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

Update: The AC/Delco 12T11B is a standard 160 degree thermostat for the 235 style housing. Just got it in today in an original box. If anyone knows of a thermostat that fits the bypass housing, please share.

Deve,

Does that thermostat have a 2.125" diameter?

I'll measure my 261 thermostats tomorrow.

The 1960 Shop Manual show the following 261 thermostat: Dole DV-50 (Dole Pellet Bypass - supposedly also the Harrison T180 thermostat).

It looks like the by-pass was a part of the thermostat?

http://www.google.com/patents/US3157357

" . . . a thermostat valve arrangement including a valve casing adapted to be mounted in a two-way housing for conducting fluid, a main valve being adapted to control flow through one way leading through the housing and a movable portion of a pellet-type motor being arranged not only to operate the main valve but also to control fluid flow through the second way leading through the housing . . "

This style thermostat started being used by GM in the 283 (and in later 261) engines.

Yes. The diameter is 2-1/8" which is for the small 235 thermostat housing.

The one we need for those larger housings needs to be 2.5 inches in diameter. There were no numbers on the two different styles of the larger ones that were helpful in the investigation.

http://devestechnet.com/Images/Projects/Venerable261/261Thermostat023lg.jpg

The middle one is the 12T11B.

The NAPA THM155 thermostat has a 2.5" diameter. It is 180 degrees. Here is a link to pictures showing the thermostat and how it sits in the upper and lower 261 bypass housing. I also took pictures of the NAPA box showing the specs and the UPC code.

http://s844.photobucket.com/user/ma...1367_zps84yrah8s.jpg.html?sort=3&o=0

Matt

Here is a web page with photos of a 1954 235 thermostat housing and a 1959/1960 261 thermostat housing.

If anyone sees a mistake, please let me know.

Tim, thank you. Even though I have a 261 engine (848 head) I personally have never seen the 261 housings. I understand a lot better what has been said. Pictures are great.

Tim,

Does the internal cup on the 261 t-stat slide down to cover the slots when it opens to the radiator? You might check it in a pan of hot water...

Slots are open to circulate coolant to prevent hot spots and then close to allow full flow to radiator...my guess.

Mike B

OK, Mike, I'll try the hot water treatment, and I will report back.

Please note, Alvin & others, it has been reported by other people in this thread that their 261 did not come with the larger diameter thermostat housing.

There are at least two possibilities:

1. Earlier 261s used the same thermostat-size housing as used on 261s.
Question for people with same size housing on their 261: is there a hose fitting in the top cover of your thermostat housing?

2. It is quite possible that people have bought a 261 from someone who swapped out the original thermostat housing for a 235 housing?

My two housings have a "documented" history of never being used in a small truck:

- one came out of a 1960 school bus in 1972 (by my hands)
- one spent its entire life (1959-2010) in a high school auto-shop class/lab/garage (what a beauty)

The NAPA THM 155 thermostat fits the 261 bypass housing as reported earlier, but I had the same question that Mike B raises above about how the bypass is closed off when the thermostat opens so that all coolant can flow to the radiator.

The thermostat fits, but looks different than the slotted ones that both Deve and Tim have shown.

The THM 155 comes in three pieces as shown in this picture--a gasket, a thermostat and a base.

http://s844.photobucket.com/user/ma...thermostat/IMG_1373_zpsyfobn6ez.jpg.html

One of the pieces looks like a 'regular' 235 themostat and even measures 2 1/8" as seen in this picture.

http://s844.photobucket.com/user/ma...thermostat/IMG_1372_zpsdfeb2kio.jpg.html

The brass colored base has raised ears on both sides that the spring loaded thermostat fits into. The ears create slots on both sides that allow coolant to flow through the bypass when the thermostat is closed. The thermostat fits tightly between the ears. Here are a couple pictures that show the ears and the slots.

http://s844.photobucket.com/user/ma...thermostat/IMG_1374_zpslm0bpv5b.jpg.html

http://s844.photobucket.com/user/ma...thermostat/IMG_1375_zps5f1ea8yu.jpg.html

I did the boiling water test. When the thermostat opens, the slots remain open so some coolant will go directly to the water pump through the bypass and not to the radiator. Contrast this Tim's thermostat which looks like it has a sleeve that will close off the slots so that all coolant will go to the radiator when open. It will be interesting to hear when he confirms this.

Even though the THM 155 fits and allows for the bypass when the thermostat is closed, it appears to have a design flaw because it does not close off the flow to the bypass when the thermostat is open.

