Trek 970 Seat Post Size?
 

A few months ago I picked up a Trek 970 frame. It had a crank and derailleurs, and also a stuck seat post. It was pretty beat up, but I thought it would be a good winter project to build it into an errand bike.

I ended up cutting the seat post out after other methods failed. The seat post that came out was marked 27.2mm. When I tried to put another 27.2 seat post in, I could only insert it about 3 inches. I tried a 27.0, and that would go in about 4 inches. I tried a 26.6 and that would go all the way down, but it was fairly loose.

I got a flex-hone and honed the seat tube. The seat tube is nice and clean now, but the inside dimensions were essentially unchanged. I called the flex-hone manufacturer, and their tech support basically said that a flex hone will give you a nice surface finish, but won't remove much material.

I don't understand what is going on with the seat tube. I can't see any dents in it. I measured the outside with calipers and it doesn't seem to be ovalized. I have no idea though, of what might have been done to this seat tube in its past lives.

The frame is lugged 4130 chro-moly, and based on colors is apparently a 1990 model. The seat tube lug clamps nicely around a 27.2mm seat post.

I figure one option is to use a 26.8mm seat post that I shim out to 27.2mm at the top. The other option is to have the seat tube reamed to 27.2mm, but before I look for someone to do that I thought it might make sense to find out what the original seat post size was.

The local Trek dealers weren't able to help with information on the correct seat post size, and when I emailed Trek I got a polite brush-off.

Does anyone know what size seat post this frame came with? Also, are there any reasons why this frame shouldn't be reamed to 27.2mm?

Trek used 27.2 mm seatposts nearly across the board on their bikes so 27.2 is almost certainly what your frame should use. Perhaps the corrosion on the orignal post constricted the seat tube a bit. If a 27.2 post doesn't fit properly, have the tube reamed until it does.

Thanks, HillRider. I think you're probably correct about the size.

I can't make sense though of why the lower part of the seat tube is tight; I honed the tube pretty thoroughly so there shouldn't be any corrosion in there. The seat post stuck was only stuck about 3" and I can get a 27.2 seat post in about that much.

I would think reaming would be a pretty simple operation, but none of the LBS's seem to have the tools or know anyone set up to do it. One shop suggested a machine shop they knew about; I guess I'll go talk to them.

Anyone have an idea of what a reasonable charge is for reaming a seat tube?

Trek 970 seat post diameter
 

I just recently measured the original seat post that came with a trek 970. It was 26.8 mm.

Trek 970 seat post diameter
 

I just looked it up at the Sheldon Brown site . it says 27.2mm, for a 1995 vintage. That's about when mine was made, give or take. Go figure.

When I have a rusted up seatube I'll use a crescent file to clean off rust inside, and take sharpe edges down as well. I use it flat against the wall so I'm not enlarging the opening.

just measured my 1991 930... it is a bone stock survivor, right down to the tires... 26.8mm seatpost.

there is a possibility that a previous owner pounded the seatpost (you struggled to remove) into the frame and/or honed the tube out to KINDA Fit the 27.2 post.... !?1?

i've also owned a fade-paintjob 930 that was a newer year, with a sloping top tube,.. it had a 26.8 post also, i THINK.... been a while since that one was here... sold it to a neighbor... she may still have it around... i'll ask her if i can measure it next time i see her.

the thicker tube sets that early MTB bikes were built from were thicker walled to provide strength yet maintain a standard FD clamp size..... ergo the 26.8mm post diameters... my '91 Diamondback Ascent also has a 26.8 post.

Ummm.. What does the Seat Post Clamp look like? Is the Frame a LUGGED frame, or WELDED at the main joints?

my advice... open Facebook and search up "Vintage Trek Bicycles"... those folks will know.

Just FYI folks this thread was created and last active on 04-23-12 with only 3 posts two of them from the OP. I think the issue was probably solved.

wow.. i didn't even look... a lotta Zombies getting dredged up...........

Might be old news, but my purchased-new 1991 930 also has a 26.8mm seatpost.
My later 930 (based upon color, probably a '96) has a 27.2 seatpost.

Q: is the newer bike a WELDED frame, or still a Lugged style frame? I'm thinking that's the determining factor.

The newer bike has a welded frame. It's a deep dark blue with white/ silver decals. Looking through the Trek Archives seems to indicate it's a '96.

I once again think you should find "Vintage Trek Bicycles" on FB... and it's looking more like there is some serious Rust crystal buildup deeper down that seat tube... A "tool" i keep around is easy to make... get some 1/4" or 3/8" round stock steel... cut a 1.5" slit in one end... length should be enough to do the task, plus a couple inches... mine is 18"... then get some Emory cloth strips from the Hardware store... 120 grit is my choice... slide a 8" strip into the slit... roll the strip around the tool in a way that has it tighten when used in your electric Drill (Counter-clockwise comes to mind, opposite "forward" drill rotation....).... insert into tube BELOW where the present 27.2mm post will insert to with low resistance... start Flapping with your new Flapper Hone... Ball Hones are for light refinishing... that flapper will remove material like a champ.... run it for a minute or so, then Check Fit... BE AWARE OF HEAT BUILDUP or you WILL ruin the Paint job! Repeat until happy, and removing strip material as needed...

one final hint> you are removing Metal and rust buildup... Gravity will have that garbage falling... where? INTO THE BOTTOM BRACKET area... guess what you'll need to Clean now?

I've had a few lugged 970s, the seat post size changed during the model's lifecycle. These are the years I've had.

1989 = 26.8
1990 = 26.8
1992 = 27,2

There are times when the intended post size didn't fit a specific frame due to heat related distortion. The usual fix is to use the next size smaller post, faster then addressing the seat tube side of the problem. Often this is discovered when the bike is pulled from the shipping box and first assembled.

I would be careful when considering reaming a seat tube. Removing .2mm from the tube walls (as in from 26.8 to 27.2) could be about 1/3 the wall thickness. If there's a distorted portion to get to 27.2 would require more wall removal, than that .2mm initial assumption, at the point of deformation. Andy