RS-555 back-end shaft length

To whomever can help,

I am looking to mount an E4T US Digital encoder to the back of the 6100 RPM RS-555 motor that RobotShop sells (Product Code:RB-Wtc-06). The encoder needs at least 1/4" length of axle to grab onto. However, I was unable to find a specified length in the dimensions section of the product page.

My question is: Is there a dimension for the length of the back end of the shaft for the RS-555 and, if so, what is it?

Thanks,
Sean

The rear shaft is 13.5mm long. Sorry for the late reply.