41 vs 42 IS Headtubes and internal routing headtubes

There are two sizes for the upper diameter on IS headtubes, 41 and 42 mm (actually 41.8). Why are builders choosing one size over another? Is it headset or headtube availability? Cost? Rider preference? Do the cool kids like one size and the dorks ride the other? Would builders like to see a 42/52 IS headtube from PMW? Would builders like to see a less expensive version from PMW?

What are builder’s opinions on internal routing headtubes? As always, it’s great for carbon fiber, OK in aluminum, chunky in titanium and heavy in steel. 44/56, 52/56 and 56/56 are sizes that are being used; 52/56 seems to be the most popular. It is possible to do a less heavy/less expensive version from 3 pieces, a center tube that would be assembled and welded with machined ends by the builder. The trade off is more work for the builder. Is there value to the builder if PMW assembles and welds them for out-of-the-box readiness?

Please comment!


41 vs 42:

IS41 vs IS42 is something that I have been curious about as well… We use IS42 because we use the BFS headtubes for mountain bikes. But I have no preference either way.

Out of curiosity, why did Paragon go with the IS41 standard?

Is there some history behind these two standards? It looks like IS42 is the “Campagnolo standard”. Where did IS41 come from?

As far as I can tell, the only difference in availability is with Wolftooth (IS42 is only available in their “Premium” level)

Internal routing:

With the release of Chris King’s Aeroset headset, I imagine the EC/ZS44 headtube will continue to be the headtube of choice. I don’t think we will route the cables internally, but if we do, we would use this headset and headtube system.

From an aesthetics standpoint, in my opinion, 44/56 52/56 56/56 headtubes start to get really chunky and mismatch in size for steel and even titanium tubes on road and gravel bikes. For mountain bikes, the tubes are bigger, but I can’t imagine internal routing taking off.

What I would like to see:

I would like to see more 1.125-1.25 headtube options for road and gravel bikes. I personally think 44mm headtubes and thick carbon forks look out of proportion on small <54cm and >56cm road and gravel bikes.

I see this as a chicken and egg problem: Right now there are very few slim 1.25 road gravel and forks, so there is no motivation to shrink the headtubes from the ubiquitous 44mm headtube.

I think @PineCycles is leading the charge with the EC37 standard for tapered or straight forks.

There is also the random Pegoretti D11 Standard (EC38)

I think it would be cool to see an EC37 headtube or even a EC34/37 headtube. I think that would scale back the chunky aesthetics that we inherited from the “aero is everything” road bike phase.


I actually think the EC37 standard has some value and could replace EC34. It allows for straight 1.125 and tapered 1.125/1.25 steer tubes to be use. Not specific to the question asked, but I’d be interested in a paragon EC37/EC37 headtube. Cyclus is currently the only tool supplier making a reaming cutter, but I was able to find non bike specific 36.85 reamers that would do the trick and weren’t ridiculously expensive.

As far as 41/42, I am indifferent and as @Daniel_Y mentioned we use what is available and fits our budget. Looking toward drop bar bikes we would like something that is a little more aesthetically pleasing than the 160+mm versions of the headtube we use for our mountain bikes.

It is worth noting that the Park Tools IS42 cutter is significantly less expensive than the archaic IS41 cutter set sold by Park.


When we took over Loco Machine’s headtube line in 2017, all the IS headtubes he did were 41, so we stuck with it. 42 wasn’t common at the time, but it looks like this is continuing to evolve. I believe 41 IS came from Cane Creek, but I’m not positive.

Right! The size of these is really out of proportion to steel tubing, and heavy as well. I don’t see the need for internal routing through the steerer/headtube, but that’s subjective. If the demand is there, we’ll do something.

Forks are the driver here. Nobody’s going to put them on a bike unless they’re available. That’s your job to pester the fork makers!

This could look really good, and have a nice esthetic with steel tubing. EC37/37 could be made from 1-5/8" tubing, either leave the OD alone and relieve the ID, or turn the OD to leave some beauty rings. EC34/37 would look the best, but there’s more machining for more $, but not as much as EC34/44.


This is an easy one to do, it’s straight, and tubing is readily available. Do you prefer straight OD, inside relief, or turned OD with beauty rings?

This is huge, and if a builder is just gathering the tools, it makes the decision for 42.

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Columbus do a 1.125-1.25 Futura Disc fork and matching 36/46 swaged head tube. Cane creek does an EC34/EC44 headset to match.


I am currently working on this project on the side to source a fork. I was unable to find any open molds. There were some candidates that had a slim appearance, but new open mold forks follow the trend of internal routing: 1.5 tapered to match the 1.5 headset, and no external hose routing.

