Stupid threaded headset question
 

I'm sure this is a stupid question, but are JIS and ISO fork crown races interchangeable? I mean is everything else the same size and it's only the crown race that's got a different inner diameter (so if I have a JIS fork and and ISO headseat, I'd only need a new fork crown race) or is everything in a JIS headseat also larger to accommodate the extra 0.6 mm inner diameter of the fork crown race?

My guess is I'd need a whole new headset (or at least the bottom half), but thought I'd check to make sure. Because I have a tall 26" wheeled mtb, I'm having a hard time finding a replacement fork. Finally found one online, but am having a hard time figuring out this one dimension from online sources, so I was trying to see if it'd matter or if I should just order it because replacing the crown race would be easy.

I'd imagine that most threaded forks produced nowadays would be ISO, but you never know and I've learned not to assume if it's not spelled out.

they are different. the race seat and head tube diameter are different

head tube isn't the same diameter for all frames? Or do you mean head tube race?

JIS uses a 27mm fork crown seat, and a 30mm headtube ID
ISO is 26.4 & 30.2mm

However within the same model of the same brand headset these are the only dimensions that are different - JIS or ISO.

So, for example if you bought a JIS fork for a bike that used to be ISO, all you'd need to replace is the crown race. (staying with the same brand/model)

I hope that clears it up.

Or have a bike shop mill the crown race seat on the JIS fork down to accept an ISO crown race. That is a common procedure.

So the actual head tube diameter is different between JIS and ISO forks? I thought I'd read from Sheldon Brown and elsewhere that frames were pretty indifferent and it was only the headset and fork that made the difference and that you could even swap in and out threadless for threaded just by changing headset, fork and stem? Am I mis-remembering that?

Or am I misunderstanding what I head tube is? That's the bit of tubing that connects the toptube to the downtube and that the headset and fork go in, right?

the ID of the headtube is different. .2mm difference. i think you are confusing the steerer tube which has the same threading. fbinny has is right. you can swap a crown race within the same model of headset. crown race is the OD of where the bottom "race" is pressed on

Yes they're different 30.0 JIS vs. 30.2 ISO. How important that difference is depends on the specifics of the situation. On the fork there's no margin for error because the crown races are stout and brittle, so there's no way that a .6mm difference can be accommodated.

However it's muddier with head tubes. the standard .05mm interference (head tube is actually below the nominal by .05mm, ie 29.95mm, not 30) is based on the lugged steel frames with steel headsets in use when it was adopted. But changes in wall thickness and materials of both the headsets and headtubes changes the amount of interference both possible and desirable. So it's possible to press an ISO cup into the thinner walled head tube of a lugless Tig welded steel or aluminum frame.

However, if for example you were to force a cup into an undersized carbon head tube, there's a great chance of cracking it. Fortunately JIS carbon frames are very rare (if they exist at all).

BTW- the reverse, putting a JIS headset into an ISO frame will never work right. The cup will never be tight, and the headset will always act like it's adjusted too loose.

+ some headsets are offering better weather seals on the bottom,
if whole thing is complete.
labyrinth seals, O ring, and etc.

[QUOTE=FBinNY;14623411]
So, for example if you bought a JIS fork for a bike that used to be ISO, all you'd need to replace is the crown race. (staying with the same brand/model)...QUOTE]

+1

I think that this is the only thing that the OP needs to know, but by now he/she must know at least enough to be dangerous!

One way to think of JIS is the raw frame, before any precision machining is done.
Cheaper bikes are sold that way, which usually works well enough with well-made headset parts.
ISO is like the same frame/fork with a little bit of metal machined away, to achieve precision-machined tolerances.
Sometimes JIS frames and forks are machined after the parts are welded or brazed, but you can't count on it, so distortion can cause fit issues in some cases.
Also, some brazed frames from the old days had ISO dimensions but actually lacked the post-brazing prep cutting that would assure exacting tolerances.

This isn't the case.

JIS stands for Japan Industrial Standard, and is comparable to DIN (Germany) Norm Francais (France), BSC (England), Ansi (USA), and later on ISO - International Standards Organization which seeks to streamline things with a single worldwide standard.

The ISO headset standard is the old Italian standard probably because of Campagnolo's success in establishing it worldwide.

However whatever standard bikes are built around, the methodology is the same. Bikes intended for the Japanese home market were machined to JIS the same way others were machined to ISO or other standards. The machining after assembly was done on all better brazed bikes because these were built with excess material, which allowed for the machining to correct for distortion during the brazing process.

Some low end bikes skipped the machining operation by using tubing sized to spec (or close enough) and simply forced a headset into the ovalized tube.

