(long post, sorry but no real way to shorten it)
Not sure if I'm just unlucky and its a factory defect or if its normally small enough that people don't notice, but I ran into a weird boost leak this weekend that I wanted to share since I have never seen it mentioned before and I didn't notice it until I hooked up a smoke machine to hunt a DIFFERENT boost leak.

First, some back story on the bike. Its a 2016 Concours 14, so while its "close" to a ZX14 there are a lot more differences than most realize but I can't imagine there being different frames so probably not relevant. It has been turbo'd with a DIY setup for about 6 months now, with a TD05 w/ billet 7+7 wheel, E85, 10psi, your average supporting mods, yada yada. Did a hard pull a few days ago leaving gas station and bike fell on its face and was only making 4psi, with some oil on the forks when I got home. Tightened all the charge piping and it made no difference, so yesterday I begrudgingly pulled all the fairings and hooked up a smoke machine w/ air pressure nozzle to the turbo inlet and ran it up to a few psi with the smoke on. I quickly found the boost leak I was expecting around the drain port thats right above the front tire - no big deal, I didnt use loctite like an idiot and the bolt I ran into the (poorly tapped) hole came loose over time and fell out, leaving the 5mm hole wide open. Put a new bolt in there with some loctite this time, and hooked up the smoke machine again to double check.

Imagine my surprise when I see smoke billowing out from UNDER MY STEM BEARINGS. I had noticed recently some oily residue around my horn that I assumed was just the head stem bearings starting to go bad and leak some grease residue, but its actually leaking boost from inside the head tube, which I have never seen before. Got a camera inside the airbox expecting the worse(cracked frame??? She's a heavy girl and doesnt get babied). I found way up front right in the center of a supporting spine a tiny hole leading to the steering stem tube, which you can't see from outside the frame but allows boost to actually get inside the head tube and force its way out of the bearings. Probably doesn't make it past the bearing seals on really low ~4psi setups, and its likely a small enough leak that most don't notice it, but it was surprising to me and thought it was worth sharing. Honestly looks intentional, like a vent for welding or something, but I guess it could be a defect/void.

To seal it was annoying but nothing crazy - its a rough bumpy surface already so opened one of the airbox covers and got my hand inside with a rag and some iso to degrease it as best as I could, then mixed up some JB weld that I awkwardly ran down the spine and around the hole until it built up enough to seal it. Its far from the entrance hole and tight around the forearm even with my pretty lean and lanky arms so might not be possible for some to get in there, but I was able to seal it and after letting it cure for 24hrs I tested again and zero boost leaks.

Again, not sure if its something only Concours14s have (doubt it since the frames should be the same), something all bikes have (doubt it since I have never seen it mentioned as a place to seal), or I'm just unlucky and its a weird casting void on my particular bike but worth sticking a borescope in the airbox and checking if you're boosted and running more than the base PSI.

First picture is where the smoke was billowing out of directly over the front tire with the horn+shield removed for access.

Second pic is the hole I had to seal to stop it, view is inside the airbox, facing the right side frame blockoff/ pointing towards the steering stem. Its directly in front of the drain hole that is above the horn and is directly connected to the head tube, not visible from outside the bike.

Anyone ever seen this before, or am I just that lucky??

EDIT: Known location from the RCC guide that I read before even starting the build but somehow missed this one and forgot about. I remember the guide mentioning the one on the floor thats visible from outside the bike, but did not remember that one on the spline and since its not visible I never caught it during assembly. I used 2part instead of the putty since it typically sticks much better, but basically sealed it the same as RCC recommend.

* Last updated by: Puck on 4/28/2026 @ 9:15 AM *

In the first pic you can see the bolt in the top right sealing the "known" drain hole in the airbox above the front tire which is on the FLOOR of the airbox - this is a completely different hole on the front WALL of the airbox, that leads inside the head tube so it would never be found if looking from outside the bike. If I wasn't testing with a smoke machine I never would have found it.

Seems to have improved my spool by ~100rpm by sealing it, but its so hard to tell from one ride that it could be placebo. Either way, it was leaking before and now its not so I'm happy :). Now to figure out why its flashing "FI" with the red light for a split second when I do hard pulls...did it twice today while testing, and wasn't doing that before.

Page 2 of the RCC turbo kit calls it out, all frames have it.

Bingo Fordtech58. I was looking for the same thing to share. Get a tube of TankWeld JB Weld. Clean the frame with spirits, and use the tankweld on the holes inside the airbox. Use a bolt for the front hole of the airbox.

Derp, I literally read that guide before starting my setup and I could have sworn it only called out the lower hole above the wheel that can be accessed from below, not the one inside the airbox in the spline.

Disregard then, Oops!