I have twin Mark IV 454s and just installed 140 deg tstats (drilled 3 3/16" holes). Ran for 40 mins at fast cruise. Water held steady at 165 all day. 3500 RPMs were seeing oil temp @ 265 deg and climbing. I pulled back to 3000 RPMs and it held steady at about 260. I have the stock coolers in the bell housings. I want to move to external coolers but have no idea what size to get.

Any suggestions or input?

add'l info.. the motors are 400HP and push a 30' Scarab

Not being stupid but buy the biggest ones you can afford. Mine is the size used for a tractor. It's almost 3 ft long temps never go over 140. 600cu in motor 6k rpm. Not saying you need tha big but it's insurance fo your bearings.

Your oil temps never go over 140º F???? That is not good for your engine.

Yeah, and that's why I'm asking. I don't want to go too big and not get enough heat. I was just hoping someone would have a similar setup and tell me what they were using with their oil temps.

Go to the New Products and Members discount forums.
Cp Performance and Mr. Cool are both having sales now and either can help you out.

Just call them, thats what they do for a living.

I just went through this....What you need to do is get coolers that will keep your oil temp at 140 and then buy oil thermostats, its really the only way to go, that way oil will always be at 212 which is what the hp500 oil thermostats are or you can order them from summit or KE makes them on CP too. That way the oil is only going through the cooler whwn needed. Mine only run through coolers when im wide open most of the time its at 180-190 then never more than 225 WO for a few miles. I ended up going with HP500 oil t-stat just because its a nice clean install and oil filter mounts to it on the back side of right head. You will also pick up oil pressure with short lines, most of your oil will just go to filter then back to block unless your hammering it over 212 and that where the larger cooler comes into play. hope this helps. I run a 16 cooler with -12am hoses from CP performance on 450hp 502.

Gentlemen, Thank you!

These will work fine!

http://www.hardin-marine.com/detail~ID~849.aspx

That is the one i have. Couldn't get more than 140 degrees with 450hp 502 out of the oil temp, so i had to do the hp500 oil thermostat. It does depend on the water temp too if you dont have oil thermstat, Here in new england, salt early spring water temp cause alot of low oil temps so having an oil thermostat solve it.