The engine compartment in a VW, including the tinware surrounding the engine, is a closed system designed to keep air flowing around the oil cooler and cylinder and head fins. Anything that defeats this system will make the engine run hot. Air-cooled VW engines run quite close to their heat limits to begin with, so the margin of safety is small. Make sure that ALL of the tin is in place, properly fit, well secured and in good condition.
The Volkswagen cooling system is neat and simple. A multi-bladed turbo fan is mounted on a shaft which is simply an extension of the generator armature shaft. The cooling fan is designed to suck air into the fan housing. It rotates in a sheet steel, semi-circular housing, drawing in air through the fan centre and directing it down to each pair of finned cylinders. Cooling air then directed by the cooling flaps and tinware over the cylinder heads and cylinders.
The cooling tinware in the sketch above is shown in red. The cylinder heads and cylinders are shrouded by carefully designed sheet steel covers (the engine "tin" or "tinware"). The tinware directs the cooling air from the fan housing. Below each pair of cylinders a contoured deflection plate is mounted centrally. Thus the air is directed over the full surface area of the cylinder cooling fins.
In order to shorten the warming up time a thermostat is mounted below the right hand pair of cylinders. This is a conventional bellows type and it operates a restriction on the through flow of air when the engine is cold. The thermostat opens flaps in the fan housing when the engine warms up, so allowing the full airflow to pass round the cylinders.
Also in this fan housing, bolted to the top of the crankcase, is an oil cooler that stands up in the stream of air like a radiator and cools the oil, which is pumped through it.
Apart from the large piece of cooling tinware at the rear of the engine, the remainder of the cooling components and cover plates fitted to the engine cannot be removed unless the engine is removed from the vehicle.
There is a tendency for the screws holding the tin work to disappear over time, but these are commonly available from almost any air-cooled VW parts outlet for a few cents each. Replace them when you find them missing.
Rob wrote in response to a query about the piece of tin that wraps around the lower part of the engine pulley (Part #38) -
I bought my 1970 Beetle brand new, and it has the #38 piece on it -- it's just a cover which stops you dropping screwdrivers, etc. down under the engine pulley, as it fits very closely around the pulley and belt, exposing only the top of the engine pulley. Absolutely no effect on the cooling if it's not there. I haven't seen it on later engines.
One piece I do see missing on many engines is Part #11 and it's counterpart on the right side (not shown in the diagram).
These fit on the rear side of the cylinder tins and force the cooling air down past the cylinder fins, and encourage airflow past the very hot exhaust ports in the heads, before allowing the air to turn rearwards through the air exit.
They might not seem like much, but they are a very important part of the cooling system -- almost as important as the under-cylinder deflectors (Part #5).
Another piece often missing is the small deflector plate under, and in the centre of the heads - it works like #5 to force air fore and aft through the finning - without it the heads run very hot underneath, and without #5 the cylinders run hot underneath.
Disclaimer stuff: Rob and Dave have prepared this information from their own experiences. We have not assumed any specialised mechanical knowledge, but we DO assume that anyone using this information has at least some basic mechanical ability.
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Last revised 26 May 2004.