My cranks have started to wobble and it looks like the Sram GXP bottom bracket is shot.

I’ve got the non-drive side off okay, but when I go to remove the drive side, it just moved a couple of mm each way then froze solid. Further efforts just chew up the soft metal of the outside notches. They’re pretty chewed now actually.

Anyone had this problem before? Any suggestions as to how to proceed, especially in a time of staying at home?

I’ve sprayed all around it with GT85 and from the inside, just in case that miraculously solves the problem, but it looks like it’s stuck solid.

Non drive side is left handed thread. I’m sure you were undoing it the correct way, but as you didn’t mention it….

Check firstly you’re trying to turn it the correct way.

GT85 / WD40 probably won’t help – you need proper penetrating oil. Spray in around liberally and leave it for a few hours then have another go. Also make sure the tool you’re using is the correct one and a decent one at that.

I was turning it the opposite way to the arrow printed on the outer shell. That’s right isn’t it?

From memory yes, the arrow shows the tighten direction. I always remember it by unscrewing towards the back of the frame, (with the bike stood on its wheels)

Heat. Does not have to be a naked flame ( assuming its not a plastic frame) tie rag around it a few times. Pour boiling water over it. Or a hot air gun

From memory yes, the arrow shows the tighten direction. I always remember it by unscrewing towards the back of the frame, (with the bike stood on its wheels)

I can’t help thinking you undo by turning it towards the front of the bike ?

OMG those outer shells are made from soft metal!

I was turning the wrong way like a complete idiot, then when I tried to force it back the other way I’ve just rounded off the dimples and started to shred it. Blast.

Looks like it’s stuck until I can get it on a vice and turn the bike around it, which of course I don’t have.

I think it’s the other side where the bearing is wobbling, so if I get a replacement, presumably I can just replace that half and leave the stuck half in place? There’s no reason they both need to be changed at the same time is there?

You may be able to get it off with some large stilsons if you have messed up the grooves

Yeah, you can just replace half.

OMG those outer shells are made from soft metal!

I was turning the wrong way like a complete idiot,

They’re probably soft(ish) for a reason (at least part of it) – i.e. to stop you stripping the threads out of the BB shell if you over-tighten it / try and undo it the wrong way. See also the collar on Maxles..

Before you go replacing either cup make sure it is not the type of GXP bottom bracket that has the stupid **** tophat insert thing that sits in the bearing on the axle. I cannot remember which side it is (drive I think). It is supposed to stay in place by some kind of voodoo as there is a gap to the start of the spider.

These inserts wander out and then, unsurprisingly shear in half. This leaves the axle loose in the bearing and it wobbles. The bearings in gxp cups are actually pretty hard wearing IME, which is a shame, because the overall design is shite.

So my next stupid question is – how do I tell which BB I need to replace it. The only information on the manufacturer’s site (Marin) is external threaded. There’s no information on the existing BB.

I assume it’s English / British threaded rather than Italian on an American bike, but how do you measure it for replacement? Wiggle sell 73/68 and 83mm variants.

You’ll want a GXP threaded(BSA) bb, and most likely 68/73, 83mm is typically DH bikes only. You can measure the width of the frame where the bb goes in to check.

The 68/73/83mm refers to the length of the BB shell that the BB screws into – 83mm is generally used on DH bikes, most others will be either 68mm or 73mm (which usually use the same BB but with extra spacers for the shorter length).

If it is an apex chainset then you’ll have a 68mm shell.

(ah, it was the other GXP thread that mentioned apex)

That’s all very helpful, thanks. I did have to look up what stilsons are.

That cup has to come out sometime so better to do it before it gets even worse, a friend once had a mangled cup issue so he filed two flatish edges into the cup and used a stilson to remove.

I figured I could bodge a solution until a grown up can help me.

Massive pipe wrench fixed it – thanks for the tip!

Hopefully you greased/copper slipped the replacement before installation….?

I can recommend the Praxis Works replacement GXP BB. Far better made, no silly plastic top hat, and includes a wavy washer to keep the alloy end cap in place. Dead easy to re-grease.