Restrictors vs thermostats

**Merc 2.5** · 10-08-2025, 06:53 AM

So is anyone runnin washers instead stats on stock poppet cooling system? Im on the fence with stock cooling runnin 7k but I usually dont hold it there for real long periods of time. I do have a dump off each head and I like being able see instant tell tale and knowing i dont have a sticking thermostat or something. If I was to run washers , I know it takes little longer to get to temp but what is a good size hole to control idle and wot temps?

**mercrazy** · 10-08-2025, 07:35 AM

I’ve always been told to use thermostats with steel sleeves. I’ve seen a lot of optimaxs that cold seized so I run em in my stuff.

**rjdubiel** · 10-08-2025, 07:58 AM

Originally Posted by Merc 2.5

So is anyone runnin washers instead stats on stock poppet cooling system? Im on the fence with stock cooling runnin 7k but I usually dont hold it there for real long periods of time. I do have a dump off each head and I like being able see instant tell tale and knowing i dont have a sticking thermostat or something. If I was to run washers , I know it takes little longer to get to temp but what is a good size hole to control idle and wot temps?

At only 7K and not for a long time leave it as is for my opinion. 1/8 inch fender washers work, have a 3/16 hole in them and will still hold some water in the block as the hoses off the heads are for bleeding air VS dumping water for cooling. If you remove the poppet your issue is now you dump water all the time VS holding water in the block to warm it up until you are on the throttle at which time the poppet opens and dumps max water. I dont think you will see much difference between stats and washers once the motor is at temp.

https://www.screamandfly.com/showthr...d-Poppet-Valve

http://www.us1.cc/

**VkingMike** · 10-08-2025, 08:07 AM

Good info there.

If you're running a stock cooling system, use stock parts (thermostats) They work..

**KIRCHNER** · 10-08-2025, 05:43 PM

as far as cooling goes, i put a red led, and a cheap-o temp alarm on all my 3 merc powered hp boats. today it saved my butt a 3rd time. as soon as i planned off, the alarm went off. i dont like starring at the gauges, at high speed. the audible alarm was [is] a must. sure enough my impellor was trashed. i was all set for a speed run, new injectors. i would of cooked it for sure. got the tiny alarms on ebay, and super bright led a blind man could see,lol livin the dream Art.

**tlwjkw** · 10-08-2025, 10:21 PM

knew several guys with stock coolin' that ran washers with 3/8" holes in tha warmer water and summer time without any problems.. cold water, not so much...........