stretched prints

Hi there,
just wondering if any one could advise iv just built a rep rap Mendel but scaled the frame up just for ease of access the bed size is the same i just wanted to be able to print higher and also make it easier to get round the machine, but when i print the object is being stretched in the z axis (the height), for instance i printed a fan mount for my tricolour Mendel and the height should be about 45mm tall but instead its 70mm tall. the thing i can not work out is that its not jumping or any thing its printing perfectly just stretching upwards. any ideas would be greatly appreciated.
this is not my first printer its about my tenth and never had this problem before.

Have you calibrated the printer?

A good place to start is this guide: [reprap.org]

Hi there,
i have just realised i have used M8 rod for the z motors and not the normal M5 so need to change the STEPS_PER_MM in the firmware's config.h, any one advise on how to go about this.

Hello,

Indeed, the pitch of M5 screw is 0.8mm while M8 screws have 1.25mm pitch, which explain the problem. Here is a tool enabling you to calculate your new STEPS_PER_MM parameter.
[calculator.josefprusa.cz]

In order to change your STEP_PER_MM, you have two methods:
- Using gcode : [reprap.org]
It is simple and useful in order to check that the new configuration is fine before storing it in the firmware.
Just prompt "M92 Z2560.00" in the start.gcode of your slicer (2560 is just an assumption based on the common reprap hardware, use the calibration link to get the number right for you)

Compiling a new firmware:
Save your current firmware and compile one with the right parameter. Which firmware and electronic do you currently have in your prusa?