REPAIRING THE SEAT POST LUG

Improper maintenance often leads to excessive torque on the seat post clamp bolt.  This torque leads to a distorted bolt hole, at best.  It is also likely that the seat tube opening will have been forced into an out of round shape.  An out of round seat post cavity opening will make seat post insertion difficult, if not almost impossible.

The first thing to do with the seat post clamp bolt hole is get it round again.  It will have to be "cold set" or stretched back into it original shape (or pretty close to it, anyway).  The proper tool to use for this task is a taper drift pin, which is a long tapered pin made of hard tool steel.  The taper is very gradual and the pin itself can be pried, this way and that, without fear of it breaking (there are, of course limits to this).  It is, however, unlikely that everyone will have immediate access to such a tool.  But most people can get their hands on a 1/4" Ratchet drive set.

Insert the square end of the extension piece into the distorted bolt hole.  Move it around until it slides into place.  Now simply and slowly turn in the clockwise direction - slowly and gently.  As the hypotenuse of the square end of the 1/4" drive extension turns, it will actually help to make the lug hole round again.  As you get the feel for this simple procedure, you can begin to experiment a bit in an effort to shape the hole back to roundness.  Take your time and try the seat post clamp bolt for fit frequently.  Once the bolt fits through the hole smoothly, it is time to stop and consider if any more stretching is a good idea.  Remember, the hole might still be a bit out of round but that situation will be hidden, once the seat post bolt is in place.

That just about covers the things that need to be addressed when considering the seat post cavity.  It must be clean of debris and smooth inside.  The seat post itself must also be clean, smooth and have no high spots.  There should be no burrs inside the tube cavity.  The slot, in the back of the seat post lug, needs to be bevelled to ensure that no sharp edge can catch and gouge soft alloy seat posts.  Finally, both the cavity and the seat post need to be coated with a light layer of grease, prior to assembly.

And now would be a great time to test assembly.  Try inserting your seat post.  It should be a snug fit into the opening but it should slide smoothly into the cavity.  Just insert a wee bit at first and then rotate the post.  If it feels like it is catching on something, remove the post and check the cavity carefully.  There might still be a sharp burr inside that you missed.  If so, clean the inside again by repeating the entire process.  Test with the seat post again until it fits the way it should.

If your seat post and seat post cavity meet those criteria, seat post bolt clamp pressure will only have to be minimal to ensure a solid grip on the post.  A grip that will neither allow the saddle to rotate back and forth nor slide down while being ridden.

NEXT - PREPARING THE BOTTOM BRACKET CAVITY