Hollowtech II play - is the preload cap too long?

[Fatal Swan]

Edit - now sorted, thanks for the contributions!

I'm in the process of replacing an Ultegra 6800 chainset with a Dura Ace 9000 on a road bike. I have plenty of experience with Hollowtech II cranks and BBs but I don't think I've come across this issue. There are no complicating compatibility issues - it's a road frame, running a pressfit road BB, so no spacer issues etc. The original Ultegra chainset was fine, I replaced it with another Ultegra chainset which was also fine.

The issue is that with the Dura Ace chainset, when I tighten up the preload bolt it stops turning any further before it actually presses tight against the left crank. So while I can push the crank snugly into position against the BB cups, the preload bolt refuses to tighten to secure it there further. I can't work out why since all the parts are present - I assume that the long plastic preload cap (the original one - the Ultegra and DA ones are the same) has used up all of the internal thread on the axle. The difference between the chainsets must be on the driveside, since when I put the left Ultegra crank on with the Dura Ace driveside, the same thing happens.

I've noticed that other aftermarket preload bolts have a shorter thread than the original one like in the photos below, so I could try that (new bolt or hacksaw??), but I'd like to know if anyone's come across this before?

 

[Mikey08]

When you slide the crank arm on all the way does the spindle protrude at all? If so you need to run a spacer or 2. Otherwise the shorter preload adjuster should do the trick.

 

[Fatal Swan]

Yes, the axle end still sits inside the crank. Funnily enough I remembered this morning that I must have had a couple of thin plastic 24mm spacers left over from when I needed to install Shimano MTB cranks onto a Wheels Mfg BB adapter a couple of years ago. After quarter of an hour hunting them down in the garage I've got them on and they've done the trick, so the preload cap loads the bearings before the thread runs out. Still can't quite figure out why two chainsets with the same spec should have resulted in the difference, but there you go!

 

[was8v]

Assuming road frame and road bb, does it measure 90mm across the faces of the cups? if not you need an axle spacer. Oressfit BBs use axle spacers not shell spacers or you are asking for creaks.

 

[mattr]

So now your indexing is all to shit...
And you have less engagement of the spline. I'd just change the preload cap from one of the shorter ones you already have. Because spline engagement and indexing are both more important than making full using of a too long preload cap...

 

[JoeH]

This is getting proper techy.

Any chance of a photo or two of your frame and chainset to bring it all to life?

 

[Fatal Swan]

Thanks everyone - yes mattr, what you said makes sense. We're talking about 1mm difference on the driveside spacing (the 2 spacers are each 1mm and I put one on either side), so it's true that that would affect the existing indexing slightly but I'll be setting up the gears in any case with new cables etc as part of the switch. But - you make a good point that what I have is a workaround, and since I've now I've diagnosed the issue through using the spacers I agree that a new preload bolt for a few quid is the right way to go before I set up the front mech.

JoeH, the reason I hadn't taken a photo was that there wasn't much to actually see that would help the problem! What you would see on the photo is a left crank fitted as normal onto the axle, only the preload cap won't fit quite as far into the crank as it should! I will try and take some photos for reference though now I'll be disassembling again, and fitting a shorter bolt.