Matt

http://forums.devestechnet.com/foru...-tip-of-the-week-25-for-march-20-26-2016

If I said anything wrong, please point it out. Tim, I would like permission to include the pictures of the larger radiator hose opening for an update to The Venerable 261 document. At least it appears bigger than the 235. What is the measurement comparisons? Your casting numbers are exactly the same as mine which means.. what? Mine came out of a CLS60 from 1961, so not sure why they would make two different diameters with same casting number.

Matt.. I don't think it needs to turn off the bypass when at temperature. I think the way it works is, when cool, water flows around through the holes on the side and then when it's at temp, the thermostat opens giving it even more flow. Am I wrong? I do not think there is ever a condition where there is no flow.

My pics of the two I have are here:
http://devestechnet.com/Home/MorePix#261Thermostatpics

Deve,

Your interpretation of the flow pattern before and after warm-up does not make sense, but your interpretation is the same as mine.

Feel free to use the photos, just give an attribution-reference somewhere.

The measurement of the diameters of the two different thermostats are posted above, I think:

2.125" (2 1/8") for the 235 (all years) and
2.500" (2 1/2") for the 261, at least for my years (1959-1960 -- my guess is 1958-1963, but I have no documentation to back this up).

Do you want other measurements?

Yes. The diameter of the radiator hose ID on your top housing. My ID is a tad less than 1-1/4 inches the very same as the 235 housing. Both of my examples are like that. Thanks and always happy to give you credit. Take a look at the Casting numbers page on my site. (last sentence)

I want to eventually talk to you when you have time about that page as well. I spent several hours the other day working with all of the GM Heritage site literature for each year to compile the last two digit decipher for the engine serial numbers. Your expertise in getting this page as complete as possible would be invaluable.

The inner diameters of the radiator hoses (235 vs 262) would be different (at least for 59-60 and maybe 58-63). My Master Parts catalogs are in boxes in NC, sorry.

I'll try to remember to measure the diameter of the radiator hose "fittings" on the top of the thermostat housings (235 vs 261).

Deve,

You wrote "In addition some extra engineering took place since they wanted the thermostat to be bypassed to a small extent on startup before the engine reached its temperature. Lots of guys do this on their 216/235 engines by drilling an 1/8" hole or two in the thermostat."

The Bypass thermostat was designed to circulate coolant through the engine as soon as it was started to prevent hot spots/steam pockets from forming. This was done by allowing coolant to flow through the lower ports in the T-stat into the lower half of the upper t-stat housing and back to the water pump via the Bypass Hose while the engine was cold. As the engine warms up the bypass ports will close and the main port will opened to allow full flow of coolant through the radiator.

The 1/8" holes drilled in normal 216/235 T-stats are there to bleed off any trapped air, not to act as a radiator bypass.

So what changed in the '59-62 engines requiring this Bypass to be added?

Can you give me the casting numbers you and Tim have on your upper housings.

Also confirm the inside top of the upper housing has a machined surface to axcept the top T-stat seal/gasket.

Thanks,

Mike B

Hello, Mike,

Here are codes I posted:

"Both of my 261 thermostat housings show the following codes:
GM part number for base: 3764748
GM part number for top: 3764749
Both parts are listed in the 1959 Parts Catalog - 60 series with 6 cylinder engine (261)"

I will check those codes again, now that I have cleaned one of the housings.

Regarding your explanation of the bypass: "As the engine warms up the bypass ports will close and the main port will opened to allow full flow of coolant through the radiator."

That is also my understanding, however, I do not see any "by pass" ports on the thermostat. I did not try the hot water test that you suggested above, but I will try to remember to do it today and I will post a few more photos. (I am regularly getting kicked-out of house and garage for prospective buyers to view our house/garage - each time I have to put things away).

Thanks,

Deve, the NAPA thermostat appears to work just as you describe above. It didn't make sense to me that the open thermostat would allow some coolant to bypass the radiator and go directly back to the engine 'uncooled' via the bypass to the water pump.

That left me scratching my head. Then I saw the pix of the thermostat that Tim posted with the perforations in the side. In his pictures #8 and #9, it appears that there is a sleeve that slides down when the thermostat is open to block the perforations and thereby send all coolant to the radiator.

That got me to scratching my head harder and made me think that the NAPA thermostat might have a design flaw.

