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Mafac "Racer" off-centre-pull

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Mafac "Racer" off-centre-pull

Old 05-05-22, 02:35 PM
  #51  
markk900
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If the end result is a new set of calipers anyway (which indeed it might well be, and frankly Mafac stuff is still cheap where I live), I would not focus on thinning the plastic bushing but probably doing a really good polish on the bolt that holds the arm and provides the bearing surface. A nice polish with a very fine wet/dry paper (say 600 then up to 1500) might be a cheap and achievable way to get the clearance you need. And if it doesn't work no big deal.
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Old 05-05-22, 03:02 PM
  #52  
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Originally Posted by markk900 View Post
If the end result is a new set of calipers anyway (which indeed it might well be, and frankly Mafac stuff is still cheap where I live), I would not focus on thinning the plastic bushing but probably doing a really good polish on the bolt that holds the arm and provides the bearing surface. A nice polish with a very fine wet/dry paper (say 600 then up to 1500) might be a cheap and achievable way to get the clearance you need. And if it doesn't work no big deal.
I would reconsider the terminology-
the arms are retained by a bolt, chrome or cadmium plated, there is a pair of flanged plastic washers, and a tubular core- that core takes the compression load onto the machined pivot threaded stud, internally threaded like a nut. If this assembly is tight, tubular core won't slide on, not long enough to allow free arm rotation, then you are indeed correct that messing around successfully with the plastic bits is not a good idea.

I easily accept that one arm is balky, there are kits to address problems, usually when the arm is loose. I also agree that a take off caliper that functionally is in better shape to start is most likely the least expensive and fastest solution.
without the parts in hand precise guidance will not be.
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Old 05-05-22, 03:05 PM
  #53  
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Originally Posted by microcord View Post
OP speaking. Quick version: What should I bear in mind when attempting to ream out a slightly greater internal diameter of a plastic bushing?

Blow-by-blow version: Wiggle-wiggle-wiggle worked: the right/outer arm (the troublesome one) of my Mafac "Racer" came off; and then the left/inner arm (easily) came off too. Nothing looked odd about the right boss. The red plastic bushing that goes on it looked as if it was very slightly dirty, or perhaps just stained: I cleaned it up but this didn't bring any improvement in pivoting. I guessed that either the boss was too big or the internal diameter of the bushing was too small. So I tried each arm, back to front, on the "wrong" boss. (Of course the brake shoe pointed outwards, but all I wanted to check was the ease of pivoting.) The left arm pivoted easily on the right boss; the right arm didn't pivot at all easily on the left boss. I infer that the right boss is OK but the bushing of the right arm is too tight. So I'd like to increase its internal diameter very slightly. My limited understanding of Wikipedia articles on related matters suggests that I should use a burr mounted on a milling machine, but I'm just a regular ignoramus with a cheap set of hand files. (I don't even have a vise.) I could just go ahead and try; but I wonder if there's any advice I might benefit from.
Is the boss clean and smooth? I haven't heard of anyone milling a brake boss. Not to say that the madmen and madwoman here haven't done it, but using a mill is overkill. Twist an abrasive pad (We call them Scotch pads. They are a brand made by 3M. I don't know if they are available in Japan, but I am sure you have a similar product) or even fine sandpaper around the boss to buff it. After polishing for a little bit, take a break, clean the boss, grease it, and test fit the brake arm to see if you get the same tension as a cleaned and greased right side boss. You should be able to get them both to have an equally smooth action. It's common for bosses to get just enough corrosion, old grease or be slightly off from the factory and need a little work to clean up. I would stay away from files as they can't conform to the round boss and you risk getting the boss out of round.
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Old 05-05-22, 03:27 PM
  #54  
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Originally Posted by repechage View Post
I would reconsider the terminology-
the arms are retained by a bolt, chrome or cadmium plated, there is a pair of flanged plastic washers, and a tubular core- that core takes the compression load onto the machined pivot threaded stud, internally threaded like a nut. If this assembly is tight, tubular core won't slide on, not long enough to allow free arm rotation, then you are indeed correct that messing around successfully with the plastic bits is not a good idea.

I easily accept that one arm is balky, there are kits to address problems, usually when the arm is loose. I also agree that a take off caliper that functionally is in better shape to start is most likely the least expensive and fastest solution.
without the parts in hand precise guidance will not be.
You are totally correct about terminology, and I apologize if I made things less clear (don't respond on a phone - i will remember!)..... I meant polishing the bosses on the back plate of the caliper - see picture.....


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Old 05-05-22, 04:44 PM
  #55  
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Many thanks to you all for the comments you made (while I was enjoying a deep sleep after a demanding ride on a much newer bike).

Originally Posted by ehcoplex View Post
Those plastic bushings are so thin I doubt you'd have any luck trying to make it any thinner... It also seems unlikely that's the problem. Were these brakes ever working properly? If they were, than it seems it ought to just be a matter of cleaning things up and lightly greasing the pivot points. Have you pushed the bushings out of the arms?
Point taken about any attempt to thin a bushing. (And it occurs to me that even if such an attempt didn't destroy it, any inaccuracy would introduce wobble. The pivoting should only be in a plane.)

Even before you asked me, I'd been asking myself about whether the brake had been working properly. Surely it had done so. But I can't see any sign of degradation, so maybe it hadn't: maybe I was so delighted with my (otherwise lovely) newly assembled old bike that I was somehow blind to a defective front brake. I'll never know for sure.

I haven't attempted to push the bushings out of the arms.

