring and it has a gap, doing a quick visual it looks like it might around 0.20 mm but will measure all rings and post results !

Yes, peace of mind is the thing. I realized it was nagging me and it just wouldn't have felt right to start the engine without knowing

Is pretty good
 

Very good, not bad

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Measured gap bottom, mid stroke and top of cylinder bore and found all ring gaps to be between 0,3 and 0,4 mm. No variance along the cylinder bores.

0.3 mm = 0.012"
0.4 mm = 0.016"

Trond:

Thank you for posting the results.

I appreciate your reply, since it helps me understand this better and learn from other's experience.SmileWavy

Peace of mind is an important thing. last time i had mine apart i had not intention of having to open the bottom end again although I knew the top was coming apart again for new heads when i went to EFI. but i had an issue with one of the conrod bolts thats bugged me ever since. So when it comes out for EFI i'l be pulling the whole thing apart again(also decided on a gt-3 oil pump)while i am in there lol.

Ken

Are you going to tighten your new rod bolts with a stretch gauge this time? Use superstrength aftermarket bolts?

You might almost pay for the GT3 pump by selling your Turbo pump.

p&c assembled with nice bead blasted and por15 painted engine tin. When disassembling engine I found one scuffed rocker shaft and a partially blocked spray bar nozzle so today I drilled out the alu plugs and pulled the spray bars to ensure completely clean.

I was in doubt whether to use sealant on the cylinder base gaskets and chose not to. Then I tightened up the cylinder heads before fitting the cam housings so might have to replace at least the head ring gaskets.. What do you advice ? Leave or replace ?

When I haven't run a motor, I just reuse the CE rings should something require removing heads before startup. I've not kept notes on this, but I've done it a couple of times. Haven't noticed any problems (blowby, damage to sealing surfaces, or the broken, fracured CE rings along with other damage some have posted pictures of).

My theory is that until heat (which is significant) is applied, these rings don't lose their spring qualities and thus their sealing capacity. But it is just a theory, and not tested in any scientific way.

But if I know I am doing a test fitting, I leave them out.