John Barton continues hos article on the GPW engine

rebuild this time he tackles the timing.

In a previous part, I mentioned that it was not necessary to

know the 4-cycle engine theory to rebuild this engine. It

does, however, help a little to understand some of it when

dealing with setting up the timing. In a 4 cycle (also called 4

stroke) engine, the usual description of this cycle starts with:

the intake, then compression, then power, then exhaust

stroke. During these 4 strokes the crankshaft rotates twice.

Turn the crankshaft clockwise, looking from front, (nut on

front/or with flywheel bolts) until the front- #1 piston is at the

top. #4 (back) is also at the top.

Both are at the position called TDC- top dead center. You

don’t need to worry which stroke is involved at this point.

Until the crankshaft and camshaft are in time, just realize

that the crankshaft turns twice during the four stroke cycle,

and that the camshaft turns once- (half speed).

When the cam gear is installed- temporarily as in one of the

pictures below, you can see the relationship. Cam gear is

twice as large as crank sprocket. When connected by chain,

the crankshaft must turn twice to each cam revolution. This

is because while the pistons must go up and down twice (2

crankshaft revolutions) to complete the 4 stroke cycle, the

valves must only work once for each cylinder (1 camshaft

revolution) during the 4 stroke cycle. I have all the camshaft

parts laid out.

Both types, rope and the newer modern timing cover seal

are shown. When the old steel retainer is removed, the

asbestos rope seal must be removed. Clean up the sur-

faces and install new type with seal driver…a large ˝ drive

socket that fits diameter of seal, also works.

By

John Barton, USA

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