I was fortunate to have several good crankshafts around from which to choose in my rebuild. Of course I had some bad ones too!
One thing I was curious about at this point where the differences, if any, between the YZ465, IT465, IT490 and YZ490 crankshaft assemblies.
This is what I found from the parts books.
First, the exploded view of the YZ465 crankshaft assembly,
Cross referencing the part numbers shows the crank to be interchangeable across all YZ465's and YZ490's.
The rod assembly is interchangeable for all of the YZ's and IT's.
There is a difference, however, in the crank flywheels between the YZ's and IT's. The YZ cranks have lightening hole in the flywheels that allow them to spin up more quickly. The IT cranks are solid making the IT465/490 more tractable and less likely to stall in a slow corner.
Here some photos of the IT490 crank.
Compare these with the YZ465 crank shown below.
For this build, I have stayed with the YZ465 crank.
Now, even with what appears to be a good crank, there are several potential issues to watch for. For instance it is not uncommon for a crank seal lip to cut a grove into the crank over the course of time. Here is an example where a grove has worn into the ignition side of the crank bearing surface. This crank was fine otherwise.
It is not necessary to repair this. The solution is to assemble the motor without the crank seal. Then, after careful measurement, you insert the crank seal into the case at a depth that avoids placing the seal lip into the groove.
Other issues to watch for are keyway condition and galling on the crank taper. Galling can occur when a flywheel key shears allowing the flywheel to move on the taper.
The taper should also be checked on the flywheel,especially when mixing and matching parts for motors you do not know the history of. One of my flywheels was severly damage on the internal taper in addition to having an extra key picked up by one of the magnets!
With a good crank selected, it is important to install it into the case without disturbing its alignment. From one of the previous pictures you can see that I chose to assemble the transmission and crank into the left hand side case first ( I think either side is OK though). When installing the crank I used a stack up of large sockets, washers and nuts to pull the crank through the (new) crank bearing.
The idea is to pull against the crank bearing inner race as you do not want to put a side load on the crank bearing. And make sure your sockets or spacer do not impinge on the crank taper as you tighten so as not to damage it.
more to come...