I have just completed a partial rebuild of a Yanmar 1GM (6.5 HP), or
at least I though I had! Some background: A bent con rod was causing
very difficult cold starting, although once started it ran very well.
The new con rod (plus big end bearing shells) has reduced the piston
top clearance to 0.9mm (from more than 1.2 mm originally, the
thickness of my solder wire) and the piston height above the cyclinder
top surface has increased from 0.15mm to 0.4mm. This has considerably
improved cold starting. The rings were replaced at the same time. Oil
pressure is fine

The injector has been serviced, the engineer said that it was seized
(the engine had been stored for 4 years) but he had managed to free it
up and it should now be OK.

With the new con rod fitted, on the bench the engine starts and runs
fine most of the time. However,it occassionally and suddenly develops
a very severe knocking which sounds mechanical as if the engine is
being hit with a hammer. So far this has only happened at low revs and
I have immediately stopped the engine. On restarting everything is
fine again until it happens again. It does not seem to be associated
with anything else.

One thought which occurrs to me is that the reason the con rod was
bent originally could be due to water entering the cylinder, possibly
via a cracked head. The knocking could then be the sound of the
resulting hydrolicing with the piston hammering into the water at TDC.
Having taken the head off again I find nothing to suggest this.
Everything is evenly covered with a thin layer of carbon. If it realy
was something mechanical, worn crankshaft bearing for example, I do
not understand why I do not hear the knocking all the time. It must be
something to do with the reduced piston top clearance and consequent
increased compression as before replacing the con rod no knocking was
evident.

Any suggestions for the cause of this problem would be gratefully
received.

Thanks,
Phil