old failed rotary valve on GTR

[old smokey]

I removed this rotary valve from the right cylinder of my GTR back in 1986. No reason why I never threw it away, but since i still have it I'm wondering if anyone has suggestions as to why it cracked.
The photos aren't real crisp, but I think it shows enough of what happened. I didn't really know it was bad, I just happened to remove the covers and turned it over by hand and saw this crack.

[Jeff Bar]

That is normal rotary valve failure, as the fail from flexing, therefore the fibers weaken in that fashion, just replace it, I would do both sides

Jeff Bar

[Bridgestone Man]

I am not sure this is normal!  Looks like some foreign object went through the system, just my idea
Sam Keys

[old smokey]

the side facing the cylinder (I called it the backside) has more of a look of flexing and tear, but the front side has almost a look of a machined groove that starts deepest by the opening and tapers out. if it was a foreign object it looks like it only made part of one revolution. I've never had the heads off or done a compression check, but it always seemed strong.