after market heat exchanger? I have my regulator heat exchanger and my "warm water" heat exchanger. When we head south into warmer water I install the additional heat exchanger into the loop. It's just a matter of running a few hosed between the engine and extra heat exchanger that I have mounted on the bulkhead. When we get back into cold water we change back. Actually, I could just leave it (the second one) in place all the time but I don't because I still need to replace zinks and do routine maintenance on it so I take it out of the loop. I had the same situation. Not over propped, great running engine, runs fine at usual cruising speed but in warmer water if you pushed it past what "usually feels like a comfortable cruising speed" it all of a sudden just takes off and 2 minutes later the alarm is going off. The second heat exchanger fixed that. You still have to do heat exchanger maintenance like cleaning and changing zinks, in fact I'm doing that today, but the over heating is a thing of the past. In an emergency I can push it as hard as I want and it never gets past 180. I was coming up the coast from Cabo San Lucas 2 years ago and the weather turned so bad that with a double reefed main, stay sail and the engine running at full normal RPM's we were actually going backwards over the ground. I had to push the engine past normal in order to make the point but the engine never heated up. By the time we got into the lee of Cabo Colonet where we were going to anchor the wind dropped back to 25 and we were able to keep going. It's nice to know you can push your engine past normal once in a while.