I had a recurring problem of cooling system debris plugging up the coolant passages in the upper radiator tank of my 1940 1/2 ton pickup. I had flushed the block quite well by removing the freeze plugs on the side of the block and using a nozzle made from 1/4" copper tubing adapted to the end of a garden hose. This removed all the sediment (and there was plenty) in the trough below the freeze plugs. I thought the block/head were pretty much cleaned of loose debris, but after my radiator plugged up the 2nd time, I decided I needed to install some kind of filter screen in the coolant return line to the radiator.
I went to the local hardware store in search of some wire mesh or screen, which I thought I might be able to fashion into a dome or cone shape to fit in the thermostat housing. The only thing they had was a huge roll of aluminum window screen material, which was not satisfactory, but on my way out, I decided to make a pass through the plumbing section and see if I could find something that I might be able to adapt. What I found was a bathtub drain strainer, which appeared to be the perfect solution to my problem. The following photos show how it went together. Note that the thermostat fits nicely under the dome of the screen. The strainers come in two sizes, and you'll want the smaller size (Ace Hardware P/N 4202693).
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I cut two simple round gaskets from 1/32" general purpose gasket material, between which to sandwich the screen. Note that the lower gasket has a somewhat smaller inside diameter, which serves to hold the thermostat in its seat in the housing.
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The photo below shows the gaskets and strainer ready for assembly. I used a very thin layer of silicone adhesive on both sides of both gaskets.
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Shown below is the completed assembly. When installing, tighten the thermostat housing bolts evenly, and don't over-torque them, as the ears of the thermostat housing are cantilevered over the water neck, and you could break the ears off the housing or the water neck by excessive torquing.
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To check the cleanliness of the screen, it is only necessary to drain about a gallon of coolant from the radiator, then remove the upper radiator hose. You can look at the screen with a mirror and flashlight to see if is clean or if it requires removal for cleaning.
On my truck, an inspection of the screen after the first test run of about 30 miles showed the screen to be about 40% blocked with debris. After removing, cleaning, and reinstalling, subsequent test runs have shown the screen to be completely free of debris. I believe the debris after the first test run accumulated because the entire cooling system had been drained prior to the screen installation, and, just like when your municipal water system is shut down and drained for repairs, on startup, there is considerable dirt and debris in the lines, but once cleared, they run clean.
Mark