balancing a 277 rephased crankshaft?

[KentMoney]

Hey guys,

So I am in the middle of a 1981 XS650 Special project and right now I have the engine all apart and finally got access to the crankshaft. My intentions are to rephase this sucker 277 degrees; I already bought the camshaft from Mr. Riggs so now I need to rephase the crankshaft.

The thing is, I never hear about people balancing the crankshaft/rods/pistons after a rephase, is this necessary? If I can get it done at a precision machine shop, should I do it or do I run the risk of getting crazy vibrations? I am looking to get my bike good for city riding (stop-go, quick acceleration, between 2k-5k RPMs), but I take 2-6 hour trips along California so I also want it to be comfy for cruising from 65-80mph. I am putting in the 5th overdrive so that should help, but any opinions on this?

Thanks yall. I also plan on starting a build thread soon.

 

[hotdog]

KentMoney, You need to "true" the crank after disassembly, to ensure both flywheels are running parallel to each other otherwise the crankshaft on each end of the crank webs wont be running true to each other.
Balancing is not required as you are not changing the mass of the individual cranks, only rotating them on their centre spline, most bike shops should be able to "true" your crank for you, you will have to ring around.

Cheers.

 

[KentMoney]

Thanks for the reply, hotdog! Ah yes, I already knew about the truing and was having the shop do this as well as I don't have the dial gauges. But i thought that now since the pistons would be going up at different times that it may be 'out of balance'.

But I suppose what youre saying is that the 'balance' is a function of the piston being counterbalanced by the flywheel it is attached to, therefore no need to balance even if it is not a 360 phase anymore?

What if I change the piston rings? Or go to a one-oversized piston?

 

[Sundie]

screw one oversize pistons. just get the 750 kit. in for a penny, in for a pound.

 

[Jack]

Hey guys,

So I am in the middle of a 1981 XS650 Special project and right now I have the engine all apart and finally got access to the crankshaft. My intentions are to rephase this sucker 277 degrees; I already bought the camshaft from Mr. Riggs so now I need to rephase the crankshaft.

The thing is, I never hear about people balancing the crankshaft/rods/pistons after a rephase, is this necessary? If I can get it done at a precision machine shop, should I do it or do I run the risk of getting crazy vibrations? I am looking to get my bike good for city riding (stop-go, quick acceleration, between 2k-5k RPMs), but I take 2-6 hour trips along California so I also want it to be comfy for cruising from 65-80mph. I am putting in the 5th overdrive so that should help, but any opinions on this?

Thanks yall. I also plan on starting a build thread soon.

The rephased engines will run smoother with heavier piston mass ,balance factor should be in the neighborhood of 50 to 53%. 3M from 650Central has more knowledgeable experience when it comes to rephasing the XS.

 

[Punkskalar]

Listen to Jack, he really knows his stuff.

For your goals, even the stock bore will work fine with the Rephase. Cruising speed will be easily attained with minimal vibration. Just make sure to weld the crank as you build it, the tend to seperate when pushed hard...