|The red Mercian is one of my "go-to" bikes.|
I thought about sending the frame off for a professional repair. Had the damage been worse, or been on any other frame tube, I probably would have sent it out. Being that it was pretty shallow and on the upper part of the seat-tube, which is open at the top, I thought I could probably try fixing it myself first. I hope nobody with frame building and repairing experience cringes when they read that, but I had a pretty good idea of the mechanics and the process of it and decided it was worth a try.
First, it's good to have an understanding of what a dent or crimp like this actually does to a tube.
For the frame blocks, I took a block of wood and bored a nice straight 1-1/8-inch hole through it with a forstner bit, like this one:
To perform my operation (and again - try not to cringe), I started by inserting the extra long 27.2 mm post into the seat tube. When the post started to encounter resistance, I kept twisting and lightly tapping it in. I made progress very slowly, tapping and twisting until I knew the bottom of the post was well past the crimped part of the seat tube. Would I be able to get it back out? Well, it seemed to me that as long as I could twist it (even if it took some effort) I'd be able to get it out. But I also calculated that if I successfully removed the crimping, it would come out easier than it went in.
To work the outside of the crimp, I heavily greased my wooden blocks and placed them around the dent and put them into my vice. It is important not to tighten the blocks all the way, but to get them to where there is just a slight bit of resistance, and then twist the frame back and forth between the blocks. You have to work slowly, twisting the frame, slightly tightening the blocks - continually twisting, tightening, etc.. It takes some patience as you don't want to force anything or do too much too quickly. I didn't have much trouble because my dent was not particularly deep or sharp - and it was definitely lessened by tapping the seatpost down through the dent.
After I got the blocks tightened all the way and could twist the frame freely between them, I could tell it was done as well as I was going to get it. I took the blocks off and removed the seatpost. I knew immediately that the crimping was better because getting the seatpost out was much easier than getting it in. That shouldn't be a surprise - the seat tube was now much rounder than when I started.
It's also important to note that the frame block operation can often destroy the paint. I was lucky. Whether it was because I was using wooden blocks, or lots of grease, or because the damage was so minor to begin with, but my paint only got a bit "scuffed" but wasn't badly damaged. I was able to buff it back to where you could hardly tell. Even in good light, it's really hard to see there was any damage at all. Best of all, I can get the proper seatpost inserted without trouble.
I want to point out again that I normally leave frame repairs up to the experts, and I don't necessarily recommend other people try to do what I did here. Unless someone feels really confident in their home-wrenching skills, I might say this is one of those "Don't try this at home" situations. As I'd said earlier, if the damage had been worse than it was, I might not have even tried it myself, and even as I was doing it, I proceeded slowly and with a lot of caution. But in the end, I was happy with the result, and satisfied knowing I did it myself.