Download Building a VW Engine

had no form of oil filtration and despite myths to the contrary, the average service life
seldom exceeded 60,000 miles before major repairs were required. ('Major Repair' here
means any repair requiring the removal & dismantling of the engine.) Catastrophic
failures (ie, swallowing a vlave, usually leading to throwing that rod) were quite
common.
Many people advocate use of pre-1956 engine overhaul techniques on Volkswagens (ie,
the classic ring & valve job). Unfortunately, not only do these techniques not apply to
horizontally opposed AIRCOOLED engines, the tooling and skills are simply no longer
available at an economic price; it is less expensive to install new jugs & cylinders.
Assuming the crankcase is still sound.
Early Type I VW engines are cast of AS21 magnesium-aluminum alloy. Due to the
magnesium extraction process used by Dowmetal, the alloy contains trace amounts of
niobium and tin. If the alloy is raised above a certain critical temperature, the metal will
display 'memory' qualities. Unfortunately, the 'read' temperature is much lower than
the 'write' temperature.
What all that means is that an early VW engine subjected to catastrophic failure
(typically required to raise the temperture above the 'memory-write' threshold), will
tend to return to its heat-stressed dimensions when raised to the 'memory-read'
threshold. Unfortunately, the memory-read threshold is within the NORMAL range of
operating temperatures.
This odd metallurgical quirk means that you can remachine a used crankcase and return
it to spec with regard to alignment & bore only to see those specs CHANGE when you
run the engine. And those changing specs typically result in a catastrophic loss of oil
pressure which usually trashes the engine in short order.
Having only three main bearings (#4 is actually a pulley bearing) and being of the 'boxer'
type, catastrophic failure tends to overheat (and over stress) the center main-bearing
web. This is expecially critical in the VW engine because the center main bearing web
provides the only passageway for oil to reach the right side of the engine. Once that
web has been heat-stressed, normal operating temperatures is enough to cause
sufficient distortion to open up that critical oil passage and prevent sufficient lubricant
from reaching the right-hand side of the engine.
Once the memory property was recognized, the alloy used in the crankcase was
changed. Crankcases cast of AS41 do not display the memory property.
If you do not know the provenance of an older, used VW engine (ie, of AS21 alloy) it
would be extremely unwise to consider it as a candidate for use in an airplane. Even if
align-bored and decked so as to return the crankcase to spec, if you don't know the
Page 2 of 119