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

I know that the top part of the thermostat housing with the bypass for a 261 is different than the top part without the bypass for a 235. Do both use the same bottom bottom part or is that different too?

Flying blind here because I don't have a 261 bypass housing to refer to. Have located the top section of a 261 bypass housing, but not the bottom.

Thanks,

Matt

They both use the same bottom. I have a set in front of me. Casting No. on the bottom part is 3757233 which is different from the bottom ones I have on my other 261 (8392320 and 59 235, 3736568) but they bolt up the same. (casting on the top is 3757230 for the one with the port)

I have two of the 261 thermostat housings and original thermostats. The 261 bottom (and top) is distinctly different from the 235, but the 235 top might fit onto a thermostat on the 261 bottom (the bolt locations look to be the same).

I'll look at them tomorrow.

They both bolt up the same to the head.
They both do not use the same bottom.

Mixing one engine's bottom-housing to the other engine's top-housing will not work.
The larger diameter thermostat in the 261 bottom will not align appropriately with a 235 top, and vice versa.

Most likely, you know the outlet hose diameters are also different.

No kidding? The rim inside the bottom to hold the standard 235 thermostat is a different diameter? N/M it IS a different diameter. Unless you need that extra port, it would be easier in the long run to get a 235 housing SET. Are the 261 thermostats even available any more? If so, is there some reason a person would want to go with the bigger thermostat? Since the inlet on the block is the same diameter as the 235, I cant imagine it being better water flow.

Yes, 261 style thermostats are available. They have been referenced in other recent posts on this subject (in the Swap Meet - by Matt, I think).

NAPA THM 155
ACDelco 12T11B Professional 160 Degrees Engine Coolant Thermostat

Regarding water flow restrictions, I am not interested in participating in a discussion regarding hydraulics and flow restrictors (meaningless with respect to the original post in this thread). Measure the cross-sectional area of the smallest opening.

(these are smaller diameter thermostats than were used in later/larger 261 thermostat housings)

Right. Good this is a great addition to the 261 document. I will be adding it soon. I was fortunate to have a friend walk in with a set so I can take the proper pictures and everything.

All this time I had no idea there was a different setup since both my 261's and all my 235's have the 235 kind. It was recent posts that got me to thinking about it. There is no penalty for going with the more plentiful 235 type but a person interested in uber correctness would want that setup for sure.

So Matt, disregard my WRONG previous post, sorry, and thanks Tim for doing the proper research.

Deve,

Measure and post the following cross-sectional areas:

- the size of the opening in the head where the thermostat housing attaches
- the size of the opening at the outlet in the top part of the thermostat housing

Head = 3-1/4 inches wide by approx 3/4" on both 235 and 261
Bottom Part of the Thermostat Housing almost the same with the difference being 1/8" in width smaller. (3-1/4' x 5/8")

Top Part Outlet a little less than 1-1/4" ID on both 235 and 261 versions of the Housings.

The internal outlet for the Thermostat Housings are the difference.. Here we find..

Bottom Part internal where the Thermostat seats: 2-1/4" ID
Top Part is the same.

The 261 Housing Assembly (both parts are beefier using 1/2" NPT outlet to the Heater (or electric fan) (TRUE FOR BOTH OUTLETS) as opposed to the normal 3/8" NPT for the 235 version.

I will post pics here tomorrow once I clean these parts up a bit.

Tim and Deve, thanks for all of the help.

The thermostat that I found is NAPA THM 155. The price was $8.37. There were two at the local Phoenix warehouse, so I bought both.

The NAPA website states that they are out of stock and there is some scuttlebutt on other websites that it has been discontinued. I checked with my local NAPA store and they told me that it is an open stock item regardless of what it says on the NAPA website. My advice? Use the above part number and call your local NAPA if you are looking for one.

Thanks again for the help,

Matt

Go to RockAuto.com and choose 1961 C40 with 261. They show all sorts of options for a 2-1/8" Thermostat. The ones we get for our 235's are about 1-5/8" or so?? (maybe not) Not sure if those are the same ones as NAPA.

After researching every year at Rockauto, I am more confused than anything. I don't think any of those are the ones you are looking for. So, anecdotally, it is going to be very hard in the future to find thermostats and gaskets for that housing. The 235 ones work just as good and are very plentiful. Just something to think about.

The AC Delco thermostat for the 261 housing is readily available (part number is given in a post above).

Nonetheless, I switched to the 235 housings on my 261s in 3100 trucks, so I would not have to fiddle with a hose-diameter adapter.

My 261 gasket sets ( a few years old) came with both size thermostat housing gaskets.