You posted a picture of a perforated thermostat that looks similar to Tim's but does not have what appears to be the sliding sleeve. It would operate as the NAPA one does and allow coolant to still go through the bypass to the engine when the thermostat is open.

http://devestechnet.com/Images/Projects/Venerable261/261Thermostat010lg.jpg

Tim, I empathize with you having to keep the house neat and tidy all the time when trying to sell. Been there, done that...too many times. Good luck. If you get a chance to throw the thermostat into hot water, it would be interesting to see if it closes off the bypass access when it opens.

Now wouldn't it be neat if we could all sit around a table with the various housings and thermostats we have collected to compare them in person?!!

Matt

PS--I checked with my parts friend at my local NAPA. He confirmed that the THM 155 is open stock and they are back in inventory at the Phoenix warehouse after I ran them out by buying two about a month ago. He confirmed that they should be available nationwide in the NAPA system even thought the website lists them as out of stock.

Yeah, I am not seeing any closing sleeves anywhere. Mike that is great verbage to use in the document if you don't mind. But again, no closing sleeves. All I see is it's open via the many holes around the shell all the time. As far as I can tell from looking, the only thing the valve does is add even more flow.

My casting numbers are the same as Tims. There is a machined surface. And.. Don't the holes we drill in the thermostat allow water to pass the thermostat as well? I do not think anything from an engine design perspective changed much in 1958. There are differences, but I would think the reason for the change of thermostat housing was GMs experience with them overheating and it was 1958 when they finally fixed it? Speculation of course. Matt that is exactly what that bypass thermostat looks like. It came fresh out of an engine, and I took it out, so there wasn't any evidence of closing sleeves anywhere.

Mike,

I confirmed the casting codes on base and top of the larger diameter 261 housings:

casting code on the base: 3764748
casting code on the top: 3764749

Also, the thermostat is almost at a boil and you are correct,

There is an internal cup that is open on the sides when "cold".
The cold coolant can easily move through slits around the sides.

When the coolant is hot, the cup moves downwards to almost totally restrict the "open" flow through the side slits/openings.
The heated coolant now moves through the spring-controlled "pellet" center-top-opening (which opens when the coolant is hot).
This opening now regulates flow, thus regulating the temperature of the coolant.

Thank you very much.

Ok, so I did a little more digging in the following sections...

1.153 Upper Housing
1.251 Lower Housing
1.246 T-stats

NOTE, Casting Numbers are the same as Part Numbers unless noted otherwise.

My 1959 Parts Book has;

1959 Series 6 (6-cyl)
Upper 3757230
Lower 3757233
T-stat 3144583, 160* Bellows type

My 1960 Parts Book has;

1959 Series 6 (6-cyl)
Upper 3757230
Lower 3764748
T-stat 3144583, 160* Bellows type (1st design)
T-stat 3139133 T-stat unit w/gasket 160* (Harrison) (2nd design)

1960 Series 60 (6-cyl)
Upper 3764749
Lower 3764748
T-stat 3139133 T-stat unit w/gasket 160* (Harrison)

My 1963 Parts Book has;

1959 Ser 6, 1960-62 Ser 60 (6-cyl)
Upper 3764749
Lower 3764748
T-stat 3144583, 160* Bellows type (1959 1st design)
T-stat 3150671, 160* poppet pellet type w/ gasket type (1959-61 ALL 2nd design)
T-stat 3139134 T-stat unit w/gasket 180* (Harrison)...looks like the 160* Harrison was dropped on or before 1963.

My Comments;

It apears the 1959 2nd design T-stats made them a universal size as they are now available for "ALL" series 6 & 8 Cylinder trucks.

I also found in the 1960 book that the Upper Housing Casting was used twice;

1959 Ser 6 (6-cyl) Cast 3757230 (pn 3757230)
1959 ALL (exc. 10F) (1st design) Cast 3757230 (pn 3757238) use with T-stat 3144583.


Deve asked "Don't the holes we drill in the thermostat allow water to pass the thermostat as well?"

Yes, technically that is a T-stat bypass, but it's not flowing coolant back to the engine, it's flowing it to the radiator which is not aiding to engine warn-up or preventing hot spots (flow is to small to do that). It's always been my understanding that the small hole in the T-stat was the bleed off any trapped air when refilling the engine with coolant.

It's my guess that if you use the Bypass hose then you would use the Bi-Valve T-stat and if you blank the bypass then you would use a standard T-stat. I bet there was a 1959 Service Bulletin that covered all of this.

Clear as mud, right?

