I was reading an article about a classic VW air cooled Beetle, and they wanted the piston to move a greater distance (presumably on the compression stroke,) than the crankshaft they had on hand was designed for, so they did something called “offset grinding” to the crankshaft. What they heck does that mean, and how would it make the piston move a greater distance?
They weld the crankshaft journals completely up with beads of weld. The journal is then reground but on a different centerline than it was originally. They usually move the centerline out a small amount and this leads to an increased stroke distance.
As a crude analogy, think of moving the pedals out further on a bicycle.
This can often mean alterations to the pistons, use of a different type of piston and/or connecting rod, or stroker plates underneath the cylinders to keep the pistons from banging into the valves and/or cylinder heads.
Ok, I think I understand the process now. Thanks. I thought they were just grinding away the existing journal to move the center out, and wouldn’t the smaller journal diameter make it weak? But it looks like they can keep the same diameter and still move it out by adding material first.
It’s usually not cost effective, but in some cases involving a rare or difficult to obtain crankshaft which has suffered major damage to one journal only is often welded up and reground to the same standard the other worn but undamaged journals are machined to.
In some cases even camshafts have had lobes welded and then reground to the proper or even a different spec. This is a pricy procedure and seldom done but I’m just pointing out that it’s possible and has been done when necessary.
I have read about grinding smaller connecting rod journals offset to increase stroke/displacement (and compression ratio if nothing else done).
There are aftermarket stroker cranks for the air cooled VW.
It was a popular engine for modding.
They didn’t have to “roll their own”.
Offset grinding is an old hot-rodder’s trick to increase displacement. It can also be used to increase compression (and a little displacement) by getting the piston a little closer to the head at top-dead-center with small offsets (0.020 to 0.030 inches). You can then use the factory connecting rods with undersize bearings. If you are grinding the crank, already, it is very easy to do at that time for very little extra cost.
Offset grinding the crank can be done either of the ways described above, weld the outside of the journal and grind to increase the stroke OR reduce the crankpin diameter to increase the stroke but decrease the rod bearing size, too.
Popular engines, like VW flat 4’s, Chevy big and small block V8’s, flathead and 302 Fords have a variety of long stroke cranks made for them so there is no need.