David,
The "smoke-filled bubbles" may not be the answer. I even tried buying some of the fluid that's supposed to detect exhaust gasses in the coolant. No joy.
If the o-ring/groove-wall failure is in the compression area you won't detect any gasses.
If you remove spark plugs and see green (or red) stuff coming out when you turn over the engine, you have the problem. Overhaul time.

My temporary solution until it got so bad that the green stuff came out of the sparkplug holes, was to use a windshieldwiper bottle (uncluding the built-in pump) as the overflow bottle. The pump feeds a check valve threaded into the waterpump inlet.

I left it in place after the overhaul. Makes it easy to get the air out, start the pump, easy the pressure cap half-way off and lift up the tail of the airplane (I have high spots in hoses to radiators when tail is on the ground (RV-3).

Finn

David Leonard wrote:

Bob, I have heard that expansion is about 4-5% of the volume of coolant that you have.  Mine expands about 1.5 cups on 2 gal of coolant.
--
Dave Leonard