I tested all 4 of my t-stats (1978 8V71TIs) in a pan of heated water with 2 thermometers. They all started to open at 160 and were not fully open until about 180. If DD specifies 160s, I'll go with 160s. I think that it's likely that when you're running below 1200 the engine won't get above 160 with the thermostats completely closed. It just doesn't develop that much heat and there is a small amount of bypass designed into the cooling system even with the T-stats closed.

The big concern about running over 185 is primarily for 92 series engines, which have wet cylinder liners with o-rings at the top of the sleeves that can be cooked by high heat. 71 series dry liner engines don't have that problem. In fact, DD specs that it's OK to run them at 190.

Re: "clean cooling system" is critical, as you say, but so is coolant circulation. The condition of your raw water pump (ie good wear plates and cams) is critical. Also, somebody on this forum pointed out that the tolerances in your coolant pumps are also critical. I have had one engine with temp creep for 4 years. I have improved it a lot, but I still can't run it above 2000 RPMs in warm seas for over 30 minutes without hitting 190, so my next step this winter is to replace my old, likely original coolant pumps.

I thought that a centrifugal pump didn't have close tolerances and would just continue to run. Duhh! Since I've cleaned the cooling system, I'll bet that's the reason.

Doug Shuman