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Has anyone explored the possibility of adapting a true bypass thermostat like those used on most modern engines to a 235/261?

I am building a 261 at this time and I am having a difficult time finding the parts for the original big "bypass" 261 thermostat. Plus after looking at what pictures I can find I don't think it is a true bypass system.

Since the thermostat housing is a separate piece on these engines it looks like it wouldn't be too difficult to adapt one. But, I don't want to reinvent the wheel if someone has already done it.

I know about drilling holes in the thermostat to keep water flowing and prevent hot spots from forming. However, the water passing through those holes is returned to the radiator and is chilled again before returning to the engine thus doing nothing to prevent thermal shock. On a cold morning the 235 engine in my pickup (with a drilled thermostat) the engine temperature will go to about 195 degrees (180 degree thermostat) then plunge to less than a 100 degrees when the thermostat finally opens. I know this is not good for the engine and causes unnecessary wear and tear especially if you are under heavy load at that time.

What I am proposing is a bypass port under the thermostat that returns the engine water back to the water pump to be recirculated through the engine until the thermostat opens which closes the bypass port and forces the water through the radiator.

Any thoughts?

The heater hoses already recirculate the water just like you want it to. Out the top of the heat to the heater core and back to the water pump inlet. This lets the cab heat up as fast as the engine does.

It sounds like your thermostat was just bad, it should be opening a little before 180 and fully open by 180. When I see one stick like yours does, I just replace them. A 180 should never spike at 195, it may climb up there over time on a hot day, but never on a start up.

Just go down to the parts store and buy a replacement. As double check, put the old one and new one in a pan of water and watch them as the water is heated up. If possible, hang them from a rod across the pan so they do not touch the bottom of the pan, let only water touch them.

The small, drilled hole is only for letting air out of the head when filling it full of coolant. There is not enough water circulating through the hole to make any difference in the cooling of the engine. The hole should be 1/8" or less in diameter.

Thanks Joe,

No offense, but this isn't my first rodeo. I am 65 years old and have been doing this stuff since I was a teenager. Also, I have been driving this truck daily for 6 years and I have had several thermostats in it. Currently I am running a Robertshaw "balanced" thermostat.

The problem is that the thermostat on a stove bolt is mounted in the airstream in front of the engine. With the thermostat closed and no water circulation the engine block and cylinder head will heat considerably above the thermostat setting before it sees enough heat to respond. Then the engine receives a huge amount of ice cold water, but the thermostat is still seeing the overheated water that is being pushed out by the inrush of cold water so it goes wide open. When the cold water reaches the thermostat it slams closed and the cycle starts again.

I'm not really looking for simple solutions to a problem I could just as easily ignore. I am trying to add modern improvements to these engines to help them perform better and last longer. And, I really don't want to leave my heater on all the time.

But thanks anyway.

well you still have a problem, I have NEVER experienced that issue in my truck in 35 years. It just goes up to temp and stops, that is it. If you have had several thermostats in 6 years, you have a problem as well. I ran the same thermostat for about 20 years and as far as I know it still works, I just got a new one because I rebuilt the engine.

Diceman, is the fluctuation showing in the temp gauge?

Dennis

If you watch it really closely you can see it. The stock temperature gauge does not react quick enough to catch most of the fluctuations. Where it is very evident is on a data log.

Look guys, I know the engine will work just fine as it is. All of them do it whether you are aware of it or not. If you tear down and measured the wear on any engine you will find more wear on the front cylinders where the cold water shrinks them more than the others. If you can minimize water temperature fluctuations you can minimize that wear.

I know what the problem is and I know the fix for it. The question was and still is " Has anyone explored the possibility of adapting a true bypass thermostat like those used on most modern engines to a 235/261?"

As technology advances the OEM engineers have determined that bypass thermostats are an advantage to the longevity of engines and I want to add this technology to mine.

...are we missing something here? Are you running a 261 with a 235 head?? I'm putting together a 261 and just had the 235 head drilled with the extra holes to match the 261 block.

You stated "I know about drilling holes in the thermostat to keep water flowing and prevent hot spots from forming." As Joe H. mentioned the small hole in the thermostat is to let the air out....but the holes we drill in the 235 heads for a 261 are, as you say, to "prevent hot spots".

I was just a little confused but that's not hard for me to be!!!

...kudos, for trying to make things better.

I am in the same state of confusion, Alvin.

The original 261 thermostat (on my 1960 261) was a special/original thermostat/housing (and an hard/impossible to find thermostat). It had a hole drilled it in.

