The Atomic 4 Thermostat Housing Fix 
The Atomic 4 Thermostat Housing Fix The late model Atomic 4 incorporates a thermostatically controlled bypass
in the cooling system to assure that there is always water to cool the
hot exhaust gases. It also makes possible a quicker engine warm-up. The
thermostat is located in a dome shaped housing on the head.
Water entering the pump is pumped through a hose running along the starboard
side of the engine to a T on a water manifold plate where it may enter
the engine or go straight to an inlet on the thermostat housing. When the
engine is cold the thermostat is closed. Water stays in the block and head.
This facilitates a quicker warm up of the engine. While the engine warms
up, water is being pumped through the bypass hose to the thermostat housing,
then to the water chamber in the exhaust manifold, and finally to an injection
point on the exhaust line where it is mixed with exhaust gases.
On a raw water cooled engine the thermostat opens at about 145 degrees.
When it opens, water from the block and head is forced into the thermostat
housing by the pressure of the water being pumped in through the water
manifold. This cools and regulates the temperature of the engine. At the
same time as the thermostat is opening it is closing against the boss or
nipple which allows the bypass water to flow into the thermostat housing.
When the bypass is closed off, all of the water is being pumped through
the engine and head, etc.
A common problem with this cooling system arises when the boss in the thermostat
housing becomes corroded to the extent that the thermostat can no longer
close off the bypass. When this happens the engine overheats. Left unchecked,
the small passages between the block and head will become encrusted with
deposits and the engine will eventually fail.
When the thermostat is fully open it should close off the bypass.
If the boss (nipple) is corroded you can restore it to spec by drilling
out the opening (the inside of the nipple) to accept a 1/2" piece
of copper pipe. Jam the pipe in the hole and mark it at 0.4"-0.5"
below the flat surface of the thermostat housing. Spec is .04 in.
and is more critical with the older Holly thermostat. The thermostats'
operating ranges are: Original OEM Holley 3 spring: closed (cold) 0.325
in / open (hot) 0.625 in and replacement OEM Westerbeke single spring:
closed (cold) 0.2 in / open (hot) 0.8 in. Cut off the short
length of pipe and epoxy it into the hole. Grind it off perfectly
flat on the end that will mate with the thermostat when it opens.
Build up a "little volcano" around the pipe with epoxy putty.
Drill out a hole thru the bypass entrance into the pipe, or cut it
out before you jam the pipe into the hole. That's it!!
If your engine is over heating, you can test to see whether your bypass
is being closed off by putting a rag on the bypass hose (between the water
manifold and the inlet to the thermostat housing) and clamping it with
a vice-grip. If the temperature drops after clamping off you know that
either your thermostat is defective or the boss is corroded. Clamping the
bypass will get you home where you can fix the problem. Good Luck!!!!
 |
 |
| Late Model Atomic 4 Head with Thermostat | Thermostat housing with "the fix" and Holly Thermostat |