The Stovebolt Site Forums The Shop Area Engine Shop 261 bypass thermostat housing question

The water pump with two 'heater' ports is made so that each port just shares the flow. The bottom thermostat housing is the same way sharing the flow with the top thermostat housing. It is customarily connected:

1: Upper T-stat housing to upper waterpump connection #1
2: Lower T-stat housing to heater
3: Heater to waterpump connection #2

I think that is right. Others will say if it's wrong. The Heater Valve you refer to is for closing off the flow/heat during the summer months. It is really a duplicate circuit with the true bypass thermostat housings. You can run two heaters if you choose, or you can run two bypasses.

The way Deve describes the installation is correct based on pictures that I have seen from another website. I copied the pictures and posted them earlier in this thread if you want to see the connections.

Matt

Isn't the lower t stat housing just going to continually by-pass hot coolant directly back into the water pump ,wouldn't this cause an overheating situation.

I think this is the installation picture that Maybelline refers to?

http://i269.photobucket.com/albums/jj52/Kustom52Mopar/1523C745-39C5-4FC8-A9A2-E17F1BDB9888.jpg

No because the thermostat opens before then. Once the thermostat opens, the entire system won't see more than 200 degrees or so depending on the rating of the thermostat plus about 10-20 degrees in soak temperature. Soak being after you turn the engine off, temperature rises a bit before it goes down.

So you say your Canadian GMC 920 (3/4 ton) came stock with a 261? Are you sure? I know they put the 261 in Canadian Pontiac cars, but always thought GMC had their own stream of engines.

Canadian GMCs used Chevrolet engines - up to big trucks (in GMC terms, Chevrolet did not sell big trucks). Canadian GMC big trucks used what we (USAers) think of as GMC engines.

As you know (but, other may not): Canadian-car 261s came with hydraulic lifters - all USA 261s came from the factory with mechanical lifters. I assume the 58-63 Canadian GMC truck 261s came with mechanical lifters (at least the 1954 Canadian truck 261 has mechanical lifters).

GMC Kid, the concern that you mention above about the bypass always sending coolant to the water pump and bypassing the radiator whether the thermostat is open or not is correct from what I can tell with the NAPA thermostat. The one that Tim showed closes off the bypass, the NAPA one does not.

As Deve points out, it might not matter. It just doesn't make sense to me to send any coolant from a hot engine directly back to the engine without going through the radiator first.

If I could fine a copy of Tim's thermostat, I would use it on my 261. I really like the design of how the bypass works with Tim's thermostat. Without it, I will use a 235 thermostat and housing. I'm not comfortable circulating some hot coolant back to a hot engine by skipping the radiator which the NAPA thermostat would do.

Matt

When I look at the picture of the 261 with the bypass hose going from the water pump to the top side of the thermostat, I don't see it the same way as you do.
What I see is that there is water circulating around the closed thermostat via the bypass hose from the water pump, sending water to the radiator via the upper radiator hose. When the thermostat is closed, it cannot send any water back into the engine via the bypass hose. It all would go to the radiator upper hose.
Carl

The Canadian GMCs came stock with 261s, the engine in my truck is a numbers matching truck and engine,it is a solid lifter engine. So if you look at the picture the upper thermo housing has a hose that goes to the water pump ,the flow goes from the housing to the pump , the lower thermo housing goes through the heater back to the water pump.Would this be correct? If so would the heater circuit always be acting like a by-pass no matter what position the thermostat was in? I have been a Diesel engine mechanic for 25 years the engines I have worked on all had by-pass circuits on them ,but the thermostats were double acting,as they opened to rad they simultaneously closed off the by-pass until it was full open to rad and the by-pass was completely closed off. If the seal blocking off the by-pass leaked the engines were prone to overheat.

GMC Kid, the installation is as you describe. The heater circuit only acts as a bypass if the heater temperature control is moved toward hot. If set to cold, the valve that opens the coolant flow to the heater core will be closed, so coolant will not flow through the loop.

The way you describe the double acting thermostats you are familiar with work the way Tim's does by closing off the bypass when the thermostat opens. The NAPA one does not.

Matt

The Bypass T-stat is very tall and it only closes off the actual radiator hose outlet in the upper housing. The coolant in the lower part of the upper housing is on the high pressure side of the pump. If the radiator outlet is closed the coolant bypasses back to the low presure side of the pump.

I only find evidence the "16 hole - open bypass" T-stat was used in 1962 (261) and again in 1964 (292 & 348).

Mike B

I've been following this discussion with interest. If I could find a bypass thermostat housing and a proper bypass thermostat, I'd install it. Apparently, the NAPA number doesn't actually bypass. Did I read this right?

I also have worked with heavy truck engines and realize the importance of the bypass system. It's common on many gas and diesel engines. BMW even used such a system on cars, as did Mercedes diesels.

Here's how it looks on the road. I have been running a 235 in a pickup that's worked really hard pulling a camper out West and I've been concerned about hot spots. I had the head off for no-lead seats a few years ago and noticed scuffing on #5 cylinder that indicates it's run hot. I believe that #5 is a hot spot and that's why they put the temp. sensor at #5 on the head. hot spotting is not a problem in winter, of course because the heater valve is open. But in summer it's closed. Running on a mountain highway in summer, when you come to the bottom of a long downhill the temp gauge will read south of 160, starting up the other side the engine is a full throttle with the therostat closed and the internal temperature comes up very quickly. Often, mine hits 230 by the top of the next hill. Then, going over the top the engine cools very quickly until the thermostat has a chance to close (2-ton radiator). So with a properly designed bypass system the temperature swings would be reduced. And bet of all the hot spotting would be much reduced.

