Tip of the Week (25) for March 20-26, 2016

[Deve]

The 261 engine is a very good choice for a little added horsepower in our vintage trucks. The 261 was only available on large trucks from 1955 through 1962. They all came with solid lifters and most of them had a special thermostat on them. Since the cylinders are siamesed and they get hotter quicker, and since they are made for large trucks carrying very heavy loads, the standard 2-1/8" thermostat was not enough. 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 actual 261 thermostat was designed specially for this purpose and both upper and lower thermostat housings were made differently to accommodate the bypass thermostat that was available at the time. So far, I have searched and came up short on a new bypass thermostat. There is one that is a three piece model, but not very well designed.

For those of you who have never seen the difference between a 235 thermostat and housings and the 261 models, here are a few pictures. The two that I have were out of CLS60 models which is 1961's version of the 6000 series. Remember they only came equipped in heavy duty trucks. There is a rumor that there is a larger diameter version that can accommodate radiator hoses larger than the standard 235 ones, but I have never seen one. When asked about this, I am given the same exact casting numbers that are on the ones pictured, so if anyone has pictures of such a beast, I am always happy to include new information.

Both upper and lower housings have a 3/8" NPT port on them and what was included were 3/4" hose barbs. The reason for the two ports was because one goes to the heater and the other directly to the waterpump. The top thermostat housing port is plumbed directly to the waterpump which allows for the bypass feature. A special thermostat designed specifically for these housings is necessary for proper operation. I found one authentic thermostat in this search and one that was just a larger version of the 235 style. The thermostats are both elusive and they are both 2-1/2" thermostats. Most 261 full engine gasket sets have the proper gasket, but so far I haven't found a vendor for the taller thermostat. (Either one really). As you can see the thermostat action is higher in the housing with holes below it. This is the bypass procedure. Water comes in through the holes and once the engine is at temperature, the thermostat opens giving a non restrictive path for water flow.

To my way of thinking, there is no advantage to having this type of setup. The reason is because the flow is the exact same as the 235 since both housing styles have the same diameter openings. Even if there were a larger radiator side inlet, the block inlet is the very same size anyway. With no advantage in flow, now you are thinking, yeah, but it is a BYPASS thermostat right? Well, nothing that drilling a few small holes in your existing thermostat can't overcome and then you are not looking high and low for a very unique and hard to find thermostat. Moral of the story: Stick to the 235 version. The other advantage to the 235 version is it has a deck with a 1/4" tapped hole for your fuel line clamps. The 261 version does not. On the other hand, if you NEED a second water access, for example for an electric fan sensor, this is one way to have it. IF you can find a thermostat to fit the housing! Also on the subject of electric fans, there is no need anymore for the sensor that fits in the water stream since Thermistor technology works so much better. So, THAT is one reason why there are two ports on some water pumps! Seeya next week!

[GMC KID]

I have 1962 261 with the 261 thermostat housings and really after all is said on this topic,if this is correct, install a thermostat that fits, drill a hole in it to allow some flow during the warm up stage and block off one thermo housing port and one water pump port,using the upper thermostat port for the heater circuit and one water pump port.

[Pre68Dave]

The 261 was only available on large trucks from 1955 through 1962.

Slight correction, 261's started in 1954.
It is also listed in the parts book as being used in 1963 with four wheel drive.

[JDK]

Just ran into this problem with my 62 261, I decided to purchase a new upper and lower 235 housing with gaskets thermostat everything from ebay. It has a 170 degree thermostat, I think that should be ok? I'm really not sure what to get but when I drove the truck the temp gauge (if its correct) was running at about 180 warmed up.

my water pump has two ports so I guess I should plug one up, they are currently looped so I'm not sure which is best to use for the heater core.

[Deve]

The two ports are the same internally so it doesn't matter which one you plug. At least that's how the one I have looks. The heater core hookup from the T-stat housing and from the waterpump; it doesnt matter which you hook up to where on the heater core.

[JDK]

I bought a new (made in USA, that's neat) 235 thermostat housing for $100 and it is nice but look at the difference in size! I am having trouble finding a radiator hose to fit the inlet of the new housing. Description says it's for 1940-54 216 235 engine, is there an upper with a larger inlet? If not then what radiator hose do folks buy for this setup?

Edit: just answered my own question.. The 59-62 235 housing is different, and that's what I should have bought. I see used ones for sale, but I would buy another new like this one if possible.

[Deve]

I get the fitted hoses from Jim Carter and they are TIGHT around the housing.. hard to get on, but they work. Rockauto used to sell thermostat housings for very cheap. Check there as well.