Seat tube reaming questions

Abradable Chin · Post by **Abradable Chin** » 19 Jan 2018, 11:04pm

I have a thick-walled Al alloy seat tube that I want reamed and I have a few basic questions.
How much might a shop charge for this?
I want to go a whole mm on ID, which is to the next commonly available seat pin OD. Is there a limit on the taper of the reaming cutter so I'd have to be closer than that on ID to start with in order for the cutter to start?
Would a machine shop be able to do the job, and would they do it differently?
If I were to buy a cutter like the one below, is there anything I could do with stud rod to twirl it down a seat tube, or would I need to buy the whole tool holder set up? I think this would make it unecononomic to do at home.

: reamercutter.PNG (54.09 KiB) Viewed 1388 times

hayers · Post by **hayers** » 20 Jan 2018, 8:55am

I think the cutter you show would need to be used with a guide which is a close fit in the existing bore - without it will not self follow the bore. If this arrangement is out of true it will cut oversize.

For doing at home an adjustable reamer is probably easiest and cheapest. 1mm on the dia would be a lot to take out in one cut with a ordinary reamer - but probably just about ok in aluminium. Either option likely more cost than decent lbs for a one off.

A machine shop would have many possible methods, but you'd need it to be done as a favour rather than at commercial rate to be cheaper than buying your own reamer / lbs

Also, are you sure taking 0.5mm off the thickness will leave enough meat in the seat tube? A failed tube could have nasty results!

Valbrona · Post by **Valbrona** » 20 Jan 2018, 9:04am

A bike shop isn't going to have seat tube reamers ... a framebuilder will.

531colin · Post by **531colin** » 20 Jan 2018, 9:27am

Why risk messing up a frame for a cheap component like a seatpost?
Better to shim an undersize post, for example?

Abradable Chin · Post by **Abradable Chin** » 20 Jan 2018, 12:35pm

I appreciate your replies.
My seat tube is no longer round internally, and I have a lot of grey dust that falls out. I believe this is from fetting. I have 5mm thick walls, so I thought the better quality solution would be to ream.
Shims without adhesive 'packing' aren't going to solve the non-round problem, and shims 0.1 to 0.2mm thick can't be bough commercially, so they would be homemade, flimsy, and have no neat retenton lip on the top. I could downsize the seat pin, instead, then I could go with a commercial shim, but some form of extra packing would still be needed.
A third option might be to build up the interior of the seat tube with epoxy/chemical metal/car body filler to restore it to its original ID, but it seems a bit more cowperson.

Brucey · Post by **Brucey** » 20 Jan 2018, 2:59pm

In many cases the clamp will pull itself from non-round to round enough locally to provide a clamping action that is secure enough. Secure enough, that is, if lower down, the seat pin isn't able to wabble about inside the seat tube, in which case it doesn't matter how tight the clamp is made, you will get movement and fretting under certain conditions.

Anyway it is possible to make a shim (eg from brass shim stock, or beer can) that has a small lip that sits under the collar on a lot of aluminium frames . Just make a lot of short cuts in the end of the shim, and fold tabs over, such that they are small enough that the collar slides over them. Provided the shim is enough to prevent the lower parts of the seat pin from flapping about, it might cure the problem, without being a 'perfect solution'.

I'd probably ream it, BTW, but then I have a reamer. I'd also take as little out of the seat tube as possible, and maybe machine up a seat pin to be an excellent fit too; but then I have access to a lathe... The cutter you picture will most likely not be suitable for several reasons.

cheers

AndyA · Post by **AndyA** » 20 Jan 2018, 8:10pm

An adjustable reamer would be the most cost effective solution, the basic RDG tools ones work fine in aluminium.
https://www.rdgtools.co.uk/acatalog/ADJ ... SIZES.html
I've used them to ream out stuck seatposts (which is a terrible idea but will do the trick after 4-5 hours of brutal cranking), you'll maybe get 0.1mm per pass. Use plenty of oil. You could use a flexhone when you're very close to final size to get a better finish.
With a 5mm wall thickness I'd have no fear removing 0.5mm per side.
You also need a good quality, large tap handle to hold the reamer. I've got this one which I've reinforced to stop the jaws twisting.
https://www.rdgtools.co.uk/acatalog/ADJ ... 84756.html
Don't ream the frame with the seat tube pointing downwards - if the ream comes off the handle it can fall and get stuck in the seat tube!!!

NATURAL ANKLING · Post by **NATURAL ANKLING** » 20 Jan 2018, 8:21pm

Hi,
Remember also that a reamer should never be turned backwards to the cutting direction, this will blunt it faster.
Use plenty of cutting fluid.

