Ask a Mechanic | Indexed Headsets & ISIS/Howitzer Crank Compatibility

Welcome to our Ask a Mechanic column where our expert mechanic Daniel Slusser answers your bike maintenance questions. If you have a question for Daniel, please post it on our Facebook Wall or e-mail Daniel directly at Today’s column discusses how to fix notchy feeling headsets and ISIS/Howitzer bottom bracket and crank compatibility.


The steering on my Trek 5600 OCLV feels notchy. When I turn the handlebars away from center I can feel some resistance until I am about 3-5 degrees off of center, and then it turns freely. Do I need a new headset or will a simple adjustment fix it? From: Rudy


The problem you are experiencing is something that mechanics like to refer to as an “indexed headset,” a sarcastic reference to Shimano’s old indexed downtube shifters. The difference is that indexed shifters were a great new feature when they came to market but an indexed headset is a feature that you most definitely do not want.

On road bikes the handlebars turn so infrequently that all the tiny impacts from the road get applied to that small section of the headset’s turning radius right in the center. Once a divot is formed in the bearing race a single ball bearing will nest in there creating the indexed feeling when you turn the bars a few degrees off of center.

Cartridge headset bearings like this one can be easily replaced at home and to fix an indexed headset.

This problem can be avoided through preventative maintenance. If you have a cartridge bearing headset you should rotate the bearings in the frame or headset cups (whichever the case may be) 1/8-1/4 of a turn every six months. If you have a loose ball bearing headset you are out of luck because to accomplish the same thing would require pressing the headset cups out of the frame, rotating them, and then pressing them back in, a hassle that is not worth the effort.

In your case the opportunity to use preventative maintenance is past and you will need to replace the damaged parts. If you have a cartridge bearing headset all you need to do is get new bearing cartridges (assuming the damage doesn’t also include the fork’s crown race). If you have a traditional loose ball bearing headset then you will need to replace the entire headset assembly and that will require the help of a professional bike mechanic with the tools and experience to press the headset into the frame correctly. I recommend that you purchase the undisputed champion of headsets, the Chris King No-Threadset. The design and quality is so good on these you’ll never need to worry about preventative maintenance or rotating bearings to avoid damage. If you do ever have a problem with it, Chris King offers an amazing 10-year warranty.


Is there only the one standard of ISIS 10 spline BB? The reason I’m asking is that I’ve bought a reconditioned Solid Mission 9 frame that came with a new external ISIS BB as the frame shell is 100mm. My plan was to take my Truvativ Husselfelt ISIS crankset off my current, barely used bike and just swap it straight over but the BB seems to be slightly too large for the cranks. I’ve tried it in all 10 orientations in case of a raised spline (non visible), but no joy. I didn’t want to start persuading it with a mallet or spending more money until I’d had some proper advice. From: Graham


The outboard bearing (external) ISIS spline you are referring to is the Howitzer ISIS spline and it is slightly different than the original ISIS spline. You did the right thing by not forcing it. A Howitzer specific crank is required for your bottom bracket.

SRAM’s Crank and Bottom Bracket engineering honcho Anthony Medaglia.

To get a firsthand understanding of what makes the two spline standards different, I reached out to my friend Anthony Medaglia, an engineer at SRAM/Truvativ who helped design the two interfaces. Here is what he had to say:

“The splineform [for the Howitzer standard] is slightly different than ISIS and it was never intended or implied to be compatible.

Even if they were compatible, the crankarm has to be designed specifically for external vs. internal bearing bottom brackets so the Q-factor and chainline work out properly, so offset and spider placement of the crankarm forging define its application.  You simply can’t expect to use a crankarm designed for an internal bearing BB on an external bearing configuration.”

Daniel Slusser is a professional bicycle mechanic with over ten years of experience. He holds a bachelor’s degree in business administration from HSU and a master’s degree in history from Cal Poly University. When he is not riding, wrenching, or writing he enjoys spending time with his wife and two children.