About the mounting bolts for Shimano's flat-mount disc brake calipers.
Starting with the front.
The bolt that secures the caliper to the 160/140mm adapter
is called the "Caliper Mounting Bolt B."
By flipping the adapter's orientation top-to-bottom,
it accommodates each rotor diameter.
The bolt that secures the adapter-equipped caliper to the front fork
is called the "Caliper Mounting Bolt A."
Bolts A and B have no length variations, so nothing complicated there.
Now for the rear.
With a 160mm rotor, use bolt B along with an adapter;
with 140mm, no adapter is needed.
The bolt that secures the rear brake caliper to the frame
is called "Caliper Mounting Bolt C," and this one comes in different lengths.
It has a special shape with a T-shaped protrusion at the bolt tip that prevents it from falling out,
but since the brake's threaded hole is a blind hole rather than a through-hole,
using the longer one by mistake will bend the protrusion and prevent the brake from securing properly.
This is the "20mm length" Caliper Mounting Bolt C for the BR-R9170,
but as you can see, the 20mm mentioned here
is not the length of the threaded portion itself,
but rather the thickness of the frame.
The R9170 Mounting Bolt C comes in three lengths:
20mm, 25mm, and 30mm,
with a difference of 13mm between the frame thickness
and the bolt shank length including the protrusion.
There's a tool straightforwardly called a "Bolt Length Selector"
for determining the appropriate length.
The tip of the bolt's protrusion must fall within the step range of this gauge,
and you need to use the corresponding length.
↑On this frame, the Mounting Bolt C length differs front to rear.
The front uses this length,
and the rear was confirmed to use this length.
Both front and rear are proper lengths,
but front is 20mm and rear is 10mm.
However, the R9170 Mounting Bolt C doesn't exist in a 10mm length.
The early flat-mount rear brake calipers
(BR-RS805/505) used
"Converter Mounting Bolts" as their mounting bolts.
These are discontinued, but they're still obtainable at the moment.
The difference between frame thickness and bolt shank length is 6.8mm,
and the last three digits of the part number increase by 50 as the length increases.
Unlike Mounting Bolt C, these lack the tip protrusion,
so the Bolt Length Selector has a different shape.
Currently, even the BR-RS805 uses Mounting Bolt C,
and the 10mm, 15mm, and 35mm lengths exist only for that version.
Mounting Bolt C comes in only two types: R9170 grade and RS805 grade.
The BR-R9170/R8070 uses the R9170 series Mounting Bolt C,
while the BR-7070 uses the RS805 series Mounting Bolt C.
So here are the lengths and part numbers for the RS805 Mounting Bolt C.
As the length increases, the last two digits increase by 10... or rather,
15mm was added later as an afterthought,
so be careful.
↑On the left are all RS805 Mounting Bolt C lengths from 10–35mm,
on the right are all R9170 Mounting Bolt C lengths from 20–30mm,
each incrementing by 5mm.
The R9170 version noticeably has white threadlocker applied,
but actually the RS805 has threadlocker too, just in a less visible color.
They don't appear to differ materially,
but the RS805 is ¥505 (before tax) regardless of length,
while the R9170 is ¥722 (before tax) regardless of length.
Just as I stock rim brake mounting countersunk nuts
(excluding especially long fork-specific models)
in a full range,
I'm going to need to keep Mounting Bolt C
in stock for immediate availability from now on.
What a pain.
And unlike countersunk nuts, there's no redundancy where
"one size longer or shorter also works."
What a pain.
Starting with the front.
The bolt that secures the caliper to the 160/140mm adapter
is called the "Caliper Mounting Bolt B."
By flipping the adapter's orientation top-to-bottom,
it accommodates each rotor diameter.
The bolt that secures the adapter-equipped caliper to the front fork
is called the "Caliper Mounting Bolt A."
Bolts A and B have no length variations, so nothing complicated there.
Now for the rear.
With a 160mm rotor, use bolt B along with an adapter;
with 140mm, no adapter is needed.
The bolt that secures the rear brake caliper to the frame
is called "Caliper Mounting Bolt C," and this one comes in different lengths.
It has a special shape with a T-shaped protrusion at the bolt tip that prevents it from falling out,
but since the brake's threaded hole is a blind hole rather than a through-hole,
using the longer one by mistake will bend the protrusion and prevent the brake from securing properly.
This is the "20mm length" Caliper Mounting Bolt C for the BR-R9170,
but as you can see, the 20mm mentioned here
is not the length of the threaded portion itself,
but rather the thickness of the frame.
The R9170 Mounting Bolt C comes in three lengths:
20mm, 25mm, and 30mm,
with a difference of 13mm between the frame thickness
and the bolt shank length including the protrusion.
There's a tool straightforwardly called a "Bolt Length Selector"
for determining the appropriate length.
The tip of the bolt's protrusion must fall within the step range of this gauge,
and you need to use the corresponding length.
↑On this frame, the Mounting Bolt C length differs front to rear.
The front uses this length,
and the rear was confirmed to use this length.
Both front and rear are proper lengths,
but front is 20mm and rear is 10mm.
However, the R9170 Mounting Bolt C doesn't exist in a 10mm length.
The early flat-mount rear brake calipers
(BR-RS805/505) used
"Converter Mounting Bolts" as their mounting bolts.
These are discontinued, but they're still obtainable at the moment.
The difference between frame thickness and bolt shank length is 6.8mm,
and the last three digits of the part number increase by 50 as the length increases.
Unlike Mounting Bolt C, these lack the tip protrusion,
so the Bolt Length Selector has a different shape.
Currently, even the BR-RS805 uses Mounting Bolt C,
and the 10mm, 15mm, and 35mm lengths exist only for that version.
Mounting Bolt C comes in only two types: R9170 grade and RS805 grade.
The BR-R9170/R8070 uses the R9170 series Mounting Bolt C,
while the BR-7070 uses the RS805 series Mounting Bolt C.
So here are the lengths and part numbers for the RS805 Mounting Bolt C.
As the length increases, the last two digits increase by 10... or rather,
15mm was added later as an afterthought,
so be careful.
↑On the left are all RS805 Mounting Bolt C lengths from 10–35mm,
on the right are all R9170 Mounting Bolt C lengths from 20–30mm,
each incrementing by 5mm.
The R9170 version noticeably has white threadlocker applied,
but actually the RS805 has threadlocker too, just in a less visible color.
They don't appear to differ materially,
but the RS805 is ¥505 (before tax) regardless of length,
while the R9170 is ¥722 (before tax) regardless of length.
Just as I stock rim brake mounting countersunk nuts
(excluding especially long fork-specific models)
in a full range,
I'm going to need to keep Mounting Bolt C
in stock for immediate availability from now on.
What a pain.
And unlike countersunk nuts, there's no redundancy where
"one size longer or shorter also works."
What a pain.