Gattonero · Post by **Gattonero** » 21 Jan 2018, 11:33am

Abradable Chin wrote:I have a thick-walled Al alloy seat tube that I want reamed and I have a few basic questions.
How much might a shop charge for this?
I want to go a whole mm on ID, which is to the next commonly available seat pin OD. Is there a limit on the taper of the reaming cutter so I'd have to be closer than that on ID to start with in order for the cutter to start?
Would a machine shop be able to do the job, and would they do it differently?
If I were to buy a cutter like the one below, is there anything I could do with stud rod to twirl it down a seat tube, or would I need to buy the whole tool holder set up? I think this would make it unecononomic to do at home.
reamercutter.PNG

One mm on a bicycle tubing is like talking miles.
And especially on the seat cluster, a very stressed area.
Don't do it! To replace the seatpost is far cheaper and will keep the frame safe.

If the ID of the seat tube is not round anymore, work out what caused it, if it's form corrosion have the whole frame inspected as you may have bad surprises. If the frame is worth keeping, you may have it reamed plus a long shim bonded in, though will increase strength it will never be 100% as before

Abradable Chin · Post by **Abradable Chin** » 22 Jan 2018, 7:22pm

Several difficulties have presented themselves. If I were to minimally ream the seat tube, I'd then have to machine 0.7mm off the OD of the next size up seat pin. If I were to start with some thicker schedule pipe instead of a seat pin, I'd have to swage the end or spigot on the head of a proper seat pin in order to fit a saddle. The out of round seat tube is at the extreme limits of two adjustable imperial reamers, so I'd have to buy two in order to do the job. If I just buy the larger one, I might not be able to get it started. A metric adjustable one might bridge the gap, but metric ones seem uncommon.
To answer Gattonero, the cause of the wear is fretting, and the seat pin can no longer be kept in place with a tight binder, so a shim of different material might prevent any recurrence.
You've mentioned bonding the shim, but no recommendation has been given as to what to use. It might be that strength is not required, just volume, so silicone might do? I'd be able to extract the shim in the future if I were to use silicone rather than epoxy.

Brucey · Post by **Brucey** » 22 Jan 2018, 9:14pm

several comments;

1) have you accurately measured the ID of the seat tube?

2) you could presumably make a lipped shim to be captive under the seat collar as previously described?

3) Aluminium is not super-strong stuff, which means you have other options.

On the last point, if you get a length of steel machined to the correct nominal seat tube ID, you will be able to insert this mandrel into the top couple of inches of the seat tube with a lot of grease and (by leverage mainly) work the thing round again. The same thing can be done using a thick-walled aluminium seat pin, albeit it will get somewhat more mangled in the process.

Typically there are two forms of deformation in the seat tube; the first is within the slotted part of the seat tube, arising from clamping and use of a seat pin that is slightly undersize; this is easily rectified using the mandrel. The second is within the welded part of the seat tube and arises mainly because of the welding process. This is more difficult to remove, but is usually much smaller to start with and can be resolved by filing or burr grinder or something.

BTW if you are worried about sorting out the deformation thusly, I have seen steel frames sorted out by use of a mandrel where the entire top of the seat tube was folded back about twenty degrees as a result of there being insufficient seat pin inserted into the frame. A small deformation in an aluminium frame is usually quite easy by comparison.

NB if you buy (say) a 27.2mm seat pin, you will usually find that it is closer to 27.1mm when you measure it carefully. A 0.1mm clearance to the seat tube is unlikely to cause the seat pin to start to move around, but a 0.2mm clearance might.

cheers

foxyrider · Post by **foxyrider** » 22 Jan 2018, 9:40pm

Valbrona wrote:A bike shop isn't going to have seat tube reamers ... a framebuilder will.

A decent bike shop most certainly will - they often have to clean seat tubes up on new bikes.

sun ra · Post by **sun ra** » 25 Jan 2018, 9:02am

531colin wrote:Why risk messing up a frame for a cheap component like a seatpost?
Better to shim an undersize post, for example?

I very much agree

Abradable Chin · Post by **Abradable Chin** » 25 Jan 2018, 1:01pm

sun ra wrote:
531colin wrote:Why risk messing up a frame for a cheap component like a seatpost?
Better to shim an undersize post, for example?

I very much agree

I'm still not clear on what people are advocating to repair this. The seat tube lumen is ovalised, and 0.15 to 0.3mm larger in every 'diameter' than it should be. Are you saying a shim is all that is needed? The seat tube wall is 5mm thick 6061 Al at the top, so I'm not sure deforming it with a mandrel would be a good idea. If I were to spin the mandrel so that it moved metal around (spin forming?) then it might work, but I'm not sure that is what was suggested.

Brucey · Post by **Brucey** » 25 Jan 2018, 4:56pm

All a seat pin needs is to be clamped at the top of the collar (it will be, the clamp will pull it round enough) and for the bottom of the seat pin not to wobble around inside the frame. There are many ways of ensuring this will be the case. The primary wobble is usually a fore-aft one, so even a shim at the front of the seat pin or a slightly ovalised section at the bottom of the seat pin will often do the trick.

So if you plan to ditch the seat pin anyway, you have little to lose by trying a shim or by giving the thing a smack with a hammer to make it oval at the bottom.

BTW 6061 is reasonably ductile even after it has been welded. I doubt very much that a mandrel will do any harm.

There are lots of ways of fixing your frame.

cheers

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