Originally Posted by repechage View Post
cheapest solution- different caliper as you don't have the tools.
Originally Posted by markk900 View Post
If the end result is a new set of calipers anyway (which indeed it might well be, and frankly Mafac stuff is still cheap where I live) [...]
Opportunities to buy old parts are very limited here. (Not so surprising when you consider the heat and humidity, plus the tight limits on most people's capacity for storing old stuff.) But there's a rare (twice a year) "flea market" for old bike bits the day after tomorrow. (Elegant and somewhat pricey fleas....) I'll certainly go along, and I'll willingly shell out the going rate for a Mafac "Racer" in good nick. (I could even splurge on a "Competition".)

Originally Posted by markk900 View Post
I would not focus on thinning the plastic bushing but probably doing a really good polish on the bolt that holds the arm and provides the bearing surface. A nice polish with a very fine wet/dry paper (say 600 then up to 1500) might be a cheap and achievable way to get the clearance you need. And if it doesn't work no big deal.
Yes, this sounds very sensible. Use of emery paper is just another area within my fast wasteland of ignorance, but --

Originally Posted by tricky View Post
Twist an abrasive pad (We call them Scotch pads. They are a brand made by 3M. I don't know if they are available in Japan, but I am sure you have a similar product) or even fine sandpaper around the boss to buff it. [...]
Yes, yes, we have those. I'm very much accustomed to using them, too. (Of course, I'd use a fresh one for this job.)
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Old 05-05-22, 05:04 PM
  #56  
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Originally Posted by markk900 View Post
You are totally correct about terminology, and I apologize if I made things less clear (don't respond on a phone - i will remember!)..... I meant polishing the bosses on the back plate of the caliper - see picture.....


Yeah, what Mark said!
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Old 05-05-22, 06:19 PM
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0000 steel wool is also good for 'polishing' the posts..
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Old 05-06-22, 01:03 AM
  #58  
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Done!

I squeezed the scotchbrite (or scotchbrite knock-off) side of a sponge around the boss, hard, and swivelled. Again and again. And again (etc). The boss (which had looked pretty good to start with) became distinctly shinier, but there was no noticeable improvement to function. But I realized that too much of my forefinger-and-thumb force had been wasted on compressing the sponge, so I went to an ironmonger's in search of steel wool. None there, but the man sent me on to a drugstore, which had it and also had plain scotchbrite (sans sponge). I bought both. A few dozen twirls of the scotchbrite, pressing it hard, did nothing. I resolved to give it a hundred twirls. Still poor. And another hundred. Perhaps slightly better. And another hundred. Fixed!

An hour later, my right thumb still feels very tired. Fettling is a cruel mistress.

Again, many thanks to you all for your suggestions and comments.
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Old 05-06-22, 10:29 AM
  #59  
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Originally Posted by microcord View Post
Done!

I squeezed the scotchbrite (or scotchbrite knock-off) side of a sponge around the boss, hard, and swivelled. Again and again. And again (etc). The boss (which had looked pretty good to start with) became distinctly shinier, but there was no noticeable improvement to function. But I realized that too much of my forefinger-and-thumb force had been wasted on compressing the sponge, so I went to an ironmonger's in search of steel wool. None there, but the man sent me on to a drugstore, which had it and also had plain scotchbrite (sans sponge). I bought both. A few dozen twirls of the scotchbrite, pressing it hard, did nothing. I resolved to give it a hundred twirls. Still poor. And another hundred. Perhaps slightly better. And another hundred. Fixed!

An hour later, my right thumb still feels very tired. Fettling is a cruel mistress.

Again, many thanks to you all for your suggestions and comments.
Chapeau! Nice job sticking to it. That's honestly more I've ever had to attack a brake boss, but I can see this happening. Some brake bosses end up having surprisingly different sizes.

Let us know how it works when you get it back together! Oh, and post pics of the bike in question!
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Old 05-07-22, 02:51 AM
  #60  
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Originally Posted by tricky View Post
Let us know how it works when you get it back together! Oh, and post pics of the bike in question!
I took it for a little (27 km) ride this afternoon. (And a slow one, as I quickly discovered that I was wearing too much.)

Smooth! The front brake works symmetrically. (I don't say "flawlessly" -- that's a matter for a new thread.) I'm happy. Pics? Can do:



Yesterday, after only a few dozen rubs with scotchbrite

Today, on my little ride

And again

Obligatory tourist photo, very close to Haneda airport
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Old 05-09-22, 04:47 PM
  #61  
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Congrats, @microcord ! I'm happy the brake is working now. The bike looks great, thank you for the pics. What is the make and model? I also like that frame pump.

Btw, I love the tourist photo of with the traditional gate. It gives a nice sense of place.
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Old 05-09-22, 06:33 PM
  #62  
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Originally Posted by tricky View Post
The bike looks great, thank you for the pics. What is the make and model? I also like that frame pump.

Btw, I love the tourist photo of with the traditional gate. It gives a nice sense of place.
It's a hodge-podge of parts expertly assembled (i.e. by a pro, not me) just four months ago. The frame is branded Ken Ryall (the suburban London retailer), and also, "Legere" (no, not "Légère"), I suppose because it has Nervex Légère lugs. It's from 1961 and is said to have been built by Stan Pike (in Somerset). OTOH it's also said to take a 27.2 mm seatpost; but it most definitely does not, instead happily and snugly taking a 26.8 mm seatpost.

Frame aside, the only British bits are the GB hiduminium stem and the Brooks saddle. The pump is branded Lapize.



In a convenience store parking lot during the return leg of a "century" ride, 12 March
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