What are the two cross-sectional areas (square inches) for the 235 thermostat housing, Deve?

- the size (sqare-inch area) of the opening in the head where the thermostat housing attaches
- the size (square-inch area ) of the opening at the outlet in the top part of the thermostat housing (where the hose attaches).

They might help people understand why the 261 has a different housing.

I gave you the dimensions above. I do not understand what you are getting at. The head is the same, the diameters for radiator hoses are the same. The only difference is internal to the housing to accommodate a larger thermostat.

The AC Delco version is available by part number at RockAuto. $4.38 but shipping is high.

ACDelco 12T11B - I think this might fit in the 261 housing
(this is too small a diameter for the later/later 261 thermostat housing, tcl)
"If so, is there some reason a person would want to go with the bigger thermostat?" (posted by Deve)

area = sq inches - I do not want to measure/calculate them

If you calculate them and post them, they might help to understand the purpose of the 261 vs 235 housing (I am guessing).

I ordered one. When it comes in I will check the housing to make sure.

With the dimensions given, I do not understand, other than the 261 housing holds more water. Inlets and outlets are identical.

However, since there is a second port, it makes me think the whole reason for it is because one of the ports is above the thermostat, and one is below. Maybe this allowed for more options in heater design? The one below (engine side of the thermostat) would be hotter quicker. No idea, just guessing.

I know nowadays guys want them for that second port to install an electric fan sensor. But 'nowadays' we have electronic thermistor solutions that do not require access to the water stream. If you have any ideas Tim...

"Inlets and outlets are identical. "

The inlets and outlets are not the same.

The outlets are different. I thought you said you had a 261 thermostat housing?

Yup. I have one at the shop and measured them side by side. You can even put them against each other to see that the radiator hose diameter is identical, and the head openings are also identical. The only differences are when you take them apart, the internal hole is quite a bit larger. I will take pics tomorrow so you can see they are exactly the same as far as IN/OUT. You cant mix and match them like I thought because of the halves being different, other than that. Pics tomorrow afternoon!

I'm sharing this as I learn in preparation for my 261 project, so please correct me if I have this wrong.

The 261 also had a water pump with two inlets, one for the bypass thermostat and one for the heater. My 235 has a single inlet for the heater. The 261 water pump can be used with a 235 thermostat without the bypass if the second hole on the water pump is plugged. This is how mine was set up when I found it.

Both water pumps are available and can be seen at the NAPA or Autozone websites if you look up a water pump for a '57 with a 235.

Here is a link showing pictures of a 261 with a bypass thermostat connected to a two inlet water pump with the bypass hose and then after it is converted to a 235 thermostat without the bypass with the second hole on the water pump plugged.

Scroll down to post #19 to see the pictures.

http://67-72chevytrucks.com/vboard/showthread.php?p=6386977

Matt

Deve,

Your 261 housing outlet is different from both of my 261 outlets (1959 & 1960).

I'd guess the 235 outlet (radiator hose fitting) has a 1.5" outer diameter, and both my 261 outlets (radiator hose fitting) have a 1.75" outer diameter.

Furthermore:
My 235 "cover" will bolt to the 261 base, but it cannot hold/enclose the 261 thermostat.
The 235 base and 235 cover cannot hold a 261 thermostat.

The diameter of the 261 thermostat is wider than the diameter of the 235 thermostat.

If I have time tomorrow, I'll post some photos.

Your 261 thermostat housing is distinctly different from my two 261 thermostat housings.

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)

Yeah, it's all clicking now. I had no idea why the extra port was even there. When you install the 261 on an AD, to do it right you get Dave's water pump adapter plate, then order a 1953 water pump. This puts the fan in the center of the radiator allowing for better cooling. Since the 1953 water pump does not have two ports on it, its a mute point for us AD enthusiasts.

Nonetheless, our 261 document is not about the AD, so I am in the process of getting the parts together to show exactly how your system works. Thanks for that Matt.

Why a 1953 water pump?

The 1953-54 water pumps had a 3/8' belt.

Earlier water pumps had wider belts.

Use the earlier (1955-1st or earlier) water pump that matches the belt-width you need for your Advance-Design (or earlier) truck.

Why are you discussing/bring-up the water pump here? I thought this post started with a question about the 261 thermostat housing (on a 1959 engine)?

Regarding the thermostat housing:
I would not use the 1959 261 thermostat housing on a 261 being installed in a small truck. I'd use a 235 thermostat housing so that there would be no need for a hose-diameter adapter. The 1959 261 (the engine being discussed) was used in trucks with "big-truck" radiator hoses (the radiator to water pump hose and the radiator to thermostat housing hose were larger diameter than the hoses used in small trucks).

