This may be a really dumb question, t I'd rather ask it before I blow up my motor.

How necessary is setting piston ring gaps? I put in some NOS Goetze rings that seemed to fit fine. The engine is harder to turn over by hand now. Maybe just good compression?

If setting the gaps is necessary, why don't they just make the right size at the factory?

Thanks,
Konrad

And thanks to @iancarr for this up in the latest video.

You need to verify the gap based on piston manufacturer recommendations and increase if necessary. Rings expand when they heat up, and you want them just right to create a good seal against the cylinder wall when hot. If the gap is not big enough, the ring ends can interfere with each other. The rings may get wonky in the ring groove and not create a good seal, or, worse, break. At that point you've lost compression on that cylinder, and bits of the ring may be find their way past the cylinder/piston and do damage somewhere else.

The rings may come out of the box at or close to spec. But it's very important to check.

Put rings piston and cylinder in the freezer overnight, press the rings one at a time into the cylinder using the piston to square it up, check each ring gap when cold, if gaps are to small adjust.

Too small a gap could be very Bad. too large, not so much.

KB pistons call for a larger top ring gap.

Jus sayin'

Like everyone said above, they have to be checked and adjusted to spec. Don't wing it.

Glad you like the video. Everyone here has given you great advice....

@mgphoto I haven't heard of freezing the rings. What's the advantage of cold vs room temp?

The gap will be at its smallest, everything will expand from there, I checked my notes from my build biggest gap was .013 and the smallest was .005 at a temperature of 13-22* F.

It also depends on the manufacturer.

I have never had to change the ring gap on JE Pistons in LN Engineering Nikie cylinders.

Conversely, the AA pistons and cylinders I bought through Wal-Mart to put on Betty's engine had to have every one of them filed off because they were all too tight.


But I still checked every single one of them.

Clay

Thanks for the advice! It’s going to suck pulling the heads, jugs, and pistons, but it will suck a lot less than a blown up motor.

Wouldn't that be the largest gap when cold? Also...pretty sure the manufacturers' instructions on setting gaps is based on room temperature...but maybe the engine gurus can chime in on this.

Largest gap occurs cold. Physics 101.

Gaps specified at room temp.

Tightest gap occurs when hot and gap is established such that ends of the ring never touch when hot. If they touch when hot, things go south very quickly.

I’ve only seen 911 engine builders do it at room temp. That’s probably the temp the manufacturer bases it on.

Whatever the gap is that is the difference every part and every material will grow when hot.

Note on aircooled nickies I run a .005 gap on a 102mm piston because I can, the pistons and cylinders have a similar expansion rate.

My 3.0/6 has Mahle pistons and re-plated cylinders, I run factory spec gaps, but an interesting note is the piston is way tighter in the bore than a JE piston in a nickie. So tight I was worried about it seizing, but it runs perfectly.

First law of thermodynamics, it has to do with expansion rates.
Remember metal gets smaller when cold (bridge expansion grates), your barrels, pistons and rings expand at near the same rate, if not then ka-boom.
Probably 102...

First law of thermodynamics pertains to the conservation of energy. Open to discussion of exactly how that affects setting of ring gaps as measured at a constant temperature when setting them.

I think we might be talking about different things? You are correct that the cylinder bore shrinks as it gets cold. Same with the ring. When I stated largest gap is cold, I was referring to transient effects. Ring gap does get tighter during tranient conditions from cold start to operating temperature and under high load transient conditons that heat the ring quicker than the cylinder. The whole purpose of the gap in the first place is to account for these transients.

If the gaps is set at ambient temperature (both ring & cylinder), the relative gap stays the same regardless of cold or hot temp on a T4 engine with cast iron cylinders and steel rings since the coefficent of thermal expansion is nearly idential.

If you were installing cold rings into an cold cylinder of the same temperature and then checking ring gaps it would all work out too. I belive this is what you were recommending.

Freezing the rings but not the cylinders and then setting the ring gap will result in a tighter ring gap once the rings come to equalibrium temperature with the cylinder. I hope no one is doing this. In practice the difference isn't going to be huge. About 0.006", assuming a 50F temperature delta between freezing and ambient.

Click to view attachment

As the example, if there were a minimum gap of 0.005" with frozen rings and an ambient temperaure cylinder, there would be almost no gap when installed and the ring eventually comes up to ambient temperature of the cylinder.

Here's why that can work out on a set of Nickies.

Piston and Cylinder are Aluminum. Aluminum coefficient of thermal expansion is almost two times higher than that of steel (i.e. the rings).

As you state the piston and the cylinder are expanding at identical rates. This keeps pistons from scuffing and/or collapsing the skirts with your relatively tight Mahle pistons. If you had those kind of tight piston fits in a steel cylinder you would have problems. Not so with Nickies and/or Porsche OEM 2.7L cylinders and larger that are aluminum.

Since the aluminum piston bore is expanding faster than the steel ring and you'll get a slight increase in ring gap as things are at operating temprature. I have a set of Nickies and JE pistons on reserve for a future project. They are still recommending a .004" gap / inch of piston bore though. Have you ever done leakdown on that engine? It must be awesome! Gotta' be close to using Total Seal Ring!

In typical situation for T4 engine with cast iron cylinder and steel ring. Agree completely with your previous comment that ring gap stays roughtly the same since the steel ring and the cast iron cylinder have nearly identical expansion rates.

Interesting fact. Formula 1 engines run almost zero ring gap at ambient temperature.

Same principle as Nickies, aluminum block, steel ring. In order to make this work, you'll notice the engines are pre-heated prior with an external heating pack that pumps hot coolant though the block. This warms the block and provides operating clearance for the ring at operating temperature. This pre-heat sequence is done prior to firing up the engine.

Otherwise the thermal expansion resulting from combustion will heat the top ring much faster (initially at start up) than the aluminum block. You woud get ring breakage since the ring was starting with zero ring gap at ambient and would initially expand the ring faster than the block (which has a lot of thermal mass as compared to a piston ring).

Another intersting fact is that the rings are the main heat transfer mechanism from piston dome to cylinder walls. In a 911 engine oil squirters are another mechanism to cool the piston dome but most engines don't have piston oil squirters and the rings are the main heat transfer mechanism.

Rings seem really simple but they serve multiple functions. Sealing, heat transfer, and oil control. A lot to ask from a little piece of spring steel.