Washer behind #3 cyl?

I am currently working on the baffles. A quick search with "washer behind cylinder" reveals plenty of advice to put one behind #3 to even out or even control the CHT. (I undertand we are talking about a 1/16" gap.)

My question is why? Mostly we are trying to force air BETWEEN the fins on the cylinders, not let it escape down other routes. What is the idea here?

I take it this advice applies to the -4?

Thanks.

CHT#3

Hi Steve,

I think it is an airflow mass thing: the washer lets more air flow down this route to carry away the heat, rather than choking the flow. I don't have a multi channell CHT/EGT gizmo in my RV-4, but after reading all this some time ago I moved my single CHT probe from #4 to #3. There was no readable difference at the same power settings.

I'd do nothing until test flying, and make your decision once you have some figures.

The problem is that the fins have zero depth right where the baffle normally hits the aft side of #3 or the front side of #2. Not all the way left and right, but you'll see the area of interest. The design is quite dumb, but that is the way it is. So, without any spacing, no air can get to the fins below centerline at that location. Look at the fin design and it will make sense why some sort of spacer is needed in those locations.

I put about a .200" gap there, and it dropped #3's temp by about 30F degrees. I already had a gap in front of #2. I used RTV to force the air from this gap to the fins of interest.

I put the washer there too and have had a subsequent and significant INCREASE in CHT on no 3.

Washer coming out at next MPI.

My #2 was the hottest.

So I put a washer in front of the baffle there. It was very hard to install, but did help even out the temps.

#3 is ok on my engine.

Kent

I did the washer thing on my O-290 AND put a 1/2" strip of aluminum tape from the Aviation isle at Lowes on the front two cylinders. In cruise my CHT's are within 3 degrees of each other.

While climbing, #3 is always the hottest.

Thanks for the replies and the explanation. Having had another closer look at the fins, I understand a bit more now. I think I will a) build it with a washer behind #3, b)fly it, and take it from there.

A bit of other work between a) and b).

Thanks.