I brought up the water pump inlets in relation to connecting the thermostat housing as an fyi that I wanted to get posted just in case anyone else tries to figure out where the bypass connects to. By getting it posted, it becomes a permanent part of any internet search for anyone curious about the same thing in the future. I thought that the link that I posted to the pictures would be helpful. I didn't mean to wander off topic.

Hooking up a 261 with an original thermostat becomes more complicated than just finding the correct thermostat and housing...hose sizes different, different water pump, etc. I am learning! I might end up just installing a 235 housing for reasons you mention.

Two other small things that might be worth memorializing for future searches with this post. One, the 235 top radiator housing has a threaded hole that holds the clip that secures the fuel line and vacuum line as it crosses in front of the valve cover. I don't think that the 261 housing has this based on pictures that I have seen. Two, the 261 (at least in 1957...my truck year and my engine year) not only came with the larger hoses that Tim mentions above, the different water pump and the different thermostat and housing but it also came with a larger radiator, a fan shroud and a 20" five blade fan. The radiator is about two inches wider on each side. It fits the same bolt pattern on the core support, but the sides are at 90 degree angles to the bolts instead of being smaller than the length between the bolts with angled brackets to the core support.

The entire 261 cooling set up seems like it was engineered for extra cooling. Here is a picture of the radiator, shroud and fan as I found them on the '57 6400 that I pulled the 261 from. The radiator was shot, but I did scavenge the brackets from it that hold the shroud. The brackets, shroud and fan were easy bolt-on's for my 235.

http://s844.photobucket.com/user/ma...1170_zpsla45xxqs.jpg.html?sort=3&o=6

All that being said, I am considering just using a 235 thermostat housing since I can't recreate entirely how the GM engineers designed the cooling for a 261without the radiator. The other parts of accessible.

I have learned quite a bit about the 261's cooling system along the way and appreciate everyone's help and advice.

Thanks,

Matt

Good luck, Matt

Thanks for that last post.

Yeah, its all coming together now. I went out to the back forty and pulled another 261's thermostat assembly just so that I was sure about my numbers. Both are from 1959 and 1962 261 engines. My engines did not come with trucks so I have no idea what model they are off of. I am cleaning them up and pics are forthcoming. Maybe tomorrow since I let the day get away from me.

Interesting about the two port water pumps. I have two of them, different styles. It doesn't matter how you connect them because the ports are in a cavity together. You can put your finger in one hole and out the other.

Tim if yours are a different size than described above, please take pictures so they can end up in the document too. Both of mine have the exact same dimensions as the 235 IN/OUT.

261 Housings Casting Numbers (same look but different numbers)

Set 1:
Bottom #1 is Casting No. 3764748
Top #1 is Casting No. 3764749

Set 2:
Bottom #2 is Casting No. 3757233
Top #2 is Casting No. 3757230

Both have 1/2" NPT ports and came with 3/4" hose barbs.

Also, there are two water pumps listed by Dave that work with his adapter. One is the 1953/54 one I mentioned (3/8" belt) and the other is 1941-52 (5/8" belt). Get them at RockAuto because they only cost $60 or so instead of the $130 the vendors charge.

Thanks to both of you for helping get this documentation squared away. Accuracy is everything.

I took these today as promised..

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

If there is anything I left out prior to writing a new section into this document, please let me know. I would like to have pics of the larger radiator hose opening and anything else pertaining to the Thermostat Housing, the reason it is different from the 235 including the Water Pump info, etc, etc. Thanks again!

http://devestechnet.com/Home/TheVenerable261

I've looked through the posts here, but may not see an answer to my question.

My understanding was the bypass thermostat was required on the 261 to help prevent parts of the engine heating unequally since the cylinders are siamesed.

The 261 I had had the larger thermostat which had a small circular metal piece that would seal against an opening in the lower part of the the thermostat. This engine was out of a 1960 Chevy school bus that had a Viking 60 Chevy frame and drive train.

I guess my question is, did the smaller trucks (pickups and such) not need the bypass feature on the 261?

"I guess my question is, did the smaller trucks (pickups and such) not need the bypass feature on the 261?"

The 261 was not available as an option on smaller trucks.

You can safely use the 261 without that feature.
The 261 allowed the by-pass feature, but the use of that feature was not required .

That feature not used on either of my 261 engines (1959 & 1960).
One engine was originally used in a school bus.
One engine was originally used in a high school truck-shop class (never used in a truck).
On both engines, the holes were plugged on the 261 thermostat housings and on the matching 261 water pumps.

Both engines are now in 1954 3100 trucks - one for over 10 years and one for a few decades.

