Hi all
I just bought some new pedals for my bike but when installing them I accidentally put the left pedal in the right crank and the right one in the left crank. Now when I put them in the right way the just keep falling out. Please could you tell me what might be wrong.
Thanks

Depending on the amount of force you put into your effort, trying to install them wrong, you might have damaged the crank arm threads. Try to give your local bike shop a call, they might have a tap in the right size for "opening" the crank arm treads again.
Chek what size your pedal treads are (proberly 9/16"). See if you can find some cheap taps on Ebay, remember you need both left and right. But check if your crank arms are worth the tool investment... Will you need it in your garage in the future?

Or call your local bike shop.

Thanks looks like I'm taking to the bike shop tomorrow.

You didn't just cross thread the pedals, you completely jammed in thread that are turned the opposite way LH vs RH and RH vs LH.
The pedal spindle is steel and much harder that most crank arms which are aluminum so if you put them in all the way, you destroyed most of the threads in the crank arms.
I would advise against trying to clean them up and put the pedals in. Too much material is missing and one day when you hit a bump hard, it is likely your pedal is just going to fall out and you'll take a tumble or plant you crotch on the top tube.

Most of us don't consider that a good way to save a few $$, but it's up to you.

So should I buy new crank arms

Sure, if the pedal spinle have been completely seated against the face of the crank arm, you messed up your arms good. I read it as he had just "blocked" the start of the thread.

Adam, can you screw the pedals all the way in? If so, as Android says, you messed up your crank arms.

Yeah I messed up bad

welcome to the club... we've all pretty much done this at some point... just not as bad as you did, probably...

It depends on the bike and components.

One can helicoil the cranks, but it isn't cheap. The result, however, might be as good or better than original.

Note, you'll need bike specific parts, not available from a local auto shop.

You may have also damaged the pedals.

As mentioned above, talk to your local bike shop.

Not necessarily. There are helicoil and bushing thread repair kits for this type of problem. If your bike shop has such a kit, the repair may cost less than a new pair of crank arms.

...if you can get someone knowledgeable to look at it, who has the means to repair them with a threadsert on each side, they can pretty much tell you how much it will cost to do it, and whether your particular crank arms are worth the trouble, as opposed to just buying a new set online somewhere.

Sometimes it's just not worth the repair, sometimes it is.