Repeated Failure of MK4i Intermediate Shaft Screw

This weekend at Week Zero, Team 3467 experienced back-to-back failures in separate matches on our new MK4i modules. These showed initially as diminishing drivebase control in playoff matches which led to lengthy repairs between matches upon further inspection in the pit. These failures eventually led to our robot barely being able to drive in our final playoff match.

We believe there is an incorrectly spec’d screw in the kits teams received that contributed to this failure. Our intent is simply to raise awareness for all teams using these modules to prevent similar failures that are unrepairable without a collection of spare parts or extra modules on-hand.

The source of this failure was identified as the 12-24 countersunk screw that secures the offset gear to the intermediate drive shaft. This screw tore out of the shaft, taking interior aluminum threads from the shaft with it. The torn out threads are the little circle on the pulley flange pictured below. This then renders the shaft useless without a way to retain this gear.

This ultimately caused the steering of the module to bind when the screw jammed against the gear, locking the rotation of the module.

It’s important to note that these bolts were not over- or under-tightened and that Loctite 243 had been applied correctly to each one.

The bolt did not back out or shear - rather, the interior threads ripped out of the shaft due to inadequate thread engagement. The actual full thread engagement is ~1/4" since the bolt has a taper at the end and doesn’t engage near the top due to the countersink. With the intermediate shaft being machined 6061-T6, we’d expect at least 2x the major thread diameter of engagement, which is not achieved in this case. The use of a countersunk screw was necessitated by the clearance to the bottom of the idler gear (0.06375" of clearance as currently designed when measured in CAD). However, it is odd that this hole was drilled and tapped as deep as it was, but the hardware used in the CAD model made use of ~half of these threads and the bolt actually shipped out with the modules makes use of even less.

(As a general note - countersunk bolts are measured from the top of the head when the length spec is listed so a 1/2" countersunk screw actually only has 0.365" thread length to begin with.)

This image is of the MK4i CAD of the assembly with a 1/2” long screw that has been sent in the kits as specified on the hardware page and what was received with the modules. We suspect this wasn’t a one-off screw issue and more teams have these 1/2” bolts.

0.5 screw

This image is the 5/8" long bolt that is in the CAD model provided by SDS which while this thread engagement still seems low to me would probably have been adequate.

This image is of the 3/4" long bolt that we have ordered to replace all of these bolts in every module we own to hopefully avoid this in the future. Because of the odd hardware size of 12-24, we highly recommend other teams with these modules order them as well.

Based on analysis of the CAD model and some crude caliper measurements before leaving the competition, there should be more than enough thread depth in this shaft to accommodate the entire bolt as the threads in the hole did seem to get very close to the bottom.

We are also very curious if any other teams have seen this issue either of the threads ripping out of the shaft or constant tightening of these screws. Talking with other teams, we have heard reports of frequent maintenance on this bolt in addition to Loctite being applied.

Also curious if @PatrickW could provide any insight to the inconsistencies between the CAD model and the actual hardware shipped with the modules or has any solutions to offer to aid in this very critical failure.


The trick we have used to prevent that failure is to apply green loctite (loctite 680) to the gear itself so it retains itself. We have thrown that over the charging station for hours without issue.

A few teams (including mine) have had this exact issue. As I posted in this thread, we also purchased longer screws (5/8" and 3/4") but found that 3/4" bolts could not be installed without taking the module apart.

We had nearly the exact same failure mode just prior to departing for Week0. Luckily our bolts seem to be missing and the threads aren’t destroyed.

we just had the same issue in the middle of a Scrimmage with 3506 YETI Robotics, and realized the screws laying on the field were from all 4 modules gears

Really appreciate you posting this!


Thanks for posting this. Good for teams to know.
We can confirm that ours completely loosens out, even with blue loctite. We have since switched to red and will see how it goes.
Unfortunately for us, one of the screws came out and ruined one of our steering gears.

We had no such issues over 2 events and lots of practice hours on the SDS set we got and used this past Fall.