The Athoes and Ultimate are trending toward slim forks. I want custom frames to beat the big companies at their own game!

The key difference with these forks is that the leg immediately tapers below the crown:

The slim fork, slim headtube, and EC37 headsets are an entire ecosystem that I am slowly building the pieces for.

Just for reference, here is a size comparison of an EC37 system and a ZS/EC44 system:


Inside relief, with beauty rings. We can’t make your job too easy.


Ive run the Futura SLX fork in the taper head tube as well. The SLX is straight 1 1/8 but on runs only as a rim brake. Makes for a really elegant modern front end. If they did a disc version I’d be all over that for my road builds instead of the 1 1/4.

Ritchey WCS Carbon Road and gravel disc is another option with it’s nice curves and 46mm crown seat diameter. Only comes in 46 or 47mm offset though. Straight 1 1/8 steerer.

I’ve always built with IS41 because y’all make those :sweat_smile: I’ll just hop on whatever bandwagon PMW drives.


Thanks Daniel! @mark_pmw and I have discussed the EC37/EC37 Headtube and the goal is to order from PMW when we’re through our current stock of headtubes. I’d be really stoked to see these made and available to other builders sooner though! Perhaps Mark can create a dedicated post to see if there is demand. For reference below are all the compatible forks/headsets

White Ind EC37/EC37 Headsets are now available to order!

Road RIm
-ENVE Rim Brake
-Columbus Futura Caliper SLX
-No.6 Road Rim
-Parlee Road Fork

Road Disc
-ENVE Road Disc Fork
-Columbus Futura Disc SLX
-Whisky No.9 RD
-Parlee Road Disc

Allroad/Light Gravel
-Columbus Futura Gravel
-ENVE Allroad Fork

-Parlee Gravel/Cross Fork


Hey Mark, I probably talked to more builders about internal routing that anyone while I was at ENVE. Here’s My take

I think the vast majority of builders using full internal will be sticking with 44 and using headsets from ENVE, King, FSA and White Ind (soon).

I think this would be the best solution for integrated metal bikes. It maintains a slim headtube that matches the aesthetic but cleanly flows into the fork. It can just as easily be used on an externally routed bike. I think a slight variation of your IS41/IS52 wound be perfect.

IS52/IS52 Cups
I’ve seen several builders machine cups and weld them on the headtube Firefly Style to maintain a slimmer headtube profile. I’m personally not a fan of the angular aesthetic on a metal bike but several builders are doing it so that may be the preference.
Example: Rizzo & Jaegher

IS52/IS52 Internal
I’m not a fan of straight 56ish hedtubes on metal bikes. Looks too imbalanced to me
Example: Landyachtz, Legend & Angel


Thanks, this is really valuable info.

For now, we have 44 covered with our current line of headtubes, at a reasonable price. If builders really want EC44/IS52, please comment. I’m in complete agreement on IS52/IS52.


Here’s what we’re thinking, get a 37/37 headtube in production soon. It seems to have broad appeal, and can really change how steel road and gravel frames are built.

We can start with 1-5/8" x .095"w tube, either 4130 or 1026. The OD will be turned, similar to our current 44 headtubes. Rings at the end will be 42 mm diameter x 5 mm long. The center section will be 1-9/16", so a standard hole saw can be used. The ID could be unmachined. This leaves .5 mm to ream, and makes a 1.58 mm wall. Headtubes with an unmachined ID can be very inexpensive. This may be a little heavy, so we could (for more $$) relieve the ID to 1 or 1.2 mm wall thickness.

Here’s the graphic:

Please comment, thanks!


My vote is for the relieved ID. The intent of this part is the road/gravel market which is more weight conscious than the mountain bike market.


@PineCycles what solution is there currently for reaming? I have found some off the shelf helical reamers that come in at 36.86mm and it looks like Cyclus has and EC37 reamer available, anything else that you know of?

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Just Cyclus and Silva. The Park Tool Reamer Set used to come with the 1 1/4" Reamer so older shops definitely have them lying around. If you called around I bet you could find one.

I had a some discussion with Calvin at Park about remaking the reamers but he wasn’t convinced there would be enough demand. But with a paragon Headtube and White Ind headset that might be enough!


OK, relieved it is. What wall thickness? 1 mm? Thicker?

I’m not opposed to having them made, and can certainly do so for myself but that doesn’t serve anyone else.

@mark_pmw 1mm wall is what I would like to see.

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All the Columbus headtubes are 1.1mm so that seems like a good target but if you think 1mm is sufficient I’d say go that route

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