Both engines have Dave's adapter plate and 1954 235 water pump, and 1954 235
engine thermostat housing (with a 1/8" hole drilled in the thermostat).

I wish I could find the GM description of the bypass, but it explained that it was to prevent overheating before the thermostat opened. This was only a concern on big trucks that could be put under a heavy load generating a lot of heat before the thermostat opened.

I think that MIke B or 3B posted information about this a few years ago?

I had thought some of the extra hose fittings was for an extra heater.

https://www.stovebolt.com/ubbthread...Head_Thermostat_Housing_By.html#Post7074
https://www.stovebolt.com/ubbthreads/ubbthreads.php/topics/31450/installing_a_261.html#Post31450
https://www.stovebolt.com/ubbthread.../235_water_pump_question.html#Post602558

They also didn't have the modern radiator core options we do today. My US Radiator brass replica has more cores and smaller tubes supposedly to cool better. I think it was just something GM tried back then as a stop-gap because of overheating problems on heavier trucks. The concept of bypass thermostats lasted a whole 4-5 years?

http://chevy.oldcarmanualproject.com/shop/1959trucksup/59ctsms0814.html

and the page after that explains how the system works.

Deve,

If you still want photos of the larger 261 thermostat housing parts and thermostat, I'll take some when I get back from NC next week.

Yes Please. That would be very helpful. If indeed the diameter of the upper radiator hose is larger on yours, that would mean that GM gave the very same casting number to two different sized housings. It wouldn't surprise me of they did that, but adds confusion when people are looking for the right part. I will add that as a statement so people can expect it.

I mentioned in a previous post that my 261 came from a '57 6400 that had a larger capacity radiator. I remember thinking that it was odd that the upper radiator hose fitting was bit larger than the one on my '57 3600. In fact, it had me scratching my head so much that I checked it twice. The 261 had a 235 thermostat housing with the smaller fitting attached to the radiator with the larger fitting. I don't have any pictures other than what has already been posted and did not save the upper tank when I stripped the shroud brackets off the radiator case.

This makes sense with Tim's posts. The larger diameter thermostat fitting would have mated to the larger diameter radiator fitting without an adapter. The hose would have looked just like a 235 hose, but a little wider. This would have allowed more volume to flow through the system.

I'm kicking myself a bit. I kinda wish that I would have investigated getting the 261 radiator recored. If I only knew then what I know now...

Matt

The only trucks these 261's came on was the larger ones with the big radiator. But so far, both casting number sets (one that is the same as Tims) show the outlet is the 1-1/4" ID (just a tad less) like the 235. I am looking forward to the pics so I can show there IS one with a larger diameter.

When you think about it, it's probably functionally moot because the restriction at the head is the same. Since smallest orifice determines flow, the 1-1/4" one is probably just fine except radiator hoses are a pain to find.

On your radiator, would the old one fit in your truck? I did two things to make sure that Roy's suggestion that the siamesed cylinders needed better cooling. I got Dave's adapter plate to move the fan to the center of the radiator, and I got a modern cored radiator that looks like the original from US Radiator. Pricey, but I do not want overheating problems.

Deve, I never tried to bolt it in but the bolt pattern and measurements indicate that it would have fit.

If you look at the 235 radiator on a TF truck, the brackets that are spot welded to the radiator case that connect the radiator to the core support are at an angle because the radiator is not as wide as the width between the bolt holes in the core support. On the larger radiator, those brackets are are 90 degrees because the edge of the wider radiator lined up with the bolt holes. The width between the mounting holes was the same for both radiators.

The three bolt holes on each side of the core support line up exactly with either radiator from a vertical standpoint. That is why I was able to use the brackets that I scavenged off the larger one to hold the fan shroud on my truck. The fan and the shroud are dead center on the radiator and the water pump and the fan blade has the proper amount of tip exposed. Other than having to repair the shroud, it was almost too easy.

Matt

Deve, I never tried to bolt it in but the bolt pattern and measurements indicate that it would have fit.

If you look at the 235 radiator on a TF truck, the brackets that are spot welded to the radiator case that connect the radiator to the core support are at an angle because the radiator is not as wide as the width between the bolt holes in the core support. On the larger radiator, those brackets are are 90 degrees because the edge of the wider radiator lined up with the bolt holes. The width between the mounting holes was the same for both radiators.

The three bolt holes on each side of the core support line up exactly with either radiator from a vertical standpoint. That is why I was able to use the brackets that I scavenged off the larger one to hold the fan shroud on my truck. The fan and the shroud are dead center on the radiator and the water pump and the fan blade has the proper amount of tip exposed. Other than having to repair the shroud, it was almost too easy.

Matt