Rear sprocket bolts Qingqi QM200GY-BA

[forchetto]

Following on from message nº10 on this thread concerning breakage of rear sprocket bolts by shearing and their possible replacement for stronger items, I've had a look at what's fitted to mine.

A simple observation of the heads of the bolts shows they're are marked "7ST". This could be something to do with their strengh. To compare, other bolts so marked include the pillion footrest hangers, engine holding bolts, engine guard bolts, etc.

Other screws for light duties like the chain guard are marked "4ST". This looks so far as if it's some sort of Chinese bolt tensile strengh code, but:
Bolts for serious duties like handlebar clamps, brake callipers, fork yokes, etc, ARE marked using the international ISO markings, and they clearly show "8.8", meaning a quite high tensile strengh grade. So, what does "7ST" mean?. Is it better than 8.8 or worse?.

This is a chart showing the US an ISO bolt strengh markings. No "7ST" there!



I removed one of the bolts after bending back the very strong security tab washer and it's a M8x30 mm with a 6 mm long unthreaded shank.
This 6 mm being the width of the sprocket.

One important note about fasteners. The threaded section of a fastener should never be given side loads or shear loads.
The threads will not be as strong as the shank (unthreaded portion), and can also chew away on the surrounding metal.
This should be taken into account on any critical aplication such as Disc brake rotors, drive coupling flanges, sprocket drives, etc,
so they MUST be replaced with a similar bolt with a 6 mm plain shank and a high tensile rating.

I'll visit my friendly screw peddlar tomorrow and see what he's got.

Here's a (bad) photo of the bolt (bloody useless Macro function on my Cannon IXUS 60...):



Have visited my screw supplier today (28th) and they don't have anything like that (I knew it!). Alternatively he can supply me with the nearest thing that can be modified and it's a DIN-931 M8x40 bolt in 12.9 grade (the strongest) and with an unthreaded shank 17 mm long.
This bolt can be first cut down to 30 mm and then a Die used to cut the thread another 10-11 mm along the plain shank until only 6 mm remains.
This is all very easy to talk about, but 12.9 steel is tough and after doing one was exhausted. (It's hot and humid here today). So I did the only thing a red-blooded Spaniard does when confronted with a difficulty ...gave up, drank some wine and had a siesta.

I did talk to a keen bicyclist in the shop and he confirms that the cause of the sprocket bolts breaking is letting the spokes come loose, it happens to bicycles all the time apparently. So I think that I'll just keep an eye on my spokes and see what happens.