Bert, I have no evidence to support the theory, however, my general opinion is that oil which cannot flow very well will not provide the film necessary to prevent metal to metal contact.

Whether this is true, and at what temperature it becomes true, is the question.

After viewing dino oil at near 0°F., I can picture that it has a difficult time to properly coat the cylinder walls and bearing journals until it warms up a bit. Now, it does not need to warm to 100°F., that is for sure. But, at very cold temperatures, I do honestly believe (without any hard evidence) that there is more wear.

Of course, this would not be evident in a typical climate of California. Another reason that running without a thermostat would not be an issue.

Some things have to be from inference... not all information is available on the web..

What is the lowest temperature thermostat you have ever seen a manufacturer suggest for their water cooled engines ?

What is the highest ?

That range is not very large in my case.... perhaps 175 to 195. Why is there such a narrow range ? For those manufacturers which choose a thermostat on the high side.... what factors do you think they ' missed' when you choose to go to the other end of the scale ?

Remember that on our vehicles we are not talking about short lived bores... on the contrary... we are talking about engines famous for long life if normal maintenance is followed.

Here are some interesting url's.... notice how focused they are on expansion rates and piston fit.

Much of our engines desired thermal characteristics are SET AT THE FACTORY... because they choose the type material used for the pistons, the design of the pistons and the material used for the bore.

Several of these sites talk about increased wear happening at that point before the expansion has brought the parts to proper fit.

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http://courses.washington.edu/engr100/All_Sections/Engine/UofWindsorManual/Piston%20Design.htm

" The piston is designed to be an elliptical shape when cold. As the engine reaches operating temperature, the piston pin bore area expands more than other thinner areas of the piston. At operating temperature, the piston shape becomes a circular shape, which matches the cylinder bore for improved sealing and combustion efficiency. "

http://www.diy-boat.com/Pages/Archives/links/eng983.html

http://piled-arms.com/tech38.html

http://abbysenior.com/mechanics/short.htm

"The skirt is the only area of the piston which contacts the cylinder walls. The clearance between the piston skirt, and the cylinder walls must be very precise, and depends on the type of pistons used in the engine. Aluminum and iron have different expansion rates. Aluminum, because it is not a very dense metal, takes on heat very easily, and gives it away again very easily. Iron, on the other hand, is very slow to heat up, and remains hot for a long time. If piston expansion was not controlled somehow, the piston skirt would grow to be larger than the cylinder its' going up and down in, and obviously, the engine would seize up. Cast pistons, because their expansion is controlled with a band of steel. can have a relatively small piston to wall clearance of .003"-.004". These bands are placed in the piston mold before the molten aluminum is poured in, and are around the wrist pin. This makes piston expansion parallel to the wrist pin only. The piston skirt is then ground to be oval shaped, or "Cam Ground", so there is the same clearance between the skirt and wall when the engine is hot, as when it is cold. There is, however, more area when the engine is hot. Pressure, and therefore wear is much greater when the engine is cold, but the engine is still quiet, with no piston slap."

and I just thought this next one was interesting.....

http://kb-silvolite.com/index2.php

from
http://www.beckracing.com/page05.htm

" Piston to Bore Clearance for KB Performance Pistons were dyno tested at wide open throttle with .0015", .0020", .0035" and .0045" piston-to-bore clearance. After 7-1/2 hours, the pistons were examined and all looked as new, except the tops had normal deposit color. Even with 320 degrees oil temperature, the inside of the piston remained shiny and completely clean. Excess clearance has been shown to be safe with KB pistons (no reported cracks in four years). The added skirt stiffness of the KB pistons reduces piston rock, even if it is set up loose. Loose KB pistons over .0020" do make noise. As they get up to temperature they still make noise because they have very restricted expansion rate and do not swell up in the bore. Our hypereutectic alloy not only expands 15% less, it insulates the skirts from combustion chamber heat. A short term Hp improvement can be had by running additional piston clearance because friction is reduced. To obtain actual piston diameter, measure the piston from skirt to skirt level with the balance pad. See The Special Clearance Requirements for KB Pistons.

Pin Oiling should be done at pin installation. Either pressed or full floating, prelube the piston pin hole with oil or liquid prelube, never use a grease (if you are using a pressed pin rod be sure to discard the spiral pin retainers). All KB Performance Piston sets supplied from the factory include a tube of Torco/MPZ engine assembly lube. A smooth honed pin bore surface with a reliable oil supply is necessary to control piston expansion. A dry pin bore will add heat to the piston rather than remove heat. Pistons are designed to run with a hot top surface, and cool skirts and pin bores. High temperature at the pin bore will quickly cause a piston to grow to the point of seizure in the cylinder.

Marine Applications generally require an extra .001"-.003" clearance because of the possible combination of high load operation and cold water to the block. A cold block with hot pistons is what dictates the need for extra marine clearance see our clearance chart on The Special Clearance Requirements for KB Pistons."

http://www.canadiandriver.com/articles/jk/020320.htm

http://64.78.42.182/sweethaven/MechTech/Automotive01/default.asp?unNum=2&lesNum=3&modNum=1

http://www.dkw.co.za/Cerametallic_Coatings.html

http://www.babcox.com/editorial/ar/eb40354.htm

Interesting reading Leathermang...

I use the KB Hypereutectic Pistons in a number of engines with very tight bore clearances (.002") and ring gaps of .014" top and .016" second. Good quality products right out of the box, although I opt to have my skirts teflon coated before delievery.

One thing I have noticed is almost all liquid cooled engines will run at 150° to 160°F even without a thermostat. The interesting part is how fast the engines come up to that temperature.

On my 7.4 Litre Pontiac, the heater is 'warm' in less than 1 minute and the engine is up to 160° in less than 5 minutes.

On a 104° day here in California, sitting in the drive-through at In-N-Out for 25 minutes with the A/C on, the coolant temperature will never go past 185°F. That is with the stock 4 row copper/brass radiator, 60/40 water/coolant ratio and stock engine driven 7 blade fan.

At 104°F ambient temperature at 70mph with A/C on, coolant temperatures stay right at 165° to 170° and steady.

My theory has always been that with the 'hollowed out' thermostat body in place, functioning as a restrictor, the coolant flow would be identical to the flow available when the thermostat was fully open.

No I certainly agree with you about the 'cold oil' theory, but in reality, the oil heats up quick in an engine, must faster than the coolant seems to.

Much like 'piston slap' that we used to experience with Forged Aluminum Pistons and very loose clearances (.007") cold, would go away after about 30 seconds because the pistons heat so quickly.

Interesting...Bert

The ' cold oil ' theory is more Brian's ...... Mine is the ' oval cut pistons made from aluminum are designed to run at a temperature already figured out by million dollar R and D departments ( and warranteed engines returned to them if necessary ) so that the expansion fits the bore correctly ( and particularly close on our Diesels ) during running.

You are correct, Leathermang and those pistons are likely up to temperature within a minute of exposure to combustion heat.

With the old TRW 'Powerforged' Racing Pistons, you had to run them really loose, about .007" clearance cold and they would 'slap' until they warmed up. It was a very hollow, distinct sound.

The pistons picked up enough heat to quiet down after 30-45 seconds of running...Bert