Until 7/1969, the oil channels on Type 1 and Type 3 engines were 8 mm in diameter, then 10 mm and 12 mm, so they're often referred to as small or large oil channels. If the oil cooler needs replacing, they're only available new with large oil channels 01906 , so in theory you couldn't install a new oil cooler on an engine with small oil channels. Volkswagen spotted this problem early on and came up with this oil cooler reducer joint which allows you to fit an oil cooler with large oil channels to an engine intended for small oil channels without any problems.
With these adapting seals, it is also possible to fit a small oil cooler on a large-channel engine, but this is not recommended, as it would restrict the oil flow of an engine with a better basic flow.
Type 1 engine
Up to 1969, oil coolers were flat on the underside and located on the crankcase with no space between them. After 1969, there is a certain amount of space between the radiator and the crankcase. Mounting an old radiator on a new crankcase without rings means you're pushing the seals too far and obstructing the passage.
Type 3 engine
When using radiators with large oil channels and mixed crankcases on Type 3 engines, you must use the reduction gaskets, but in both cases without the rings.
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