Boardman COmp 650B bottom bracket replacement

[Mickeyluv]

I've just replaced the bottom bracket on my Wife's 2017 Boardman Comp 650B and I thought a description may be helpful to others. The first job is to clean the bike and remove the chain from the chain wheel. This bike is fitted with a Suntour XCM HTII crank with a 73mm BB width and the first difficulty encountered was removing the LH crank, which is a self-extract type. The theory is to slightly loosen the lock ring with a peg spanner (it's left-hand thread and a decent circlip plier will slacken it) and use an 8mm allen key to unscrew the centre retaining bolt to draw off the crank. I quickly realized that the lock ring thread would possibly strip due to the extreme force required to remove the seized crank from its taper. A full kettle of boiling water was run slowly over the area to expand it and the crank then slowly came off without excessive force.

Take a note of the preload arrangement - there's a wavy washer, then a serrated lock plate (serrations face the BB). The serrations engage with a threaded preload adjuster, which has serrations that face away from the BB. Remove these items and withdraw the axle from the drive side.

After some research I decided that a Shimano XT BB-MT800 would be a good replacement. Removing the old BB took some difficulty. I had already got an Icetoolz HTII tool but couldn't apply enough pressure to remove the bearing carriers. As the old ones were scrap anyhow, I used a large stillson and it still needed a fair bit of effort even with penetrating fluid. Water had got inside the BB past the seals and corroded the threads because the centre plastic tube was too short to engage the O rings at both ends. The drive side bearing had completely rusted up and seized. This bike has never been pressure washed and always carefully looked after.

I cleaned up the surface of the axle with 500 wet & dry. Fortunately no serious damage had been done. The BB threads in the frame cleaned up with steel wool.

The new BB came with a plastic adapter tool. The BB-MT800 has a smaller diameter than the old HTII - the same as a SM-BBR60 (41mm/16 spline) but the adapter did not fit my tool so I machined it down in the lathe. In retrospect I would have bought another tool, so that's one to get to add to my growing collection. I measured up the shoulder of the old unbranded bearing carriers and they were very close (within 0.03mm) of the new ones. The drive side originally has a 2.5mm plastic spacer and looking at the Shimano dealer instruction sheet it shows this is correct for a 73mm BB. The new assembly comes with three metal spacers so I used one of these.

Reassembly is straightforward. I lubricated the axle before fitting and everything went together smoothly. The threaded preload adjuster only needs to be finger tight, then orient the serrated locking plate to give the best engagement. Fit the wavy washer, then the crank and tighten the 8mm centre bolt and finally tighten the lock ring. After reassembly I didn't even need to reindex and the whole thing is much smoother than its ever been.

[531colin]

...........The theory is to slightly loosen the lock ring with a peg spanner .........

Why do you do this please?
Park tools say "Check the retaining ring is secured into the crank arm..." https://www.parktool.com/blog/repair-he ... extracting

[Mickeyluv]

If the retainer is tight against the extractor bolt head there's additional friction to overcome between the contact faces of the extractor and crank, and the extractor and retainer, especially if there's some corrosion. You have the bolt tight against the crank, and the retainer tight against the bolt.

Ensuring that the ring is just slackened off (not removed or unthreaded) means that the bolt is free to rotate. I slackened off the ring just enough to remove the pressure on the extractor bolt. It's probably worthwhile slackening the ring a turn or so and testing to see if the extractor start to unscrew to make sure it isn't rusted in. If it's free, then screw it back in and rotate the retainer so it just barely touches the extractor bolt.

The basic requirement is to make sure that the effort needed to remove the crank is not being masked by other friction.

[531colin]

If the retainer is tight against the extractor bolt head ......

But it isn't.
As you say, you don't want to strip the retainer threads in the crank; so leave the retainer fully screwed in the crank.

[gregoryoftours]

You could unscrew the retainer ring a little to get some lube in between it and the crank bolt but then do it up again. It's the underside of the retainer ring that the bolt is bearing against to pull the crank off anyway. As such they are going to be hard in contact while turning even if they don't start off that way, so it's best that the ring is fully engaged and secure in the threads.