Cylinder 3 &4 cooling improvement
Seizures on earlier cars, and other piston failures, are almost always associated with cylinders 3 & 4 (especially 4).
Apparently, there is a design issue with the cyl head cooling method.
Let's look at the whole system.
Coolant comes in from the pump at the front of the block, and goes out to the thermostat and radiator from the front of the head. Coolant circulation around the back of the engine, cylinders 3 & 4, is minimal.
Heater takeoff comes from the rear of the head, which is the same for all 910 engines. If the heater switch is "ON", there is a coolant flow through the block and rear of the head to the heater matrix, which improves the flow/cooling around cyl 3 & 4.
However, with the heater switch in "OFF", there is no flow trough the matrix and coolant flow is stagnant around the rear of the block/head. Temperatures can rise leading to possible piston/liner seizure.
You are especially at risk if your heater valve is not working properly.
On 1989+ cars equipped with a water cooled turbo, some flow is maintained trough the 1/2" hose, which connects turbo CHRA to the overflow header tank. However, such flow may not be large enough to make a difference.
The obvious solution would be to create flow independently of the heater valve position or condition.
One could fit a 5/8" barbed "tee" at the horizontal spigot below the thermostat (heater matrix return hose) and route an additional 5/8"or 1/2"ID hose to the right, along the rear bulkhead and right eng. compartment partition wall to another 5/8" tee fitted into the hose from rear of the head leading to the heater matrix inlet.
Ideally, it would be more efficient to tap into the large aluminum pipe (the shepherd's hook) going to the radiator.
