I'm replacing the thermostats and housing gaskets on my '81 Formula with twin 260 Mercruisers. There is a large square opening outside of the thermostat that allows water to go around the stat before it opens. Here's the gasket that's specified for mine http://www.boatersland.com/barmc47-27-48818.html The later 260 Mercs ('82 and beyond I believe) have a housing gasket that blocks off that square water passage so that no water gets past the stat. My engine also specs a 142 degree thermostat. Why don't these have 160 degree stats, and what is the purpose of the bypass that allows water to go around the stat before it gets up to operating temp? I'm pretty sure there are hoses that feed the exhaust risers directly from the water pump, so that can't be a reason.

All Merc carbed engines have 140* Tstats.

The water bypasses the Tstat to go to the exhaust. If didn't flow there, it would build too much pressure in the hose feeding the Tstat.

I believe when the TStat opens, this water goes to the exhaust manifolds from the bottom. There is constant and separate water path to the risers which does not run through the block and is why the tops of the risers are always much cooler to the touch than the manifolds. I would say that if the stat housing has a bypass you would need a gasket for this configuration and not block off the flow.

Is there any reason that I can't (or shouldn't) use a 160 degree stat? 140 seems a bit cool, but maybe there's a specific reason for that.

30 years of running with a 140* one should tell you that its working fine and there is no reason to switch.

When I bought my boat, the 7.4 had a 160 stat in it. No harm, no foul I thought. Unitl I checked oil temp - even a moderate run at 4000 rpm got the oil temp to 250. Wide open, and it hit 270 or so in just a few minutes, and oil pressure dropped to less than 40 lbs (this is how I noticed the issue).

Following the advice of some knowledgable folks, I changed the stat to 140*. Oil temp runs no more than 240 even after 20 minutes WFO, and pressure holds at 60 lbs. I've noticed no difference in performance or fuel use.

Moral of the story: put in what Merc designed, it's a system.

Seems logical to me -- thanks.

1996 and back oem mercruiser & volvo carb motors used 140 degree t-stats. Model year 1997 thru current carb motors use 160 degree t-stat unless its closed cooling inwhich uses a 170 degree t-stat..

What are the technical/thermodynamic differences that require the use of a lower temperature thermostat in open cooling systems as compared to higher temp stats in closed systems?