Does ambient temp effect ring gap measurements?

[bpu699]

So, branching of from my other thread...

Installed rings on my 88 930. New goetze rings. Cylinders in spec, less than .0003 out of round, less than .0005 taper. And yes, that's ten thousandths...

I am working in a barn. It's wisconsin, and 30 degrees outside. Heater is on, temps about 45 in the barn...

Old ring gaps about .60 mm on a 70,000 mile motor with good compression. <5% leakdown on 4 cylinders. One was 15% or so, and one much higher. The latter had a stuck ring.

So, put in the new rings. Gap is .40 to .45mm right off the bat. Normal is .2-.4. .8 is nonserviceable.

I put the rings on the part of the cylinder that had no contact or wear, gap slightly better at .38 or so.


Now, we all know that as a motor gets hot, the gap closes. Right? Of course.

So, I figured that at room temp, the gap will be smaller. So, I put the cylinder with the ring in it in front of a heater for 5 minutes. Rechecked the gap, and it was BIGGER?!

Wtf?

Googled expansion rates, and low and behold aluminum expands faster than steel. So the cylinder expands more than the ring... increasing. Gap...

But that doesn't make sense, as we all know the gap gets smaller with heat...

In my little experiment, is it just that the case heated up faster?

And more importantly, would ring gaps be tighter at room temp than a freezing barn? The same? Worse?

[BURN-BROS]

You are splitting hairs at these temps as we are dealing with temps around 400 degrees or more when at operating temps. But your observation is correct.

The bore, when heated will expand quite a bit. For every .001" diameter the cylinder grows the gap will increase by Pi. (.00314). However once temps set in to the ring it will grow along it's length catching up quickly. You will not observe this unless the ring was heated much higher.

I suspect the ring's operating temps is somewhere in 400-500 degree range and the cylinder will vary between 300 at the head and 190 and the spigot.

If you want to play around you can put a ring on a hot plate and measure the change, just don't go crazy with temps as you may be buying rings again.

[bpu699]

Thanks... I figured the internal cylinder temp had to be higher at the piston and ring rather than at the cylinder...

Good point about the ring expanding along its length...

[911pcars]

Ambient air temperature refers to the air surrounding the engine and possibly the air entering the intake tract. The temperature inside of a running engine is engine temperature.

You can accurately measure the ring end gap once the cylinder (aluminum, cast iron or steel) expands to normal operating dimensions. Or use manufacturer's specs. They've calculated that for you.

Sherwood