[DirtyMartini]

I've got a 30mm seat tube on my Iron Horse 6Point which is an odd-ball size.
The outside diameter of the seat tube is 34.9 (a fairly common size).
Many 30.9 ID seat tubes also have a 34.9 OD.

The thought is to have a machinist drill & ream the seat tube out to 30.9mm so that the bike can have more seat post options.

Does any one have any experience drilling / enlarging seat tubes? Is there a heat-treated surface finish, or anything like that, on frames that will be compromised by drilling out the seat tube?

 

[ridiculous]

Seat post options? What are you trying to run?

Proprietary or not, put pipe in hole and call it a day. Rinse and repeat for your lady.

 

[DirtyMartini]

I'm entertaining the idea of KS i900r adjustable seatpost. Which isn't available in 30mm, but it is available in 30.9mm.

Also, the 6Point geometry is designed around a seatpost with a 1" offset, finding a 30mm seatposts with a 1" offset is difficult.

 

[JRogers]

Depending on what post you want to run, you can also get a shim and run a 27.2 or other small post. X-fusion and maybe some others as well have or will shortly have 27.2 adjustable posts. A shim isn't ideal, but it's probably cheaper and safer than drilling out the tube.

 

[Tim Blabbing]

Hey DirtyMartini,

I do not have experience with enlarging seat tubes, but I do have a welding background. The process of heat treating aluminum occurs around or above 400 deg. Fahrenheit. I'd be surprised if someone wanted to machine ream your seat tube, as opposed to doing it by hand. If done by hand, you shouldn't have any problems with excess temperatures. The drilling might be a problem. Make sure it doesn't get hot enough to boil water. Most importantly, however, your bike has that seat-tube mounted rocker. If you didn't have that rocker mount, I'd say go for it, but otherwise you're really risking ruining your frame, big time. If you do decide to try it, one tip is that you'll have to figure out how tight to put your seat q.r. AS the tube is machined. If you machine the tube without the q.r. tightened some, then the end size will be too tight to allow your seatpost to slide up and down. It might simply be a matter of tightening the q.r. bit by bit between successive reams until the post slides properly.

However, JRogers has the right idea. Another size with a shim would be the way to go.

Additionally, getting 30.0mm seatposts is not hard. Universal Cycles carries a Kalloy SP-267 UNO post that at least has 3/4" offset. Cheap and heavy, but it'll work. If you're really set on getting an adjustable post, even KS makes some 27.2mm models. Getting your requisite 1" of offset won't happen, but can't you just run your seat way back on the rails?

Finally, I will say that these manufacturers that use odd sizes are idiots. Eclipse (available in Canada) may still be using 30.2mm (wth?) seatposts. At least one guy claims his old Norco B-Line used the same size. Idiots.

 

[SLOPE]

I always wondered why they picked one size over another.
Is there a reason for one size over another?

 

[lovebunny]

i would assume its the tubes they get bulk pricing on or something

 

[w00dy]

It's an iron horse, so it's already out of warranty. Go ahead and ream it.
If you can find a good hand reamer in the right size, that's the way to go. They guide themselves and there's not enough speed to heat up and damage anything.

This won't be cheap, to buy the tool or take it to a machinist is going to be a good chunk of change.

 

[SLOPE]

But wouldn't you think that the most common sizes would be the cheapest??

I've always wondered.

 

[bullcrew]

$14 brake hone from autozone and some wd40... Did it to my old santa cruz bullit... Worked like a dream took a bit but honed perfect...

Stick it in a drill spray the post inside with WD40 and start honing...

The hone is spring loaded and theres a nut to tighten and it puts more outward pressure being its spring loaded the pressure is even on all sides as its honing and it doesnt built up heat...

 

[KavuRider]

We did that to my Karpiel Armageddon, the seat mast had cracked internally AND was an oddball size. Took it to the shop, we did just as Bullcrew said.
Takes a long time and its a messy job, but worked perfectly. Just go slow.

 

[bullcrew]

We did that to my Karpiel Armageddon, the seat mast had cracked internally AND was an oddball size. Took it to the shop, we did just as Bullcrew said.
Takes a long time and its a messy job, but worked perfectly. Just go slow.

Yeah it takes a fair bit LMAO... But having a standard and easily available post is a huge benefit...

 

[KavuRider]

Yeah it takes a fair bit LMAO... But having a standard and easily available post is a huge benefit...

We reamed it out to take a Thomson post and all was well with the world.

It was funny for people to walk into the shop and then look around and wonder where the hell that noise was coming from.

 

[bullcrew]

We reamed it out to take a Thomson post and all was well with the world.

It was funny for people to walk into the shop and then look around and wonder where the hell that noise was coming from.

LOL!

Yeah its not a common bike shop noise!!!!

 

[4130biker]

Why risk the frame? I vote shim. I had a Kona with a 30.0 and I got a shim for $6 from chain reaction to run a 27.2
No problems, worked fine. I even installed some reaaaaally tiny roll pins under the seatclamp to keep the thing from pulling out with the post. I'm pretty sure the gravity dropper comes in 27.2 as well...

 

[KavuRider]

Why risk the frame? I vote shim. I had a Kona with a 30.0 and I got a shim for $6 from chain reaction to run a 27.2
No problems, worked fine. I even installed some reaaaaally tiny roll pins under the seatclamp to keep the thing from pulling out with the post. I'm pretty sure the gravity dropper comes in 27.2 as well...

Yeah, I would consider reaming the seat tube as a very last resort.

I ran the Gravity Dropper shims on my bikes, held up great, no creaks and didn't ever come loose.

The reason we drilled out my Karpiel frame was because it was some funky weird seat tube size (they were all supposed to be 27.2, but yeah...).

 

[Tim Blabbing]

Hey DirtyMartini,

Here's your seat post solution:

http://www.rasebike.com/cart/index.php?main_page=product_info&cPath=1&products_id=2

A Rase Black Mamba: available in 30.0 mm size with an offset head (can't find spec for amount, though) and an incredible 9" of telescoping action, too! Definitely one of the pricier ones, and I don't know what the reviews are, but at least this one fits what you want.