On Jul 29, 9:03 am, Bob Kuykendall wrote:
On Jul 28, 11:47 pm, " wrote:

...I hope you realize there will be quite a bit of clearancing with a 78...

Oops, I think I slipped a groove somewhere. I thought you wrote in
_The Orphaned Engine_ that with a 78mm crank there was no machining
required:

http://bobhooversblog.blogspot.com/2...ed-engine.html

I also thought that "clearancing" was machining work on the inside of
the crankcase to remove material that would otherwise interfere with
the movement of the connecting rods. Or do you mean some other type of
machining?

In so far as I know, clearancing and other 'hand-work' is not
considered 'machining' in that it is a 'go/no-go' sort of thing. You
simply remove metal until you have adequate clearance.

As a point of interest, the last air-cooled engines made by Volkswagen
were apparently designed to accept a 74mm crankshaft, which will drop
into a 1600 crankcase without any clearancing at all. With 85.5mm jugs
this gives a displacement of 1699cc and appears to reflect
Volkswagen's assumption that the USA would not adopt the stringent
California air-pollution laws.

The normal method of increasing the VW's displacement is to replace
the stock 85.5mm cylinders with those having a larger diameter. This
requires boring a hole, centered on the existing hole in the
crankcase, and a matching hole in the heads Most don't realize it but
the accuracy of these eight is of critical importance to the
durability of the engine. This is a task best done on a milling
machine. Unfortunately, most VW conversions are done with portable
tooling and the accuracy is all over the map.

By increasing the STROKE rather than the bore you not only eliminate
the need to machine the eight holes, you have eliminated a chronic
source of compression leaks that arise from inaccurate spigot bores.

-R.S.Hoover