Mafac Racer centre bolts

[klunkrider]

Anyone replaced the centre bolts on their Mafac Racers with something a bit longer?

I think you have to remove some material from the head of a bolt if it's not an actual Mafac one but I can do this easily enough, just wonder if there's anything else to look out for?

I think the original bolts are somewhat of a press fit into the caliper so they don't turn as you tighten the nut? Might be hard to replicate but I could build up the diameter just behind the head with some silver solder or brass and I have access to a lathe so can turn it all down to size.

 

[pigman]

Not sure what the existing ones look like, but might coach bolts work? They have a square behind the head to stop the turning

https://www.toolstation.com/coach-bolt-nut/p74444

 

[Glug]

I've got some Mafac Racer here and the mounting bolt has a key that fits into a slot on the caliper.
Only works when the bolt is fairly tight which is why I remembered.

 

[bulgie]

I'm sure they changed some over the years but all the ones I've known, from the '50s through the '70s, had no key or anything else to keep them from rotating. They had a hex head that you could grab with a wrench, but in practice you never had to, they just wanted to stay put.

Rear:


Front:


Right under the hex head there's a short section that's 7 mm, then the rest of it is 6 mm. I bet you could get away with using a generic high-strength M6 bolt, but you'd have to thin the head down a lot lest it rub on the back of the moving arm. Since the generic bolt would be 6 mm where it passes through the etrier, which has a 7 mm hole, there'd be just that little amount of imprecision in where the etrier is relative to the crown hole. But you don't need anything like that kind of precision, so I predict it'll be 100% fine. Cut a thin strip of beer can and wrap it around a couple times if you want to get that last little bit of precision fit back, but I wouldn't bother.

Do use a high-strength bolt though, that's not a part that you want to break! It'll inevitably break under hard braking, the worst possible time. Metric bolt strength comes in "classes", and strong bolts are generally speaking those in class 10.9 or above. 12.9 would be better. A class 8.8 bolt might last forever, but the stronger bolt isn't that much more expensive, compared to burial plots.

Avoid generic stainless steel bolts from the hardware store for this application, they're quite weak. There are high-strength stainless bolts, but they're expensive specialty items, not for sale at the corner store. Strong bolts come with various platings, some of which are very good against rust — but even if it rusts, that's better than a weak stainless bolt that snaps.

 

[bulgie]

Not sure what the existing ones look like, but might coach bolts work? They have a square behind the head to stop the turning

https://www.toolstation.com/coach-bolt-nut/p74444

That type would work if you filed the hole in the etrier to match the square on the bolt. That's do-able if you're good with files. The etrier is soft aluminum, so filing the hole to square won't take long. Those bolts usually aren't very strong though. I'd want a heat-treated bolt there.

 

[Glug]

Looks like they changed the bolts over time.

Stamped June 1977 - you can see the key on the bolt:

 

[Woz]

As above.

Very early bolts have no key, chrome finish on the hex head. They are accessed via a cone spanner.

Later bolts are hex, or a domed head in chrome with a key. Even later no chrome finish on the dome head.

The screw thread is rolled and not cut.

I don't know, but it may be worth considering doing it all the other way round: