Help With Crank Arm/Pin Problem...

Quoted message said:

The left crank has extra play in it, so when it reaches
the tip of its arc, it loses friction and drops a bit for
the length of its play. This leads to a jerky motion on
each revolution on the left side. I looked at it, and the
bolt that connects the crank arm to the frame is the
culprit, however I have no idea how to fix it. There's one
component that almost looks like a very large hex bolt,
but it's thin, and there's another one that looks like a
rough ring.

The crank is pressed onto a square taper. If the crank gets
loose, it will be destroyed in short order. That sounds like
your problem. To remove the crank, you need a crank puller
tool. First remove the crank bolt and the washer underneath
it. Then thread the crank puller into the crank.

See: draco.acs.uci.edu8f.11.htmlOpen ↗

and

parktool.comCWP 6.shtmlOpen ↗

Art Harris

xmp333 writes anonymously:

Quoted message said:

I recently purchased a road bike from a flea market. This
is a Puch Prima frame, but with assorted components --
basically a FrankenBike. I put some work into it --
removed broken fenders, trued wheels, etc..., but it's got
a problem that I don't know how to solve. I couldn't find
any info in my bike maintenance book either.

Quoted message said:

The left crank has extra play in it, so when it reaches
the tip of its arc, it loses friction and drops a bit for
the length of its play. This leads to a jerky motion on
each revolution on the left side. I looked at it, and the
bolt that connects the crank arm to the frame is the
culprit, however I have no idea how to fix it. There's one
component that almost looks like a very large hex bolt,
but it's thin, and there's another one that looks like a
rough ring.

Quoted message said:

I may have a bike store look at it, but since the bike
only cost me $20, I'm not keen on spending too much on
fixing it.

Quoted message said:

Any advice would be greatly appreciated.

From your title, it seems you have steel cranks with
cotters, round tapered pins that engage flats on the crank
spindle. These cannot be tightened by the retaining nut on
the cotter, and the cotter cannot be secured anymore because
it has a dimple in it that will also make their removal
difficult.

Cotters are notoriously too soft for the job and in olden
times when high quality racing bicycles still used steel
cranks, only a few brands of cotters would stand up to hard
pedaling. Of these "Acier Dupratnik" was the top of the
line. Remember that torque from the left crank goes through
both left and right cotters and that they are both shot. I
don't know whether you can find reasonable replacements but
the whole bicycle is probably not worth the bother if it is
so equipped. New cranks and BB will cost more than the
bicycle is worth.

New cotters are usually not ready to install but need to be
filed to fit properly. Since this is a hand operation,
cranks were often not exactly 180 degrees apart. The flat on
the cotter should be at as low an angle as practical. I
always cut them on a milling machine to fit my Cinelli
(Magistroni) Cranks and a Campagnolo spindle.

Jobst Brandt [email hidden]

may have said:

Hi All,

I recently purchased a road bike from a flea market. This
is a Puch Prima frame, but with assorted components --
basically a FrankenBike.

The left crank has extra play in it, so when it reaches the
tip of its arc, it loses friction and drops a bit for the
length of its play. This leads to a jerky motion on each
revolution on the left side. I looked at it, and the bolt
that connects the crank arm to the frame is the culprit,
however I have no idea how to fix it. There's one component
that almost looks like a very large hex bolt, but it's
thin, and there's another one that looks like a rough ring.

If the hex bolt goes through the crank at a point
immediately next to the shaft on which the crank is mounted,
you have what is called "cottered cranks". In that case, the
cotter is most likely worn out. It will probably be
difficult to remove, and obtaining a useful replacement may
be a challenge. You can buy a pair of cotters (it may be
that the *right* cotter is actually the one that's loose)
and replace them in the hope that the problem will be
alleviated enough to make the bike pleasantly operable, but
be aware that these cotters wer supplied in a number of
sizes, and you'll need to match what's there now. If there
are no cotters, the crank's probably been loose for long
enough that the square taper hole is wallowed; if that's the
case, it's almost certainly junk.

Quoted message said:

I may have a bike store look at it, but since the bike
only cost me $20, I'm not keen on spending too much on
fixing it.

Personally, I would look at the possibility of acquiring a
more modern set of used but serviceable cranks (cotterless)
and a matching bottom bracket. (That's what the shaft-and-
bearing assembly is called, or just BB in many cases.)

Otherwise, the bike's probably little more than a potential
parts donor.

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