Tommy, you are correct about the NAPA thermostat. It properly bypasses with the thermostat closed but does not shut off the bypass when the thermostat opens.

I live in Arizona. I wish I could install the bypass system for all the reasons that you discuss. I drive the mountains in the desert heat and have added A/C, so I have the same concerns as you.

Matt

Anybody got a bypass housing that could be used as a pattern for a CNC mill program? My son-in-law could reverse-engineer one and write the program for whittling out copies from a block of aluminum. He's a CNC mill programmer for Remington in Huntsville Alabama.
Jerry

I stumbled across this diagram that shows how a bypass thermostat works, so I thought that I would share.

https://www.google.com/imgres?imgurl=http://www.agcoauto.com/content/images/cooling/engine_thermostat_flow_open_closed.jpg&imgrefurl=http://www.agcoauto.com/content/news/p2_articleid/191&h=361&w=550&tbnid=R5Dbs0qfsRtmpM:&tbnh=120&tbnw=184&docid=SyjT6_2rLUdJLM&usg=__cACZnxdDZv-8WKEbEd_Zw7ntBug=&sa=X&ved=0ahUKEwjyipzy-PrLAhUI7SYKHb-4DuEQ9QEIIDAA

Matt

That long URL you posted sure messes with the margins on the page! Good diagram of a bypass system, though. Recycling a large volume of coolant back to the water pump inlet keeps hot spots from developing around the siamesed cylinders on the 261, especially with a heavy load onboard. That's why the system was used primarily on the heavy-duty rigs.
Jerry

Click on the attached for the next chapter in this discussion on an easy way to make a bypass system out of a stock 216/235 thermostat housing.

I am adding this here to link the discussions as an aid to anyone doing a search on the subject in the future.

https://www.stovebolt.com/ubbthread...ing_a_216/235_thermosta.html#Post1163318

Matt

So leaving your heater in the hot position when first starting your engine does the exact same thing then right?

the three 261 blocks i have right in front of me have nothing that resembles a bypass in the thermostat housing.

Or instead of doing all this why not just install a nice clean looking spacer like Tom Langdon makes for the water heated intake and flip it or add a second port to the other side to allow for more flow. The spacer as it is already allows circulation within the block as the lines are connected to both the pump and the thermostat housing. I already have the spacer installed on my 235 and its a nice made unit.

Here is a link for some other install but its the same thing.

Spacer PIc

Or, just drill a hole in the thermostat "plate" and be done with it.

or that....use about a 1/8" hole?

That is what I use.

Having the heater on hot when starting the engine provides a bypass but will not turn off by itself.

The spacer will not work because it does not have a shut off. If plumbed as suggested, the bypass circuit will be open all of the time.

The 1/8" hole in the thermostat flange will allow trapped air to escape when filling but will not provide enough flow to create an effective bypass circuit. Plus it undermines the integrity of the thermostat (not much) because it always allows coolant to flow past the thermostat, open or closed. If releasing trapped air is a feature wanted in a thermostat, buying one with a jiggle pin is a better alternative.

Matt

Whats wrong with a small bypass pipe possibly 1/4" in size always being open? Mine has been running for 3 seasons on it now.

Yes, about the hole serving as an "air bleeder".

The bottom line is that GM did not use a bypass on these engines, except on some of the heaviest trucks. There is nothing wrong about using/wanting a bypass, but it is not necessary on a light truck that experiences normal driving conditions.

Wolf,

The heater is actually nothing more than a small radiator, so by opening flow to it during engine warm-up is counter productive, it's cooling the engine coolant making it take longer to get up to operating temperature.

Your spacer by-pass doesn't close off when the T-stat opens to the radiator...allowing a small amount of coolant to return to the block without being cooled by the radiator.

Mike B

My 1962 3/4 ton gmc came with this by-pass , I just recently got the truck on the road by-pass system seems to work temp sits at 185 f I used the the 3 piece Napa 180 f thermostat .It all seems to be working the way it should.

I have not read the whole post however i reciently aquired a 261 housing and although it bolts to the 235 block it uses the same gaskest from the block to the water outlet but the gaskest from the water outlet to the top thermostat cover is larger than the 235 the bottom water outlet is larger than the 235 outlet. So you can not use a water outlet from a 261 with the bypass extra port and mate it to a 235 top housing or at least with the pieces i have you cant. The tgermostat is easy to get from NAPA or the flaps usually in a day or 2 just fyi.

The thermostat housing is taller. I made my own gasket for the water housing and thermostat housing and kept a template so its easier next time. I have not found one for the 261 housing from the aotoparts store but sure they are out there.

On my application i used the bypass port for a mechanical water sensor outlet and not for the bypass hose as it was designed for. After fiquring it all out i saw last month tom from stovebolt engine company is not making a similiar after market housing also. Mine still a working in progress doing it my way. Hope the pics helps. Plus i had my 261 housing powder coated with a ceramic chrome.
Cheers

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