I am now using a 235 thermostat housing and have never had over-heating problems (spikes to 195) with the 235 thermostat/housing or the old 261 housing/thermostat.

I'm sorry if I confused anyone with my opening statement about my 261. I am building a 261 with a 235 head (although right now it is a pile of parts in my shop) and it does have the appropriate steam holes drilled in the head surface. Those steam holes will do no good if there is no water flow through the engine like when the thermostat is closed. That's why GM put a bypass type thermostat on 261's. The components for the 261 bypass thermostat are hard to come by and when the discussion comes up about what to do about it someone usually suggests drilling holes in a regular thermostat. This not only bleeds air out of the system as JoeH stated but it also keeps some coolant flow through the engine to prevent hot spots from developing. Although it takes more than a single 1/8" hole for this purpose.

By the way, some aluminum radiator manufacturers recommend drilling (3) 1/8" holes in the thermostat to prevent damage to the radiator from thermal shock. If hot water from the engine can damage the radiator then cold water from the radiator can't be good for the engine. It works both ways.

The example I gave about the temperature fluctuations I have seen are on the 235 that is currently in my truck. There is nothing wrong with it that causes these fluctuations. It works as well as it did in 1958 and after a few temperature swings it levels off and runs right on the thermostat. The condition I described is inherent in most internal combustion engines, but is exaggerated in stove bolts because of the thermostat location. Thing is most people aren't aware of it and because the effects are long term most people don't care.

The effect is probably lessened with a thermostatically controlled electric fan since you are not removing as much heat from the radiator when there is no coolant flow. I have a very efficient aluminum radiator, mechanical fan, and shroud so I am chilling the water at all times. And no I don't want to use an electric fan. (before someone suggests it)

I guess my original question has been answered since no one knows what I am talking about. I am going to do it so if anyone is interested I will show the process and the results.

http://chevy.oldcarmanualproject.com/shop/1959trucksup/59ctsms0813.html

Describes the 261 cooling system... It sounds like getting the proper t-stat is gonna be a PITA. Its much more than a simple hole in the t-stat.... but I would think that it would work.. as long as you have water moving in the block to remove/prevent air pockets/steam from forming.

Thanks pfarber, You are right about getting the proper 261 thermostat parts. People keep listing the 235 parts for the 261 on Ebay and they will fit but, they aren't really for the 261. I have a lower thermostat housing but so far I haven't been able to find the upper part. After looking at the 261 lower half I can't see how it can be a true bypass system like the new engines have. It looks just like the 235 unit except larger.

Thanks again for the info.

The 261 water pump also has a hose that goes from it to the t-stat housing to help circulate the water when the stat is closed. One hose goes to the heater core while the other small hose goes to the housing.

Some 235 water pumps don't have the extra hole for the bypass hose that the 261 uses.

Are you needing the top piece the radiator hose goes on?

Yes, I have the bottom half and I have a thermostat on the way. I hope it is the right one.

Best I remember the top piece the radiator hose slips onto has the two mounting ears that aren't quite directly across from each other. And they are hard to find. But it's been a long time since I've worked with one.

Broke one on my 261 way back in the mid 1980s and took forever to find a replacement back then because of the way the mounting holes were not directly opposing like most of the later ones.

You remember well R/R.

Anyone with a couple of brain cells to rub together and some machine tools should be able to adapt a modern bypass stat to the 261 housing. Start with a thick chunk of aluminum and whittle away anything that doesn't look like what you want. The trick will be finding one of those thermostats that opens at less than 195 degrees.
Jerry

The GMC 270 has a bypass water pump/tstat housing, but the steam tube is from the rear of the block (around cyl #5) and plugs into the water pump via fittings.

It might be worth looking at to see if they are close. But I know the t-stat in the 270 is a simple single action.

One steam relief method used on 235s in the 40s (well, the G506) was to pipe it directly back to the radiator. The water pump will circulate water through the relief tube.

Not an original set up for your truck, though.

I agree. My son does engine management for a team that races Daytona Prototypes in the TUDOR United SportsCar series. They recently adapted a bypass type thermostat to their LS based race engine and he is sending me the drawings and a part number for the thermostat they used which is a 180 degree unit.

I think I can modify a stock lower housing fairly easily to get the bypass water out. The trick is going to be getting the water back into the water pump in an elegant manner. There is not much room to work with.

I'm not to concerned about it looking original as long as it looks vintage. I will research it.

Thanks