It is NOT 'string' it is SILK. More specifically, it is #50 THREAD, although I have seen a German-born, factory-trained mechanic spinning free a few STRANDS from silk embroidery thread. This method applies ONLY to the crankcase's EXTERIOR parting flange. NOTHING is applied to the TOWERS around the cam-shaft. The interior parting line associated with the CRANKSHAFT are provided with molded rubber O- RINGS, except for the 8mm diameter studs associated with the #4 bearing. This IS NOT a main bearing as such. It was added to compensate for excessive wear of the CRANKCASE. This will cause many to frown but the fact is, the Volkswagen engine did NOT HAVE replaceable bearing shells until the late 1940's, the crankshaft AND the camshaft used the CRANKCASE as their bearings. Indeed, the camshaft was not fitted with replaceable bearing shells until the mid-1950's.
We owe the appellation of 'main bearing' to the #4 bearing to the ignorance of American technical editors who applied that name, largely because the #4 bearing was packaged with the three Main Bearings. Prior to that time the pulley-end of the crankshaft continued to use the parent metal of the crankcase as the bearing for the crankshaft. Alas, the eccentric pull of the dynamo/blower resulted in eccentric wear of those 'bearings.' This wear was exaggerated after the three main bearings were fitted with replaceable steel-backed bearing SHELLS in the mid-1950's until a replaceable bearing was provided for #4 as well. This transformation took more than a year and resulted in many an arguement among VW mechanics... which continues to this day, in that some insist on calling the #4 a 'main' bearing despite the fact this particular race was NOT initially fitted with a replaceable bearing shell until some months after the three main bearings were fitted with replaceable bearing shells. The arguements continued even after the issueance of a Servvice Bulletin giving specific dimensions for opening up the early-model crankcases to accept a replaceable ring- type bearing shell.
In the same vein, the cam was NOT provided with replaceable bearing shells until the 1960's. This was taken as 'proof' that the replaceable ring-type bearing shell was in fact a 'main' bearing.
The truth is, it doesn't really matter, does it? In overhauling the lower end, if you happen upon an engine that still has the cam running in the parent metal of the crankcase, the engine is more than fifty years old (!) and should be returned to RIMCO to have the crankcase overhauled so as to accept replaceable shells for the camshaft. Or to have the thing mounted on a pedistal as an example of German engineering, in that the engine has survived more than half a century of use.