I have a bike with a 27.2 seat tube that continues to zig zag seat posts. I have made sure that the seat lug is opened enough, de-brurred the seat tube interior with a drill operated, carbide ball hone, sanded the tube interior with 150 grit & 400 grit sand paper, sanded and filed the slot at the t back of the seat lug so there are no sharp edges.

All of this done and I am still getting the dreaded marks when a seat post is greased up and installed. This has lead me to buy an adjustable reamer. I have never used one before and looking for any tips/tricks from the experienced members of the group.

Have a set of them I use for seat tubes. For your requirement they are just the thing. With each adjustment one removes only the tiniest bit of metal so there is no danger of going oversize. They are too slow/tedious to employ if one wishes to enlarge the seat tube to accept a larger diameter pillar. Always remember to use plenty of cutting oil.

Suspect your zig-zag marks may be coming from the corners of the binder ears. When the binder is tightened these can be pushed into the pillar.

Tip: If you are dealing with a bare frame as opposed to a built-up bike it is much easier to put the reamer in a vice and turn the frame.
Brent

That's a good thought, and you may be able to get away with just gently rounding those corners with file and sandpaper at that point, rather that redoing the whole shebang.

as you cut your chips go some where ,

and the result may be a Poor seat post fit as the seat post will be undersize ..

I carefully reamed a 27.0 ID frame to exactly fit a 27.2 post in the (RB1) frame that is only a difference of 0.1 mm .
[I had the post in hand so stopped cutting when It first fit in]

soon you may need a shim sleeve to fit around your 27.2 post after shaving the frame round.

fender1 does not mention what the frame's seat binder arrangement is.

if, for example, it uses a welded seat lug with integral binder ears then this technique might be the ticket and problem may have nothing to do with seat tube itself.

It is this type:
https://www.google.com/search?q=Rive...GB1xdETDwhM%3A

I have filed and sanded the slot and ears from front and rear.

link illustrates frames with investment cast seat lugs.

sounds to me like you are proceeding just right.

equally puzzled by persistence of symptom.

if, for example, pillar is a slightly loose fit in seat tube reaming could make symptom more acute.

another option (I have used ALL these: flex-hone carbide balls, adjustable steel reamer, PLUS---) a stone honer as used for brake cylinders. The stone honer will cover a larger area than the carbide ball flex-hone, but cut less aggressively than the steel reamer (which needs blades to be super sharp to work right, too). Use oil with the stones...you MIGHT fine a cheap one in the bargain bin at your local auto parts shop, some have 2 stones and some have 3, both types I have used will fit in a bicycle seat tube.

And this is off-topic, but once you use it with OIL (to cut steel or other metal) you should NEVER use it on a brake cylinder where oil introduced to brake fluid will cause disaster...only lubricate with alcohol or brake fluid when cutting a brake part.

Thanks. The post has always been tight fitting. It got better when I opened up the ears but it still takes some effort to insert it.


TWSS!