Shimano Hollowtech/Octalink Crank Arm Shortening

175mm becomes 154mm      172.5mm becomes 151.5mm
170mm becomes 149mm      165mm becomes 144mm
Hollowtech FC-5501, 5502, 5504 and 5505 105s and 6500 series Ultegra are the only ones that can be shortened
Due to the way these thing are made, there will be some variation in how close the end of the tunnel is in relation to the end of the crank. They are forged around a steel tongue. The pedal end is fatter on the outside. Then the tongue is pulled out and the end is stomped flat, closing the tunnel. A lot of hand shaping and finishing must be required to make them look so nice, because they really aren't that symmetrical.

175mm or 170mm I've never had a problem shortening a 175 or 170 by 21 or 22mm. (Not so far anyway, but it certainly could happen.)
Recently I shortened an orphan 175 to 150 as an experiment, without hitting the tunnel.

165mm or 172.5mm
165s seem to be made from the same forging as 170s.
172.5s seem to be made from the same forging as 175s.
This makes tapping into the tunnel more likely. 

Out of 3 sets of 172.5 I did tap into the tunnel on 1 arm at 150.5. Since then, I've done about 20 sets at 151.5mm and just barely tapped into the tunnel once.
On one 165 shortened to 143, I saw a tiny black line at the tip of a thread that may have been the tip of the tunnel. On another one shortened to 144mm, I clearly hit the end of the tunnel with the tip of one thread. I've done about 15 sets of 165s.   However on one 165mm arm the opening was about 2.5mm wide, making the crank unsafe to ride.

Based on this "Too small to be statistically valid" sample, I'll leave it up to you to decide if it's worth taking the chance.

If I tap into a tunnel, because it is unusually long on your arms, I don't charge enough to be responsible for replacement.
Given that the typical failure mode for pedal eyes is on the sides, if the opening into the tunnel is a hairline crack, it seems unlikely that any failures will occur due to this. The metal there is thick, while it's amazingly thin further down the arm. 

I will NOT shorten Shimano 105 FC-5503 or FC-5500    See photo at top for Model # Location on back of crankarm.
These are a cheaper, Non-HollowTech variety that are narrower and therefore do not have sufficient metal in the area where a new eye would go.

Hollowtech MTB cranks
Someone tried shortening a Hollowtech MTB crankset and cut a big hole into the tunnel.
The 105s and Ultegras are old technology. The fact that they can be shortened indicates that they weren't taking full advantage of the idea.

Hollowtech II hollow two piece cranks  No way, not even close.

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