Cycling UK Forum

Fool that I am, I’d fondly imagined when I bought a Shimano 4-bolt 104BCD middle chainring, that it would fit on a Shimano 4-bolt 104BCD spider. But no: the span across the locating flanges on my spider was 90mm, and the gap on the ring was only 88mm. The kind of thing that’s easily fixed in five minutes if you’ve got an angle grinder, and the confidence to use it; the kind of thing that could drive you to suicide if you haven’t!

similar problem with some 64mm bcd shimano inner rings - they foul on external bearing (ht2) bb's.

Do you know the model number of the chainset?

I didn't know about what you've described, but I did buy some replacement rings for my Deore M510 chainset in the summer. I bought them from Rose (vg value). I took a considered risk of buying the later M590 rings reasoning that they really should be the same. When I received them I was surprised that they were coded as M510 rings!

However, since you've brought this up, I've just been out to the shed to measure between the flanges you describe. My old 510 spider measures 86mm from one flange to another (not 100% accurate, due to the crank being in the way. Definitely not 90mm though). The new 510 coded outer ring measures 88mm, like yours.

I don't understand

iow wrote:I don't understand

Its all about planned obsolesence. It ensures that when you need a part you have to buy the complete kit.

AB702 wrote:

iow wrote:I don't understand

Its all about planned obsolesence. It ensures that when you need a part you have to buy the complete kit.

It was actually me that said that

I'm aware of planned obsolescence, but I don't understand why I would buy a M590 ring, which was coded as M510 (correct for my M510 spider) and yet the dimensions were different. Thankfully I can fit mine.

I'd still like to know which model chainset and replacement rings the OP had.

It's also worth adding that I once had a M510C chainset, which had a wider Q factor and required a longer bottom bracket. And was made in Italy What's that all about?. Regular Deore were coded M510A.

Model numbers – crikey, I haven’t a clue. I’ll have a look in the morning and see what I can find.
And, I find myself wondering, for future reference, how would it be in the opposite situation? If the span of the flanges on the spider wa 88mm, and the gap in the chainring was 90mm, obviously it could be fitted, but with a 1mm gap. Could the flanges be intended to take some radial load, which would now bear on the bolts? I’m not an engineer, but I imagine that most of the forces involved are to do with torque being transferred from the crank to the chainring, through the bolts, and if they’re strong enough to carry that, any radial force should be insignificant. QED. Am I wrong?

I suppose what you describe would contribute something to the integrity of the chainset, but I don't feel unduly phased by having some small gaps. In normal chainset assembly, I don't think the rings are fully butted up to these flanges; I think it's more a casual fit. The bolts are doing most of the work in keeping it all together IMO.

A file will work if you have no angle grinder. And there's no need for radial contact between the flanges and the ring.

Right, the crank calls itself FC-M540. The 36 tooth ring I took off says ‘Shimano DD M9 T-26’. The 32 tooth ring I put on says ‘Shimano SG-X M9 8-32’.
Does that perhaps suggest that I’ve removed a 9-speed ring and fitted an 8-speed one? (Though why would they be different?). Would that be a reason – or at least a plausible excuse – for making them not interchangeable?

I don't think that ring is for 8 speed, so no issue there. The 8 you quoted is an S.
As somebody above said, a little grinding should fix it.

Perhaps it might be better to file the end of the flanges rather than the ring itself. Just a hunch.

Forgot to add that your chainset is a Deore model. I think the chain ring you've bought is also Deore level, possibly LX. You would think it compatible, wouldn't you?

Yes, I thought it was a Deore, though I can't find anywhere that it says so. I ground the chainring off and fitted it before my OP.

nirakaro wrote:Yes, I thought it was a Deore, though I can't find anywhere that it says so. I ground the chainring off and fitted it before my OP.

Of interest on another thread, what was it made of?

Steel