I know it's highly recommended to run a Robertshaw 333-xxx thermostat as opposed as a regular thermostat like Motorcraft RT139 on this forum and others.
Why? What's the difference? Is there a design flaw in the regular thermostat for the 351M/400? What?
I have both, and my born-out-of-wedlock 351M/400 needs a thermostat. Which one should I throw in? I live in an all-season area.
The whole issue is in the small brass flange pressed on to the bottom of the thermostat. If you go to a local parts store and get a cheap thermostat it will most likely not have that piece in the unit. The Robert-Shaw unit does and your Motorcraft probably also has it. The purpose behind that little piece is to close off the hole directly below the thermostat in the block. That hole controls the internal bypass of the coolant system. When that hole is open coolant doesn't flow much past the rear 2 cylinders in the block, thus it gives it time to get hotter and give you heat quicker in the winter months. When the hole is closed off by the thermostat opening it forces more coolant by the rear 2 cylinders and properly cools the engine. One of Ford's "better" ideas. Constant overheating of the 2 rear cylinders causes the rings to wear faster and cause a loss of compression and the burning of more oil.
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Working on my truck is like opening Pandora's Box.
it's not just highly recommended, it's the correct thermostat. any other type is the wrong thermostat. many parts books and computer systems list the wrong Windsor part number.
The RobertShaw thermo doesn't have any provisions for blocking off anything! It doesn't even move up and down outside of it's own housing, it rotates internally
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Bear Tracks
1977 F-100 Ranger XLT,400,C6,GV
Mod's Listed in Gallery
I did some homework on thermostats for our beloved (ugh) engines. I found interesting results. Read on, peons!
Here is the line up. I have labeled each thermostat so you can see the differences.
Quote:
Originally Posted by grclark351
it's not just highly recommended, it's the correct thermostat. any other type is the wrong thermostat. many parts books and computer systems list the wrong Windsor part number.
After inspecting all kinds of thermostats, I have to strongly disagree with you. If you can't find the correct thermostat, you're not doing enough. My stand has always been this: stay with OEM parts if possible. As you can see, Ford has one for the 351M/400 -
Motorcraft RT139.
That's for the 192F. Now, here is where the confusion lie. There is also a Motorcraft RT1139 which is for the 351 Windsor engine only. RT139 is for the 351 Modified. I also have a RT1139 handy, and it's an entirely different thermostat. Ford/Motorcraft does offer two other thermostat that opens at different temperatures -
Motorcraft RT351 - 180F
Motorcraft RT350 - 160F
I'm not sure if they are the EXACTLY same thermostats as the RT139.
The pellets as shown.
Close-up of the Motorcraft RT139 pellet.
As you can see, when compared to the RT139, the aftermarket one has nothing more than what is required for a thermostat. They should NEVER be used. I haven't seen an aftermarket one that's different than this. I don't know its brand. This was taken out of a used engine. Since we can't see the internals of the Robertshaw 333-192, a picture of it will not be shown.
The Motorcraft RT139 has an extra copper pellet. The Robertshaw does also from what I can see. For what purpose, I'm not sure. Perhaps it is what garypettengill stated - the extra provision to close off the internal bypass.
By the way, garypettengill - GREAT info! That explained why I lost two 351M/400 engines, both exhibiting issues with the two rear cylinders. I never did figure out why until now. One engine blew its head gasket where the rear two cylinders were; one engine burnt valves in the same location. I learned something invaluable here tonight.
I am not done. I need to do the best part - BOILING THEM FER SUPPER! Well, no...to see how they work. I'll update with pics and my conclusion. Maybe I'll get to see what the extra pellet does.
it's not just highly recommended, it's the correct thermostat. any other type is the wrong thermostat. many parts books and computer systems list the wrong Windsor part number.
Quote:
Originally Posted by Jermafenser
After inspecting all kinds of thermostats, I have to strongly disagree with you. If you can't find the correct thermostat, you're not doing enough.
i never said that i've ever had a problem finding the correct thermostat, what i said is that the countermonkey has incorrect info to work with, but it's also possible that they are incorrectly reading the info. RT139 and RT1139, can you see how a mistake might be made? blame that on Motorcraft, was there a shortage of unused part numbers that day?
i didn't spell out that by 'type' i meant internal bypass type, as i thought Gary did explain that,
however, the operation of the internal bypass has absolutely nothing to do with coolant flow at or near the rear two cylinders, which i took to mean #4 and #8, on opposite sides of the block. the internal bypass allows not yet hot coolant to flow from the thermostat to the water pump through an internal passage in the block. the not yet hot coolant from the bypass is then recirculated back into the block, in through the two water pump ports at the front of the engine.
