I bought a rear pannier rack from Halfords, assuming since they only sell one version that the mounting hardware must be universal, but the 4mm bolts are clearly too large for my bike. I don't want to buy new bolts only to find that I've got the width correct but the wrong thread or something – is it likely that what I'm looking for is another "standard" bolt or do I need to do some trial and error to figure out exactly what I need for this frame?
(yes, I'm replacing the chain and cassette too, don't worry…although maybe expect another post here when I get stuck with those 😂)
by ThatMathsyBardguy
2 Comments
They are normally a standard size. I’m guessing that if they have never been used they are full of paint from when the bike was built. Your best bet is to run a tap through to clean up the threads.
Edit – you don’t need anything expensive as you are not cutting threads from scratch, you will get a basic set off Amazon for about a fiver
Most racks come with pure shit mounting hardware – too long or too short bolts that rust really easily. Buy stainless bolts and a dollar store metal file to fine-tune any rack installation. You’ll probably have to bend the rack’s mounting legs and mounting brackets to get them in proper position too – all bike racks are a squeezy-bendy metal file affair.
Those mount holes are almost always M5, and if an M4 doesn’t fit, I’m thinking it is full of paint/clear coat. If it is actually M3, that is way way too small for a rack. it needs a bigger bolt. M3s are not good for anything but grip clamps on a bike – you need M5 bolts.
I know that bikes are different, and some are probably actually M4 (or maybe M3!) but M4 is insufficient for a rack dropout mount. A bump or a knock, and the bolt will snap like a twig.
Buy a 4.2mm drill bit and a cheap M5 tap, and drill out the two needed holes and tap the hole with some wd40 or 5-56 spray. this will take, at most, 5 minutes. Before drilling, try chasing the threads with the M5 tap – it might just be full of paint.
Tapping them will let you use standard M5 bolts that will break much less easily. Blue loctite them into position and file off the protruding end on the cassette side. Hardware stores sell the bolts in 5mm increments, so you can get it really close. The goal is to have the bolt protruding just slightly, because engaging ALL The threads is necessary to prevent the bolt from tearing out the aluminum threads over time. Ask me how I know!
For racks, I often use a greased M5 standard external hex bolt on dropout mount points (one you can turn with a generic wrench or pliers), installed backwards, from the inside of the dropout, poking through the frame outwards. Mount the rack on the two protruding studs. Put a washer and a nylock nut on the outside and tighten it up, and then file off the end of the bolt sticking out, and probably the head of the bolt near the cassette down to about 1mm. It’s threaded, so the bolt head can be quite thin – you just need to be able to barely get a spanner on it. The bolt head inside will prevent tearout of the aluminum frame by overtightening, allow you to securely mount the rack, and easily remove the bolt if it snaps in an accident – the 1mm protruding head can let you turn the greased bolt out of the frame easily.
Whatever you do, figure out how to use M5 bolts if at all possible.
Good luck with the bike!