When did you guys buy your modules? Curious if we got ours in the same batch and if its a batch issue.


We bought a mix with Mk4i kits bought on 11/1/2022 and Mk4i conversion kits purchased on 11/10/2022.

Any teams who used Mk4i know what length screw was used in the 2022 version? We used Mk4 in 2022 so we can’t corroborate what the screw length was, but we don’t remember hearing these reports.



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I saw your question so we decided to pull ours out from our offseason bot. Its a 1/2" screw same as what we have now, with blue loctite.
I’m guessing since its the same as what we had before, the red loctite that we started using yesterday should be good for preventing the screws coming loose, hopefully.

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We have experienced this failure mode as well

We have not done an investigation of our modules purchased 12/22 from Andymark, but in the 2022 season we implemented some countermeasure to ensure reliability of our swerve modules after seeing some bolts loosening on our dev chassis MK4’s.

  1. Minimize Student distraction, this was a big one for us and prevented many mistakes
  2. Cleaning bolts, this removes the grease from manufacturing which interferes with the blue 243 Loctite.
  3. Loctite/Torque all bolts to spec, this is a Tightening Nominal Torque spec not a torque to yield.
  4. Paint pen mark each bolt individually to show loosening, luckily we never saw this

Following this procedure, we never had issues with bolts backing out during the comp season.


It is one of our goals to make the most robust and reliable swerve modules available. We are taking this issue very seriously and understand the urgency of providing a reliable solution.

We (2910) had previously seen this screw slightly loosen (but not continue to back out). Because of this we always made sure to put a bit extra loctite threadlocker on this screw and it has solved the problem for us. Prior to this year I have only heard of a small number of cases of this screw backing out. It has always been able to be solved by a little more loctite. Recently there has been a significant increase in teams experiencing this issue. It is more than can be explained by the increased number of modules in the field.

To eliminate the screw issue I have designed an e-clip ring version of the intermediate shaft.

Because we are machining these parts in house, we can iterate and get parts into production very quickly. The undercutting tool has been ordered and will be arriving tomorrow. My goal is to have a batch of these shafts ready for customers before the end of this week.



Thank you for the fast reponse time and quick thinking on this issue. Appreciate how well you strive to keep shipping out top of the line products and listen to your customers. It speaks volumes.



To your knowledge, could this affect both Mk4i as well as Mk4 modules?

The MK4 and MK4i modules have the same parts in this area. I would not expect there to be a difference.



Thanks for the quick work on these! I assume you’ll be producing these continuously to keep up with demand?



It should be noted that this isn’t an issue with bolts backing out. The bolts are stripping the threads of the intermediary shaft. This isn’t an issue that could have been solved with more thread locker or better procedures, at least in 3467’s case.

The truth of the matter is that with a 1/2" length bolt, the length of thread engagement is just too short. The Safe length of thread engagement (I went through the manual calculations) is 0.442 inches which not even the 5/8th inch length bolt provides. (Although a 5/8th inch length bolt won’t fail as easily as the half inch one as it’ll get you 0.375in of LE, so 5/8ths is the minimum length that you could get away with, considering the life span of an average FRC Robot) And that isn’t even getting into the design deficiencies of how the intermediary shaft interfaces with the screw and gear. This is most likely only one of several issues that could cause thread stripping on this one part.

TLDR: No amount of loctite is going to stop the screw from stripping the threads. This isn’t an issue of a bolt backing out, this is an issue of a bolt ripping through the threads on the intermediary shaft. The two best solutions are to either replace the bolt with a 3/4in length one, or replace the intermediary shaft with one made of steel, so that the material yield strengths between the bolt and the intermediary shaft more closely match.

EDIT: I have included an excel spreadsheet with the LE calculations.

LE_Calculation.xlsx (9.5 KB)


we never saw this failure and we ran 200 matches last year and many many practice hours and had no issues but we used red Loctite.

I feel like there has to be something that different. are teams over torquing it initially? it’s not like this bolt see any real load pulling up on it