Whether a frame needs to be prepped (or can be) isn't a function of the standard it was built to, but the process and thinking that went into the construction.

Yeah, I definitely know enough to cause myself problems. I measured my old headset but the battery in my calipers was dead and I lacked any other watch batteries. If the analog markings on the calipers were correct, they looked like it was much closer to 26.4 than 27 (and the new headset I had laying around and will replace it with has 26.4 pressed into it), so I'm guessing the frame is ISO (head tube is masked off as I'm repainting the frame at the moment.

So now, I'm waiting to hear back from the people selling the fork as to whether it's ISO or JIS. Since both headsets I have access to are apparently ISO and the frame must be ISO if the headset I pulled off of it was, it sounds like I need an ISO fork, and that's that. Now, if these folks would just get back with me, I'll know.

I had just seen where people were only selling the crown race and thought I could just use that if it was a JIS fork, but since what they're selling isn't for my brand/model probably (the one I pulled was OEM on the bike and not specified and the new one is old but pristine DA - many generations ago) that's not an option. I didn't know how standardized they are. Sounds like they're not within brands.

I guess I'd seen this table on Sheldon Brown's website and thought it was possible to do more mixing and matching than it really is.

If you have a metric ruler, preferably a steel rule, you can measure the fork's crown race diameter with a piece of paper.

Cut a strip about 3-4mm wide and a few inches long. Wrap it around the crown once and mark a straight line across the point of overlap. Unwild and measure, then divide by 3.14. Since the fork is either 26.4 or 27mm the difference in circumference is shy of 2mm, and you should have a pretty positive result. Do it 2 or 3 times to be absolutely sure.

BTW- Aftermarket JIS forks are very rare, and the last bikes to come to the USA with JIS headsets were Specialized and that ended over 10 years ago, so of you have an idea of the age of the fork you might have some confidence whether it ISO or not.

Side note, as one of the last defenders of vernier calipers, I'll add - no batteries needed - to my list of virtues.

Yeah, I guess part of the reason you get cheaper prices when ordering from an internet vendor rather than getting something from the lbs is that, in cases like this where the shop doesn't respond to this sort of e-mails (though has shipped promptly in the past), you take the risk of not getting quite what you want because of lack of service and lack of ability to physically touch/measure something yourself.

Eh, it's supposedly a new fork (though cheap and from taiwan and Sheldon says cheap, asian bikes are the only ones likely to be JIS, but all cheap forks are from Asia), so it's probably what I want, and, as I mentioned it's cheap, so I'll probably take that chance on it.

My only steel rule of any quality is SAE (though you'd have thought that the plant I worked at when I got it would have had to convert to metric to get their ISO 9001 or whatever certification, but what do I know), but I will have to make sure I get a handle on what I have and what I get. As a low tech person myself (I have to be the last person I know to resist a cell phone) I can see the benefits of calipers that don't require batteries. I need to stop cheaping out on my tools and getting good stuff.

Again, and not meaning to state the intentions of any frame's builder, but any JIS fork or frame can be quickly cut to a very-precise ISO dimension.
The ISO standard as applied to cheaper bikes was problematic in that there would not be any "leftover" metal to machine to ANY standard.

A single exception is the Alloy fork, as on a Specialized Epic, which had a bonded sleeve junction with a reduced steerer tube. The alloy part of this sleeve joint interferes with the most popular crown race seat cutters, so the fork crown race seat must either be filed or lathe-turned (I've seen it done both ways).

Well ****. A couple of days before my first post in this thread, I'd found the fork I thought I wanted to use. 4 or 5 vendors online including 2 I'd bought from before. So I contacted the cheapest (who I'd used in the past) and asked if it was ISO or JIS. 2 days and no response, so I contacted the other seller and asked them the same question. Their automatic form said they'd get back to me within 24 hours. 2 more days and no response from either so I figured what the hell, chances are it was ISO and it's not that expensive so I went with the cheaper option (~$40 shipped vs ~$50, not a big deal, but $10 isn't nothing) and ordered yesterday.

Then today, the more expensive seller got back to me saying that she couldn't find the info on the part so had contacted her distributor and he said it's ISO (what I want). So now I'm feeling like a tool because she went the extra mile to find out the info for me and I'd already ordered it from someone else. And with some other stuff I've ordered in the last few days, I've gone over the parts budget I've allotted myself for the month (and don't really need any other parts at the moment), so I can't even just buy something else from her as a thank you.

So yeah, I guess I'm that guy.

Edit: I should note that had she responded before I ordered, I was prepared to buy from her store (I contacted them second because I wanted to buy from whoever gave me the info as I figured the knowledge confirming it was the right thing was worth some money), just when I thought it was a gamble, I figured I'd go as cheap as possible.