So i have removed the old gear cable on my Girlfriends mountain bike, The problem is the new cable that we have will not fit in the same hole the original went through. After trying for some time we tried to put the old cable in so it was still usable for the time being but that also failed, It goes so far in and then seems to get stuck on something inside the gear and nerve actually come out the other side.

Please help i can add pictures if that would make it easier.

No pictures necessary .. we are all mind readers here. We can see you are holding 2 cables.. one in each hand.

Are you trying to use a brake cable for a gear cable replacement? Other than the end fittings not working, brake cables are thicker (1.5 mm) than gear cables (1.1 or 1.2 mm). If you are using the proper cable, the old housing may be clogged or dirty inside and may have to be replaced.

Mountain bike cables and road bike cables use different ends. When you go to buy replacement cables some come with a mountain bike cable end on one end and a road bike cable end on the other end.

When you bought the replacement cables, did they have fitting on both ends? Did you look at and match up the cable ends before cutting the other end off?

Some cables are sold with just one end. Did you buy the right cables?

And, as already been mentioned, shifter and brake cables are different sizes, and also use different ends for mountain bikes and road bikes.

Sometimes a cable needs to have a slight dogleg at it's end to better align with the shifter spool and the exit hole (hopefully with it's barrel adjuster). the old cable would have developed this minor kink over time and use. Andy.