justonwo

So with 9200 power cranks being unobtanium and with the accuracy problems reported for the 9200 power meters, I decided to swap out my 9100 power cranks for an SRM PM9 setup. I’ve been running 9200 with my 9100 power cranks for about a month.

The SRM was ordered about a month ago with a Titanium spindle. I managed to find 9200 chainrings to pair with the SRM. All the parts were finally at my house yesterday and I started the swap. Two problems.

1) I was lazy with selecting the 3 mm allen driver for the NDS pinch bolt. I used my Park socket. I guess it’s not very precise because that pinch bolt stripped way before I got to the specified 5 Nm torque. Argh. I guess I’ll be messing with extractors or even drilling out the bolt if that doesn’t work. This means I may have to also replace the barrel nuts. My own fault I suppose, but I would certainly have opted for Torx for such a small screw on such a ridiculously expensive set of cranks. Such a small screw with such a high torque rating seems like an accident waiting to happen.

2) I noticed the large chainring had a wobble after I got the crankset installed. Big Argh! I finally figured out that the chain catcher pin between the large changing and the crank arms is so long that it pushes against the Look crank arm. I’d guess it’s 50% longer than the 9100 pin. So it seems like these cranks aren’t compatible with 9200 as advertised (unless you grind down the pin, of course).

Very frustrating. When I finally get the cranks off, I’ll have to grind down the pin to get it to work.

Anyone else have these kinds of issues? Any tips or wisdom?

FBinNY 

The key to good mechanical work is be to think faster vs than you work.

You stripped the pinch bolt either because you misread the torque spec or let a printed spec overrule your common sense and instincts.

Typical torque specs for 3mm cap screws are nearer to 2Nm than 5. That's for high strength alloy bolts, so I suspect that the correct spec for the pinch bolt was actually .5Nm, which should have been written as 0.5Nm to avoid confusion.

In any case, once you started thinking "gee, that's awfully high", the internal brakes should have been applied while you triple checked.

A for the pin. Odds are it felt wrong as you were assembling it, and if you allowed yourself a moment then, you'd have saved time in the long run.

So, take the time to think so you don't end up wasting more time doing rework.

justonwo

No, it is without a doubt 5 N-m. I verified with SRM. The trouble was a rounded out Park Allen driver that should be thrown out. It certainly is a lot of torque for a 4 mm screw (3 mm Allen), but it is not unreasonable.

I didn’t notice or anticipate the longer pin. And after discussing the issue with other people that have SRM cranks, neither did anyone else. Furthermore, SRM, the manufacturer, didn’t even realize their cranks have problems with 9200 chainrings. They know now.

justonwo

SRM ordered a replacement 9200 ring and ground down the pin to a length that is compatible with their cranks. Kudos to them. I was able to be the bolt out of the NDS cranks arm with the help of my LBS.

Once I receive the new chainring I’ll install and verify not contact with the crank arm. Then use my very precise Wera hex driver to install the NDS crank arm. The replacement bolt they sent installs just fine at 5 Nm if I use the Wera driver.