I HAVE 3 525SC IN A 42 FOUNTAIN.TOOK MOTORS OUT
RE-VAMPED THEM OVER THE WINTER TIME.TOOK OFF
CIRCULATOR PUMPS AND PUT CROSS OVERS ON.
LIKE TO KNOW HOW IMPORTANT OIL COOLERS ARE,IF THE MOTORS RAN AT 135 DEGREES STOCK WITH
THEROMSTATS HOW COOL WOULD THEY RUN WITH OUT
THEROMSTATS AND OIL COOLER.PLEASE RESPOND

I think an Oil T-Stat is important to help control oil temp if for some reason your oil is running cold. I have seen the effects of too cold of oil. Not good.
Where do you do your boating?

Had the same motors, without oil thermostats. You absolutely need the coolers. The only way to know for sure if you need oil thermostats is to run the motor and check oil temp. If you get up to around 220 degrees, you're fine. I just make sure the oil gets up to that temp on all my boats every time I run it.
Gary

You can get Perma-cool oil thermostats at Summit for less than 40.00.

Oil coolers are a must on those powerplants. I would run thermostats in them.

Chris

The 135 degrees is your water temp (I assume), which while important doesn't tell your your oil temp as mentioned. You will need the oil cooler nonetheless, and should check your oil temp as well.

If you are talking about running without water thermostats, depending where you boat, you probably won't build any temp up. I have tried that, and the motors never warm up without t-stats.

Also, if you removed the circ pump for crossovers, you now should check your water pressure at full throttle on all motors, as now you can overpressure the cooling system, and start blowing intake and head gaskets if the pressure goes too high. Merc makes a dump valve to install before the crossover to relieve water pressure. Do a search on "water pressure" for more info.