Mike B

Thanks again Mike. I do not profess to know anything, rather try to keep up with the conversation. I am still a little behind since the Thermostat here:

http://devestechnet.com/Home/MorePix#261Thermostatpics

does not close those holes all the way around at any time. So I do not see any bypass action taking place, rather, the same thing would happen with NO thermostat in the housing. So that is what I have here. Tim's seems to work as advertised and Matt seems to have one that would work. At least now I think I have the theory behind it down and can write accurate information about it. I will run the text past you guys here once I have time to sit down and write it out. It belongs in devestechnet.com/Home/TheVenerable261 document. So this Thermostat is available through NAPA. Has anyone found a 2.5 inch regular thermostat for those who wish to bypass the bypass?

One other question... If we are worried about hotspots prior to the thermostat opening to allow cooled water to flow (261 engine but 235 type thermostat housing), what are good steps we can take to manage that? Lower temp thermostat? Larger holes drilled in the Thermostat? Other?

I also heard in my perusing the internet that a mechanical water pump steals horsepower and an electric pump would make a substantial difference in performance. Just curious as to what comments there are on that issue as well.

Here is my stab in the dark.
The 261 was designed for big trucks and were pushed to their limits often.
The 261 has Siamese cylinders due to larger bore diameter similar to a 400 cubic inch small block. They both share the extra holes in the head to aid in keeping them cooler.
One of the challenges of keeping a 400 SB running has been trying to keep it from over-heating. This engine was probably the originator of running an engine with out a thermostat.
My guess would then be, the "bypass" thermostat housing design was developed for the same reason that gearheads pulled the thermostats on race day, to keep them from overheating. The "bypass design" would clearly be a less severe solution compared to the "no thermostat" option.
Carl

My thoughts are the bypass served two purposes, first it helps with hot spots and second it helps get the engine uniformly up to operating tempature as fast as possible, most of your wear happens when the engine is cold.

My 0.02 cents...

Mike B

Tim, thanks for testing the thermostat. You appear to be the only one with a 'true' bypass thermostat.

Sure wish a true bypass thermostat was still available. It would be nice to replicate the original cooling system designed for the 261 by the GM engineers.

The best solution seems to be to use a 235 housing with a couple holes drilled through the thermostat...or use the NAPA thermostat in the 261 housing with the bypass hole plugged. Either solution would function the same way. Both would allow some coolant to flow through the radiator to the engine with the thermostat closed. The 261 version preserves the original housing but sacrifices the screw that secures the fuel and vacuum lines that are part of the 235 housing.

NOTE: THE 261 HOUSING WITH THE NAPA THERMOSTAT STATEMENT IN THE ABOVE PARAGRAPH IS INCORRECT. COOLANT WOULD NOT GET THROUGH THE CLOSED THERMOSTAT TO THE RADIATOR AS IT WOULD WITH A DRILLED 235 THERMOSTAT.

Matt

More info...

The T-stat Deve has that looks like a kitchen sink strainer is found in my 1963 Parts Book.

1962 Ser 60-80 (exc. Diesel)...T-stat assembly (180*) (Harrison) (16-1/4" dia bypass holes)...3151250

My 1967 Parts Book has this same listing for 1962 but add it's for (261, 348). It also list it being used for the 1964 Ser 50-60 (292).

Mike B

CORRECTION--I was wrong with my above post about using the 261 housing with the NAPA thermostat and the bypass plugged. None of the coolant would get past the closed thermostat as it would with a drilled 235 thermostat.

Matt

So far, we haven't found any vendors that sell the bypass thermostat now that the NAPA one isn't right either?

Really, there are only a handful of people that it would affect now anyway, but it looks like availability is going to be a serious issue. VERY valuable thread for those effected.

So, why did Chevrolet design the '62 and '64 trucks to run the 16 hole bypass T-stat that doesn't ever close off the bypass??? Is this a simple flow thing where the coolant just naturally goes to the radiator as soon as it opens the main (top) port (path of least resistance)??? If that's the case, why did they not use that T-stat in '63???

Lots of question that we may never have answers to...

Who would have thought there could be this much discussion about T-stat housings for a 261...LOL

Mike B

Mike, I had the same questions because the NAPA thermostat works like the 16 hole bypass one, both of which work different than Tim's. I can't figure out why some years had a true bypass and some didn't.

Here is another trivia question...why did GM eliminate the hole for the screw that secures the fuel and vacuum lines on the 261 housing? The lines pass in front of the valve cover just like on a 235.

One bit of good news from all this discussion...if you have a '62 to '64 truck with a 261, then the NAPA THM 155 thermostat is available and should be a good replacement for you!

