Hello guys, me again.

So I bought new pedals (shimano SPD-SL ultegra) a week ago and today I checked to see if the pedal had come loose and it had, so i tightened it, this time with more force.
My question is: Is it easy to overtighten pedals, I'm not using a pedal wrench, I'm using a standard 15 mm wrench and it is 19 cm long.
If you do overtighten the pedals can the threads in the crank arm strip? Or will the threads on the pedals strip first?

No, you're unlikely to strip the threads using any hand tool, but if you rode for any length of time while the pedal was loose, the crank arm threads could have been damaged. If the damage is slight, you may be able to mitigate it by wrapping teflon plumber's tape around the pedal threads before installing the pedal. If that doesn't help, bring the crank arm in to a bike shop for replacement or helicoil repair.

It is very unusual for pedals to loosen if properly installed. Tighten them firmly. Since the pedal threads are steel and the crank arm threads are aluminum, it is the crank arm that you have to worry about. Plus the crank arm is more expensive to replace. So be careful, but don't be afraid to make the pedals firmly tight. If you don't go overboard, there is little danger of damaging the crank arm threads either.

Thanks! The pedals weren't exactly loose, they just weren't as tight as they were when I installed them.

That's odd. They usually get tighter because of progression. that's why the left pedal has left handed threads.

19 cm is only 7.5 inches. that is NOT long enough to properly tighten pedals, and it's pretty much impossible to overtighten them. I would want to see you using at least a 30 cm wrench/lever, so that you can get adequate tightening with only about 30 lbs of pressure on the wrench (pedal wrenches are typically about 30 cm long) . Either get something you can use to add leverage to that wrench or get it tightened down properly with a pedal wrench before any more riding is done. As for progression, that is a small effect over time, not enough to offset inadequate tightening.

Unless you got a package labeled "L. Armstrong or current resident" recently, I'd get a longer wrench. I find, that if it lines up right, I can get it tighter by squeezing it against the arm tan by turning it with a baby wrench, but I eat all my spinach, better yet, use a pedal wrench, or find a pipe (seatpost will work) that will fit over the wrench, just don't scratch the arm...

as mentioned above, with that wrench i wouldn't think you would be able to strip the crank arm threads even if you tried. OTOH, i often use a wrench of that length (3/8" drive craftsman ratchet) on them just so i can't, but i put about two grunts worth of force on them. if i didn't i wouldn't be surprised if they loosened a bit.

When installing and removing pedals I use the technique of squeezing the wrench and crank arm together between my fingers and palm. I get surprising force for tightening and loosening the pedals that way. No cheater pipe or long wrench is needed. Even with a 5 inch Allen wrench I can fully tighten my pedals (from behind) and never have trouble loosening them the same (reverse) way.

Although some people can have success with a shorter tool it's my practice to make suggestions that will work for the population at large. The fact that the OP was not able to tighten properly the 1st time with the wrench at hand calls for a change in approach. Of course if one does not use good technique for tightening the pedals, such as pointing the crank toward the back and the wrench forward, it will make proper tightening more difficult even with the correct tool.

Of course. My point was that 90 lb weaklings like me have a hidden resource in the surprising power of the hand grip. Particularly for trying to tighten pedals onto a rotating crank the hand grip technique is the best method I have found.

you are right. with both hands at the end of even a short wrench handle and also around crank arm, it is possible to put a real hurttin' on the pedal threads. it almost always pops loose. i can just about feel it now!