Underlying premise: The VW engine is cooled by AIR.
The engine compartment designe in a VW, including the tinware surrounding the engine, is an important feature of the VW beetle. The tinware around the engine is designed to seal the upper side of the engine from the underside. Fresh cooling air enters the upper engine bay through the under-window slots (and the engine lid slots where fitted), goes through the fan and is blown down over the cylinders and heads, and out under the rear of car. The tinware around the engine is designed to keep these air paths separate so you never get used hot cooling air sucked into the cooling fan. 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 tinware is in place, properly fit, well secured and in good condition.
Volkswagen Engine Tin
Volkswagen cooling system is neat and simple. A multi-bladed centrifugal
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 called the fan shorud,
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.
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. This makes the engine both directly air cooled, and also oil cooled, and a LOT of heat is removed via the oil cooler. There are two different oil cooler designs. The arly version (pre 1971) has the oil cooler stnading up in the cooling airstream for the left cylinders, so they tend to run a little hotter than the right side cylinders. That worked Ok until the engine horsepower increased towards 60hp, then they started having problems with occasional broken exhaust valves, particularly No3 (left front) cylinder. In 1971 they introduced the "doghouse" cooling system, which moved the oil cooler out of the main cooling airstream and into it's own "doghouse" with a separate supply of cooling air from the now larger cooling fan. All cylinders then got nice cool cooling air, and more of it too.
Apart from the large piece of cooling tinware around the rear of the engine (the rear breastplate), 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 "cheesehead" 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.
installing the fan shroud, the doghouse oil cooler tin, and the firewall
tin (front-most), you must install at least the cylinder cover plates.
However, you want to install the oil cooler before installing the
left cylinder cover plate; other wise access to the oil cooler adapter
nuts is very difficult.
The engine tin is installed at the longblock buildup proceeds – not all at once. Make sure all of the fastening bolts are in place and snugged down tight. Here’s the approximate sequence –
- Rear air deflector plates. These must be installed first - the engine breast plate and cylinder cover plates go on top of the deflector plates. If you fail to do this, the deflector plates will be very difficult to install (that's why most people leave them off - a mistake).
- Do not fail to install the two rear deflector tins! (No 11 in the exploded pic above shows the left side version. The right side is not shown in that pic. They are also pictured below). These go on the rear of the engine, under and to the left and right of the engine pulley. They should attach to the tinware that covers the cylinders, and they form the rear bottom section of the cylinder cover so the cooling air has to travel to the bottom of the finning before turning towards the rear of the car. Without them attached the #2 and #4 exhaust ports get reduced cooling air through the fins around them as the air will spill out to the rear just above the ports.
- From Aircooled.Net - These are the often missing and lost pieces of tin that keep cooling air tight against the head, at the rear of the engine. If these are missing, head temperatures shoot up. Commonly missing because butcher “mechanics” neglect to install them, or by sheetmetal screws that vibrate loose, and they just fall off. We can’t stress enough how important these are.
- You MUST install the rear deflector tins before installing the muffler header. With the header in place, the deflector tins are almost impossible to install.
- The deflector tins each have two captive nuts; the engine breast plate is secured with two cheesehead bolts through the breast plate on either side, and into the captive nuts in the deflector tins.
- You will probably find it necessary to bend the deflector tins a bit to make them fit. They are equipped with tabs that are supposed to fit around the cylinder fins. Installation of the deflector tins is a bit difficult. The hardest part (for me at least) was starting the bolts through the rear engine tin and into the captive nuts on the deflector tins.
Left and Right Rear Deflector Tins
Engine breast plate.
Cylinder cover plates.
Note: The two cover plates are a right and a left, so make sure that you get the correct orientations. You may have to do some minor modification so that the manifolds will clear the lip of the tin.
Again – Install the oil cooler adapter before installing the left cylinder cover plate.
Cooling air channels (following installation of the heater boxes).
Oil cooler tin and exhaust tin.
Rob wrote in response to a query about the piece of tin that wraps around the lower part of the engine pulley (Part #38 - see the engine tin diagram above). 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 (deflector tin) 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). The deflector tins are easily installed with the rear engine breast plate (Part #30) removed - one of the bolts (on each side) that secures the breast plate also secures the deflector tin.
Another piece often missing is the small deflector plate under, and in the center of the heads - it works like Part #5 to force air fore and aft through the finning. Without it the heads run very hot underneath, and without Part #5 the cylinders run hot underneath.
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