when the coolant reaches operating temp, the Tstat opens and blocks off the bypass, then all coolant is routed to the radiator.
how can the bypass being left open after reaching operating temp affect only the rear cylinders if the hot bypass water re-enters the block at the front?
if the cooling system is low on coolant, there may not be much if any coolant going to the radiator, the low-level coolant will stay down in the block and not be forced upward through the Tstat by the system being full.
overall engine temp will increase, but there's no way to isolate the rear cylinders to feel the excess heat. the holes in the intake gasket determine how much coolant flows and where.
I am not done. I need to do the best part - BOILING THEM FER SUPPER! Well, no...to see how they work. I'll update with pics and my conclusion. Maybe I'll get to see what the extra pellet does.
Stay tuned, folks.
What happened to our updates?
Based on the info found in this thread, I am taking off the O'Reilly Murray 3469 (store brand) thermostat that I bought yesterday. After further research, I see that O'Reilly references RT1139 for both the 400M & 351M, so their store brand is likely not the proper thermostat for my truck. It's not worth risking my motor on a $7 store brand thermostat. I will try to get my hands on the RT139 and put it in my truck.
Thanks for all the info in this thread!
__________________ Gallery 1978 F250 2WD Pickup built at the Norfolk plant in March 1978 and powered by a 8 cyl 400 cid 2-bbl engine.
I wrote a long post on this very subject a couple years ago. I've owned my '77 F-150 4x4 351M since 1986.
With the 351M / 400s, the bypass is cast into the block directly under the thermostat. You have to leave it open so water will circulate when the T-stat is closed, but you want restriction in the bypass when the T-stat opens so coolant is forced through the radiator to get cooled.
Of the T-stats I've looked at using over the years, only the stock Motorcraft T-stat and the Robert Shaw 333-xxx T-stats are designed to restrict the opening to the bypass when the T-stat opens, and they both do it increasingly as the T-stat opens.
Other T-stats I found listed for the 351M/400 motors are simply listed for it based on outside diameter at the flange with no appearant thought to the bypass in these motors. Run one of these other ones and even when the T-stat opens, much coolant will still take the path of least resistance through the bypass and never see a radiator, just carry their heat back into the motor, and the motor will get hotter. I suspect that with it hotter up front while some coolant gets cooled, some don't, that likely the rear end of the motor so far away might get really hot.
The other small blocks use external bypass hoses with kinks to restrict, and they restrict the same allways. The 351M/400 has a restriction that varies with need.
Do not block the bypass permanently with sealant either, you'll simply cavitate the coolant in the water pump, churn it needlessly. When the T-stat does open, the coolant may not flow well because the pump vanes are spinning in a cavitated mess and coolant has mass .... once stopped in it's tracks it must be started moving again. You want that fl;ow to be steady through the water pump, hence the bypass that varies with need.
No doubt some have lost one of these engines to heat by failing to understand it.
I got my best cooling with a Robert Shaw 333-180 and a aluminum disc riveted to the pump vanes with 3 stainless steel rivets tokeep coolant on the vanes with minimum spilloff. She's fine even on the hottest days with AC on at 70.
The Robert Shaw 333-xxx does have higher flow at same pressure than the Motorcraft one.
Found the Robertshaw on Jeff's Bronco Graveyard. My local Ford dealership wouldn't order the Motorcraft thermostat for me. Going with the Robertshaw.
Wow, I wrote that 20 months ago too. Like I said then, the Robert Shaw is all I'll run in a 351M or 400.
Well, I'm convinced that the 351M gets the same mileage as a 400 with 2/3 the power and the worn bearings as a side benifit. My '77 still has the 351 in it, with the goodies I added. Oil pressure still good now, but I don't drive her a lot, not much over 102,000 on her now, or about 27,000 on those new main and rod bearings.
Good truck, gonna make some one a fun project here when I get around to listing it and all that goes with her, including a complete rebuilt 400 shortblock, new pistons too, but less cam.
From what I see on this is , all you have to do is make sure your replacement thermostat has the flange attached to the pellet , it's the little flange that blocks off the bypass, like in the pic of the RT139 motorcraft,
my NAPA replacement has this flange,
My Ford dealership said the RT139 is obsolete
I tried a Robertshaw and the thing would not open overheated my new engine,very upset with that so guess I will not be buying another one of those EVER!!!!
Thanks
Benny
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