Originally Posted by metromizer
I have a few comments for the original poster:
Crankshaft- when destroking an engine, you usually purchase a destroked crankshaft, or modify the one you already have by a couple of methods.
a) no one makes aftermarket crankshafts for Geo engines
b) the traditional offset grinding you read about requires a different, smaller journal rod and some research.More than a couple mm is not usually possible without grinding into the oil passeges. Finding a rod with a smaller journal than a Geo my be tough, they are already tiny. Research, research, trail and error.
c) Weld offset grinding, by a process called submerged arc welding, can only be done on a steel crankshaft, and the first several cranks are research projects. The 4cyl metro crank on my bench is cast iron, I'd guess the 3 cyl crank is too. so weld stroking is out.
d) custom cranshaft machined from billet. I've personally ordered semi-custom cranks (custom stroke, different main journal and rod journal sizes, etc) for commonly modified engines. It's expensive and often takes months. But their isn't much call for semi-custom Geo cranks. Fully custom odd-ball engine custom cranks have been built, but it's painful, $$$, time, begging and pleading to stay on schedule...
Rods -you never modify rods to make them shorter, or longer. For commonly modified engines, different length rod options are available. The metro engine does not fall into that category. However, with a big budget, semi-custom connecting rods can be ordered for just about any project from places like Pauter Machine in Chula Vista, CA, Cunningham in Denver, Crower, etc. for about $250-$500/rod. The last semi-custom set I bought from Pauter, 2" chevy journal, 356 Porsche length, 22mm wrist pins, light weight chromoly race rods cost ~$800/4 rods
-pistons with custom placed wrist pins, higher for larger stroke, lower in the bore for a shorter stroke, is a common way to account for changed stroke and not modifying the block. The dimension you'd change is called 'compression height' of the piston.
So in a nut shell, you might be able to shorten the stroke by a few mm's, if you can find the right rod journal diameter, rod, and custom pistons.
I hope that is helpful.
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