Combined chainring bolt tool

A combined tool based on the VAR PE-35500 (https://www.vartools.com/en/combined-tool-for-chainring-bolts-var-p828.php).

The tool does have some limitations. If the nut is clocked at a certain angle, the throat of the tool won't be big enough to match with the nut, in which case you'll need to use an actual chainring spanner.

This could be addressed by increasing the throat depth, but doing so weakens the tool.

As a bonus though, if you only ever install your chainring bolts with this tool, the nuts will always be clocked correctly & you'll be able to remove them.

I will likely call this project done unless mine breaks.

Print Settings

Printer:

DreamMaker Overlord Pro Plus

Rafts:

No

Supports:

Yes

Resolution:

0.1 mm

Infill:

20% (I think)

Filament:

Hatchbox PLA, 1.75 mm

black

Notes:

Print with the largest face side down for strength.

How I Designed This ===================

Throat

Depth is 65 mm, width 30 mm.

I used a cut up piece of cardboard to approximate it for my chainrings (44T, 96 mm BCD & a 50T, 110 mm BCD).

Bushings

The idea was to press-fit the bushings into the body, & have the hex key rotate in these bushings. They are also there to allow the hex key to slide axially with minimal friction.

They are supposed to be replaceable when they wear out.

Chainring nut interface

Although you could make a slotted head to fit the chainring nut from regular stock (which would vastly simplify/strengthen the design), I decided to keep this project limited to readily available items.

It turns out that an ordinary North American NEMA 5-15 prong is within the range of correct thicknesses for the chainring nut.

It also turns out that standard keys often are of the right thickness too.

The prong is filed & press-fit into the body with a soldering iron.

If it needs replacement, there is a small hole on the backside of the body which allows the user to push out the spent prong with a spoke or small screwdriver.

A new prong can be press-fit provided that the slot is not worn out from the previous prong.

A copy of the NEMA standards handbook is included among the downloads.

Future improvements

Bushings should press-fit into the body

They currently fall out too easily under axial play.

The bushings aren't supposed to be such a generous slip fit. As they are in the first revision, the body will wear out first since the bushing rotatewith the hex key (I suspect it would take a lot of usage to wear out the body though). The intent was to keep the bushings stationary with the body, & have the hex key rotate inside the bushings. Then if the bushings wear out, you simply replace them. However, this still mitigates wear.

The fit is exacerbated by the fact that I (& you should, when you do) print the body with the largest face side down. Because of this, the bushing hole came out oblong.

Printing the body vertically will cause premature fracture due to layer orientation.

It's also possible to buy COTS bushings instead. However, I figured that it's easier to 3D print them with negligible loss in performance.

Swap out NEMA prong for an actual chainring wrench

You can get one for less than $2 off eBay. In retrospect, I should've done this. The design & assembly would be easier & stronger since we'd have a whole lever arm to work with.

Stronger body

I made a second version of the body which increases the amount of material (for strength) around the slotted head for the chainring nut, but this will require more supports.

Assembly

You'll need:

• 5 mm hex key (I made a convenient handle here: https://www.thingiverse.com/thing:3180264). The vast majority of chainring bolts take 5 mm. You may need a T30 instead. Either way, the handle will probably fit correctly. You can edit the SLDPRT file to match your key.
• NEMA 5-15 prong. File this down to the dimensions listed in the only drawing file.
• body. Print this largest face down.
• 2 bushings.

The slot on body for the prong is undersized deliberately. I used a soldering iron & pushed down on the heated prong with the hex key. The PLA locally bowed somewhat concerningly, but it seems fine.

I needed to use a hair dryer on the bushings to get a good fit with the hex key.

NEMA 5-15 prong dimensions.

In use

It's survived loosening 4 fully-torqued chainring bolts so far. I will have to see how well it does for tightening chainring bolts.

Category: Tools