[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?