Help - what's this doohickey called?

[MajorMantra]

I need to find a part but I've no idea what to search for and I couldn't find it by browsing the big sites. It's the little insert that goes in the gap in the bottom bracket end of a Shimano Hollowtech II XT crank on the non-drive side. I noticed when I replaced the BB on my new (old) MTB that it seems to be missing and I'm worried I'm going to damage the cranks without it.

Anyone know what it's called and who sells it? Or, failing that, does someone have one going spare from a dead chainset?

Cheers,
Matthew

 

[Gerry Attrick]

I take it that it is the little black shim which fits between the split end of the crank arm where it fits tot he bottom bracket spindle? I think it is designed to prevent the crank from parting company with the BB spindle in the event you don't tighten it sufficiently.

I doubt that by omitting it you will cause damage provided you tighten the crank arm to the recommended torque, but if you are a pedant then I'm sure an LBS could order you one.

To highlight the part, go inline to Shimano.com and obtain the exploded diagram for that crank. It will have a part number.

 

[Gerry Attrick]

Is it this bit labelled "4"?

http://techdocs.shimano.com/media/t.../FC/EV-FC-R600-2596_v1_m56577569830608905.pdf

 

[MajorMantra]

Yeah, that's the one. Silly me, I hadn't thought of looking for the diagram.

I know it's probably alright but it must place a bit of extra shear force on the bolts that shouldn't be there. With the shim the forces should be close to axial.

Cheers!

Matthew

 

[MajorMantra]

Hmmmm. It's listed as part no. Y-1FU 98120 but searching on that doesn't actually produce any results other than Shimano's own page. Any ideas?

Matthew

 

[Gerry Attrick]

It will make no difference to the tightening torques whether or not the shim is there. I suspect the part is there only because of today's obsession with avoiding legal action should the worst happen.

 

[MajorMantra]

It will make no difference to the tightening torques whether or not the shim is there. I suspect the part is there only because of today's obsession with avoiding legal action should the worst
happen.

Do you reckon it's there just to stop people over-tightening?

Matthew

 

[Gerry Attrick]

Do you reckon it's there just to stop people over-tightening?

Matthew

No. As I said, the tightening torque is unaffected by the presence or not of the shim. It is there to stop the crank arm coming off in the event the clamping bolts loosen or are not tight enaough.

 

[MajorMantra]

No. As I said, the tightening torque is unaffected by the presence or not of the shim. It is there to stop the crank arm coming off in the event the clamping bolts loosen or are not tight enaough.

Well, depending on the thickness of the shim I would have thought its absence could make it easier to over-tighten, but perhaps not.

Not trying to be dense, but I'm not entirely clear how it would prevent the crank coming off.

Matthew

 

[Gerry Attrick]

Look at the illustration. At one end there is a small protruberance which when the shim is in the correct position, engages with a radial groove cut in the BB spindle. The shim is locked in place when the clamping bolts are tightened. Thus the protruberance is locked into the radial groove and the crank arm cannot slide off the spindle. Geddit?

The tightening torque is independent of the shim in that if it is there or not, the effective resistance to turning the screw is the same. The torque wrench cannot tell whether or not the shim is there.

 

[MajorMantra]

Ah, got it. I hadn't realised it engaged with the axle. And you're quite right about the torque. I'm an engineering student who should know better, but my brain's been a bit scrambled with (ahem ) resits.

Matthew

 

[Gerry Attrick]

No problem. I was an engineering student way back in 1970! I know what you are going through. Stick with it. It'll be well worth it in the long run.

 

[MajorMantra]

No problem. I was an engineering student way back in 1970! I know what you are going through. Stick with it. It'll be well worth it in the long run.

Cheers for the encouragement. Bike maintenance is rather distracting...

Matthew