nouveau.ukiyo

I recently learned that I stripped the spindle threads in my left crank arm and need a replacement (model = Shimano Tiagra FC-4600/4603/4650). I'm having trouble finding this crank arm because the groupset is quite old. However, it's quite easy to get the newer FC-4700 parts. After doing some research, I believe a FC-4700 left crank arm should work as a replacement.


As bike mechanic novice, I have two questions: am I correct in my assessment? And if obtain an entire crankset, could I replace my current crankset? I have a FC-4603 triple and the FC-4703 is also a triple with the same chainrings (50/39/30T) and I believe both were designed to be used with the same bottom bracket. However, the 4703 is meant to be paired with a 10-speed cassette and I have an 8-speed. Does this make a difference? I'm asking because it's easier to get the 4703 crankset than just the left crank, so I might as well replace the whole thing.


Thanks!

Miele Man

10-speed chainrings will be closer together than 8-speed chainrings which means you'll probable get chain rub in a lot of gears and shifting between chainrings might be quite difficult to get the chain to mesh properly. Others can chime in and confirm, but iirc an 8-speed chain will work with 8-speed cogs.

Cheers

Crankycrank

+1 with what Miele Man mentioned but you can get chainring washers/shims to widen the space between chainrings. The 4700 crankset will work but try it first as is and get the shims if necessary. Keep your 8 speed chain to shift on the cassette properly. Sell off the good parts of the 4603 to recoup some of your losses.

dedhed

https://www.ebay.com/itm/shimano-tia...0AAOSwocFe4YhF

HillRider

You can use the FC-4703 crank with a 10-speed chain to avoid the chain rubbing and it will work fine with an 8-speed cassette. I've done just that successfully.

BTW, the FC-4603 is a 10-speed crank just like the FC-4703 so it you already are using a 4603, the 4703 will work just as well.

70sSanO

While a moot point to the crank arm question...

10 speed crank, 8 speed cassette, 9 speed chain.

it is funny but now that I think about it, I have never put calipers on a crank arm spider to see the actual spacing between chainrings between cranks designed for 7-10 speed cassettes. Then again chainrings from different companies are not manufactured identically either so c-t-c will vary as well.

People can rattle off the cassette spacing by speed but chainring spacing gets a deer in the headlights.

John

nouveau.ukiyo

Thanks, everyone. I might just try and pick up a FC-4700 or 4703 left crank (I think either will work?) and call it a day. My goal is ultimately to sell the bike.

Does this seem odd? I looked more closely at the bike and noticed the FC-4603 was designed for a 10-speed, but I have an 8-speed cassette. And the rear derailleur is a Shimano RD-4400, designed for a 9-speed. I do get chain rub along the front derailleur when in the largest chainring.

fietsbob

Just needing a new left arm? left crank does not have to match the right, it just has to work.

nouveau.ukiyo

Thanks. You think a Claris crank arm is compatible? They run in the US $20 - $30 range. From what I can tell, it'll work. I'm just worried it'll affect the resell value of the bike.

fietsbob

I'm just an old guy on a keyboard, my opinion on whether things I cannot see or touch in person fit are not valuable..

Worry on , Mate.. Can't do profitable resale assurances either... a in-person bike shop consultation out of the question , for you..?

nouveau.ukiyo

I'm just wondering about what the perception is of having different labels on each crank arm. Or if there is some kind of weight difference with different crank arms that'll be noticeable while riding, so the crank arms need to be a set.

icemilkcoffee 

Sounds like the previous owner cobbled it together with a mish mash of parts. The derailleur makes little difference but the cassette should be matched to the shifter, and ideally I would avoiding mixing components meant for different chain widths. 6,7,8 speed share the same chain. 9 speed uses its own chain. 10 speed uses its own chain. 11 speed it’s own chain.
What bike is this? Try to find out what the original configuration was.

nouveau.ukiyo

I think you're right. So here's the story of this bike: in 2004 I purchased a new Specialized Allez, base model with Claris parts, for $700. In 2012 a crack formed on one of the seat stays and I took it back to the purchase shop to have it replaced under warranty. They didn't want to do it and we fought aggressively. They eventually did it but for some reason they could only get a 2012 Specialized Allez Evo frame. I believe it was designed for Tiagra, so they cobbled together the least amount of parts possible to reduce their costs; the bike's Claris 8-speed shifters and the 8-speed cassette are from the original bike, I believe. At the time, a 2012 Specialized Allez Evo with Tiagra went for $1200 new.

HillRider

No, it's not odd. I mentioned above the FC-4603 is a 10-speed crank and can be used with an 8,9 or 10-speed cassette with no problem. Shimano road rear derailleurs from 7-speed up through 10-speed groups are generally compatible (pre-9-speed Dura Ace being the exception) with 7 through 10-speed cassettes. It is desirable to use the chain width that matches the crank. Your fd rub can probably be eliminated by judicious adjustment of the high limit screw and/or cable tension.

70sSanO

I think the chain rub mentioned above is due to a crankset with narrow chainring spacing and that could occur when on the inner ring and the chain is rubbing against the outer chaniring. On a compact crank setup this would be more likely than a typical 10 tooth difference.

If you are getting chain rub in the large chianring you are probably cross chaining by running big-big, or close to it, and the chain is rubbing against the front derailleur cage. You can help it some by using the trim on your brifters, if you have that feature. You might be able to adjust it somewhat but this is to be expected and it would only be worse with a wider 10 speed cassette.

Unfortunately your 4603 FD has the cages riveted together. When Shimano used a bolt to attached the front and rear derailleur cages together you could insert a thin washer to ever so slightly widen the back of the cage. Never a guarantee, and usually only beneficial with a small and middle chainring, it was a way to get just a skoosh more width for the chain. I would guess no improvement for your situation.

John