Matt

If you really wanted to solve this problem, one solution could be a metal reducer plate to 2-1/8 inches. (about #11 metal with a machined ID surface). Around the outside of the inside diameter you could drill holes mathematically calculated to emulate the amount of water that bypasses using the kitchen strainer thermostat. In that way, you could just use an everyday 235 thermostat and it would work (from what I understand anyway) about the same as the original. But a person would have to be highly motivated and I am not sure the amount of people who really care. Like Matt says, just order a THM 155 although, if 155 means its a 160 degree thermostat, that's a whole nother can of worms since the Kitchen Strainer is a 170 and my other example is a 195. This reducer plate I am speaking of would allow you to put whatever temp thermostat in you wanted.

The original 261 thermostat (1960) was rated as a 160 degree thermostat.

Started to open at around 160 degrees and fully open at around 180 degrees.

Your solution would not work the same as your strainer type T-stat. I believe the strainer T-stat had a gasket around the top that seated against the radiator outlet. When the main outlet valve is closed (cold engine) there will be flow through the bypass only. When it opens there is flow through both.

In your reducer with holes solution there will be flow through both as you havent closed the radiator outlet. All you have done is bypassed the T-stat, not the radiator which would make engine warm up take longer.

Mike B

But Mike, there is no restriction and no gasket around the top. It sits in there and allows all the water that wants to go through those holes to just do it. Unless that gasket deteriorated and left no evidence it was ever there. When you look at the photos, you see what I see. Holes with no way to shut them off. Maybe there is something I am not understanding. That actually would figure. Amazing we have no pictures showing the action of these thermostats. I am thinking we have over-threaded this one. I am too dense or something to understand this one. Thanks for trying! I can see it working better if it had a second regular thermostat under it, but when I took it apart, that's what was there.

Deve,

Not actually holding all of the different parts does make it hard for me.

How tight is the fit between the strainer T-stat and the upper housing machined surface at the outlet, maybe it's close enough that a gasket is not required? If it's not even close than it defeats the purpose of having a T-stat by allowing the (16) 1/4" holes to flow coolant to the radiator all the time.

I believe one of Tim's pictures shows the thick donut gasket with one of the T-stats.

The other point is that T-stat was 1962 only...what's different about the '62's???

My head hurts...lol!

Mike B

Mystery Solved! Thanks Mike. Makes total sense now. The kitchen strainer one is really a nice one to put in there. Sad our choices for new ones are so limited. I added 5 new pics to the archive to show how it works.

The housing is the exact height needed to seal off the bottom hole area from the radiator inlet. That's how it works and probably very well. When cold, the water bypasses through the bottom area where the inside surface of the top thermostat housing is machined to allow water to flow around it. The top of the thermostat housing is sealed off from the bottom part by the height of the thermostat. No gasket is needed because it is not critical to seal off ALL leakage, just that it is tight enough to flow through the bypass.

This is using the Casting No Housings Tim mentioned earlier. What a nice design afterall! Water is being circulated via the waterpump and as it gets hotter since the radiator is not involved yet, the Tstat opens. Nice.

The last 6 pics of that section here:
http://devestechnet.com/Home/MorePix#261Thermostatpics

Now that I understand what is happening, it seems to me that the 235 housing does the same thing only through the heater core. Maybe this is why the bypass thermostat idea didn't last long. It wouldn't be hard to make a bypass valve setup to bypass the heater core (since it would need full water flow all year around) if you have a 261 and worried about hot spots?

Just to throw in one thing more to think about, starting in about 1947 all water pumps have a by-pass hole in the back that aligns with a port that brings water from the head. This allows water to circulate through the engine even when the thermostat is closed.

Oh *&^ NO YOU DIDN'T!!!

(Dave is a very close friend) So Dave.. can you explain how this works and what makes this relevant? When you have a 261 in your 1950, and people are talking about hot spots created by a lack of water flow.. what exactly is your take? (thanks once again!)

Right column - 2nd paragraph down.

I'm going to wade into this ,I am just finishing rebuilding my 62. 261 it has the original thermo housings PN 3764749 & 3764748 I didn't get to see how all the hoses were routed,head was off engine when I bought the truck.I have read all the threads ,read all Deves info on his site and I'm still not sure how this all goes together. I purchased the 3 piece napa thermostat and was trying to fit it this afternoon,it seems it will work,it seals off the upper housing ,and allows coolant to by-pass from the upper housing back to water pump until the t-stat opens and then majority of coolant will flow to rad. The question I have is what about the heater circuit, it is by-passing from the lower Tstat back to water pump uncooled all the time? Or at least all the time the heater valve is open. Or am I wrong?