Steerer tube options
 

To me it looks like my steerer tube diameter is 1 3/8 and the quil stem is 1” I assume. I don’t know them to come in other sizes. It looks like a threaded headset please correct me if I’m wrong.
whats with the 1 1/38 steerer tube. After a little reading its not mentioned with the “common sizes” and there doesn’t seem to be many options when shopping around for any. Is this an odd ball size for steerer tubes?

https://cimg1.ibsrv.net/gimg/bikefor...dd40c10c2.jpeg
https://cimg2.ibsrv.net/gimg/bikefor...0ff82ccbe.jpeg

I can't tell anything from your photos, especially whether it's threaded or not as you haven't shown the stem or top of the headset. A quick (very quick) google search will show you the difference. I'm betting it's 1 1/8" whatever it is.

• It’s hard to show but it is 1 3/8 steerer tube.
• 
  I was planning on adding a springer fork but this 1 3/8 steerer tube makes me feel like I can’t do that because I think springer forks come in 1” steerer tubes.
  

springer forks are not safe *im aware

what brand of bike is this and frame material?

I am paranoid about this but have had cracked head tube with aluminum frames...

I would check those paint scratches and and make sure they are not cracks

they are just scratches.

IT is a 2010 trek drift 20”

it says it’s alpha white aluminum

300lb limit I’m 185lbs

I disagree with the claim of a 1 3/3" steerer. Where the tape (a poor way to measure diameter without wrapping it around the cylinder) is placed only tries to show the crown race seats' diameter below the step that the crown race is pressed on. Which is larger than the steerer is by a bunch. Now one could unthread the headsets lock (top) nut and again try to measure the now exposed threads. A cool trick if one has no calipers it to wrap a strip of paper around the cylinder (steerer) and mark off one circumference. Divide by 3.14 for the diameter. Much less error induced by parallax or poor technique.

Most likely being an AL frame this has a 1.125" steerer with a 30.0 crown seat milling. The bottom of this crown seat ring (which is welded onto the fork) will be a bit larger, 1.375" would be about what I would expect.

As to "odd ball size"- I think you should do a bit more homework on terms and what you reference when you write. The size of the balls in the headset have nearly nothing to do with steerer diameters. Andy

You are measuring the wrong part of the fork. The steerer is actually inserted into and welded to the section you are measuring. To measure the steerer you have to expose it. To do this you have to remove the top bearing and let the fork drop out of the frame. The bottom of the steerer has a short widened section onto which the crown race is friction-fit. Measure above this section. Use a caliper. You can buy a good cheap one at Home Depot or Ace. My money says this is a 1" steerer, (25.4 mm) with a 22.2 mm inside diameter.

No, it's not. It's 1 1/8".

Use somthing accurate to measure the ID at the top. Accepting your statement that the quill stem is 1", I infer that the steerer is 1-1/8" at the top, (based on a typical 1/16" wall thickness). That was a somewhat common size as threaded headsets began to give way to threadless systems.

Note also that there were a number of "tapered" steerers which were more like "bulged" or "stepped" and were larger at the crown than the top. These were fairly common BITD, though I'm a bit surprised to see a threaded one (if that's what you have).

Remove the fork from the frame and measure the actual steerer tube OD. The place you are measuring is likely a larger diameter sleeve that goes over the steerer tube and provides a seat for the crown race and a larger area to weld the fork legs. I've built several forks with that method of construction, before Paragon Machine Works started making machined steerer tubes with a larger diameter section at the bottom.

Rather than remove the whole fork, really all they need to do is remove the top nut and stem to expose enough steerer tube to measure as the top nut is threaded on the steerer tube.
once the top nut is removed the OP will need to preserve or readjust the bearings while reinstalling it.

Standard size steerer tubes are 1" and 1-1/8". As others have said you have to take the fork out to expose the steerer tube. Holding a ruler up to a round tube to measure diameter is a poor method. Here are some better ones:
1) Micrometer (best, but you need a 1-2" micrometer which limits your range for other uses) $$$
2) Caliper (pretty accurate, and more versatile for othe stuff) $$
3) Put the steerer tube on a hard level surface (e.g. a surface plate) and use a height gage to measure diameter. $$$$
4) Put the steerer tube on a hard level surface, stand up an accurate ruler next the tube, and use a level on top of the tube. Measure height at the ruler (poor man's height gage) $
5) Use a pi-tape to get diameter from circumference. $$$
6) Wrap a strip of paper snuggly around the tube, mark the circumference with a sharp pencil. Measure circumference with a ruler, divide by 3.142 (poor man's pi-tape, same suggestion as Andy's above).$
Methods 1-4 should be done several times and the results averaged.
Given that the tube diameters are either 1 or 1-1/8, in this case I'd go with method 4 or 6. Gets you close enough to determine which tube size you have.

It looks to me like you have a 1-1/8" threaded fork. Unless they make bearing sets that fit into a 1-1/8 head tube that fit a 1" steerer tube.

You're right. I was being a little anal to suggest the measurement be made on the unthreaded portion.