XJ550 Vrs FJ600 Crankshaft

Besides part numbers who knows the difference between the two crankshafts?

 

Different stroke, 51.8mm vs 55.7mm, FJ has shorter rods from memory to compensate as the blocks are the same height (I'd have to check that but pretty sure)

600 is basically a bored and stroked 550

 

Yes I know, but I'm interested in the crank. I am building my 600 and have a choice of cranks.

 

What's the base of your build - a 550 or a 600? Do you mean a choice of different 600 (longer stroke) cranks. I also note that while the FZ600, Radian and early FJs motors looked identical to the 550 (same ignition set-up etc.) the later XJ600s in Europe (up to 1990) had a different set-up which maybe affected the crank. Did you consult yamahapartshouse.com to cross reference numbers?

 

The rod's are different length, that controls the stroke not the crank itself. This is a FZ600 motor.

 

could you explain how that works?

 

The combination of crank journal distance from the center of the crank (crank stroke), and connecting rod length is what determines the overall piston stroke. Altering either, or both, will change the stroke.
The FZ600 crank (dame as the XJ600) has a different part number from the XJ550 for a reason, and my first guess is that they have different crank stroke lengths.

 

think about that for a few minutes

 

Con rod length has nothing to do with it, it is the crank only which determines the stroke

No it doesn't, it is all about the crank

 

FZ600 engine then use Fz600 crank, unless you want to build an FZ557 (with low compression).

 

That's what I thought, thanks.

 

If you don't change the rod length to compensate for the change in crank stroke there will be consequences. Clarity was lacking in my prior post.

 

If the rods are either too long or too short and you can't find a rod a suitlable length you can also use spacers under the barrel or shorten the barrell, we've done both on different engine builds (not XJ)

 

Plus I was not thinking about the fact that rod length changes alter the compression ratio, no the actual stroke length.
Sometimes me no smart....

 

Something else to consider when swapping 550 & 600 motors is the drive sprocket alignment. It's not clear what you're doing here, you say you're building a 600. The FJ/XJ600 I believe has the same chain alignment as the 550's. The FZ and YX I think has the drive sprocket spaced further out. Someone else may verify this.

 

Thanks for the heads up.