Bicycle Mechanics - 400 mm seat post on 17.5" frame: too much insertion into frame?

i borrowed a relatives' trek 17.5" frame and used a 400 mm seat post to make it fit me. now i am returning it to him so he doesn't need the extra height of the seat post. however, we lost the original seat post.

question: a 400 mm seat post will have extra insertion into the frame - can this damage the frame?

we more commonly hear minimum insertion but is there such a thing as "maximum insertion?"

thanks

no such thing as "maximum insertion " . but depending on the frame tube built and / or if there waterbottle cage 's bolt in the way to keep the post from going all the way down .

Can't think of a way it would hurt anything; it might not go in all the way if it hits a bottle boss or something. Deeply inserted posts can be a source of mysterious once-per-crank-revolution clicking noises and can retain water, so just grease up the end of the seat post before putting it in.

You can also just take a hacksaw and cut it down, as long as enough remains below the top of the seat tube -- if the min insert line is 4" from the end, say, you can cut it down as long as you're putting at least 4" of shortened post in the seat tube.

edit: thanks guys you were right - it is hitting the water bottle bolt. didn't realize this before. will have lbs hack saw it or buy a cheap seat post and keep the 400mm as spare.