I went to the local bike shop and the guy thinks that the fit interface is too loose, which causes the left arm to slip. He applied carbon friction paste to the splines, and although it feels stiff when installing, it is a gel and probably migrates over time. I used a marker to draw lines on the clamping bolt heads to record where they were when torqued at 16 Nm. After a couple days, I released the bolts and torqued them again, but the lines did not match up anymore and was a little further clockwise. Also there isn't a way to control the thickness of the suspended fibers in the gel once the gel migrates, so I assume the thickness will no longer be uniform.
I measured the gaps between the splines of the spindle and left arm. The outer diameter has 0.17 - 0.21 mm of tolerance. The inner surfaces do not touch. The sides of the splines have about 0.05 mm to either side (total 0.1mm). The black paint on the splines of the spindle has worn away only on one side. The opposite side still has the black paint intact, so I assume that when the left arm is clamped tight, only some of the splines are in contact. The splines with the paint still intact are on the back facing side when the left pedal is pointed down, the area that I assume experience the least amount of pressure during pedaling.
I read somewhere that Shimano spindles extend 20 mm past the left BB cup. The Sugino spindle is shorter, and in combination with the large chamfer at the end, the surfaces of the splines on the spindle in contact with the left arm only extend approximately 17.5mm from the left BB cup. The center of the outer M6 clamping bolt is 15mm from the BB cup, so the splines only extend 2.5mm past the outer clamping bolt (17.5 - 15 = 2.5 mm). Also, the splines of the left arm stop 3.5 mm short of the BB cup. So the total width of the surfaces in contact between the spindle and left arm is only 14 mm. I have a 1.2 mm spacer on the right side for my chainline, which shortens this further.
The design of the clamping surfaces are different between Shimano and Sugino. I don't have a Shimano crank in front of me to look at, but from photos, it appears that Shimano possibly has a continuous, round surface in contact between the left arm and spindle, in addition to the splines (is this correct?) which makes it much more stable. The Sugino has no continuous surface in contact, only the discontinuous deep splines with only 14 mm width in contact, which I suspect is susceptible to wiggle and slip with the large 0.2 mm tolerance.
The directional play of my Sugino BB used to be only when the left arm was pointed vertically or to the rear. After adding more preload to the BB, I was able to remove the play, but now it has come back no matter how tight I make the BB and the direction has changed so that it only happens when the left arm is pointing forward. Adding more preload has added more friction to the BB. So instead I will attempt to switch the Sugino BB with Shimano BB.
I read on this blog page that the Shimano road BB is 90 mm wide measured from the outside of the cups when mounted, and is almost the same as the Sugino BB which is around 89.7 - 90mm. The Shimano MTB cups are 1 mm thinner on each side than the road cups, and the water sheaths (inner covers) appear to be interchangeable.
tobias feltus: créateur » bottom bracket interchangeability
So there is a possibility to make the spindle 2mm longer when using MTB cups with the road sheath to lengthen the contact surfaces for my Sugino crankset and hopefully make it more stable. Mixing the spacers on my spindle, I will get a final extension of 1.8mm of the spindle the spindle. The maximum I can extend the spindle before it hits the compression bolt is 2.2mm, so if I extend by 1.8mm I should have 0.4mm remaining. I will also have to readjust my chainring spacing for the FD and thick seat tube clearance. The new position means the double rings will be closer together and instead of being able to run the chain on the inner ring up to 8th of 10 cassette, I will probably only be able to go up to 7th before the chain catches on the outer ring. I have a 46T / 26T double.
On another site I read that the MTB sheaths can be milled down at the ends to fit 66 mm shells, instead of having to buy an additional road BB for its sheath.
FAQLoad - XT M760/761 cranks - Shrinking the Q-Factor