# Shimano RX-100 crank replacement



## pw1972 (Jun 10, 2008)

I have a 1994 Shimano RX-100 crank, 7 speed, 42/53 that I want to replace with a 39/53 if possible. What are my options? Do I need to find a used crank or are there any new ones on the market that will work with the rest of the drivetrain?


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## pmf (Feb 23, 2004)

Why don't you just buy a new set of chain rings. Actually, all you need is to replace the 42 with a 39 and you'll have a 39/53 (sure it wasn't a 52 tooth ring?). Any chain ring will work fine. I don't think I'd run out and buy anything real expensive since this was the group below 105 at the time.


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## pw1972 (Jun 10, 2008)

I can give that a try. My wrenching abilities are pretty limited so I wasn't even aware I had that option, thanks. 

The specs on it say 42/53, but could be wrong. I'm trying to keep the cost relatively cheap your option sounds like a good one.


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## pmf (Feb 23, 2004)

If there's nothing wrong with the crank and all you want to do is get a smaller chainring (whether the big one is 52 or 53 makes hardly any difference) then just replace the 42 with a 39. The make 38 tooth rings too. I used to swap 39 and 42 all the time back when I rode a lot (before kids). I used a 38 tooth ring on bike tours with a lot of climbing. This was before compact gearing was around. 

All you need to do is unscrew the chainring bolts. They come apart into two pieces. You'll understand how they work when you take them apart. The chainring should have a mark on it -- on Shimano, it was a little triangle. Make sure the section with that mark goes under your crank arm. The small ring goes on the inside, the big one on the outside. Just pay attention when you take it apart and you'll be good to go. 

Here's a 38
http://www.excelsports.com/new.asp?...cription=Chainring+38T+Inner&vendorCode=SALSA

Here's a 39
http://www.excelsports.com/new.asp?...cription=Chainring+39T+Inner&vendorCode=SALSA


If you want the replace the entire crank set, you're probably going to have to get a new bottom bracket as well because RX-100 is a pretty old group, and as you know, bike components constantly get "improved".


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## Kerry Irons (Feb 25, 2002)

*Bcd?*



pmf said:


> Why don't you just buy a new set of chain rings. Actually, all you need is to replace the 42 with a 39 and you'll have a 39/53 (sure it wasn't a 52 tooth ring?). Any chain ring will work fine. I don't think I'd run out and buy anything real expensive since this was the group below 105 at the time.


Most cranks that came with a 42 small ring will not fit a 39. They had a 144mm bolt circle diameter vs. the current standard 135 or 130. You can pretty easily tell by looking at the small chain ring: if the chain ring bolts are just about touching the chain with the 42, that's all the smaller that will fit.


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## FatTireFred (Jan 31, 2005)

Kerry Irons said:


> Most cranks that came with a 42 small ring will not fit a 39. They had a 144mm bolt circle diameter vs. the current standard 135 or 130. You can pretty easily tell by looking at the small chain ring: if the chain ring bolts are just about touching the chain with the 42, that's all the smaller that will fit.




I seriously doubt that a 1994 shimano crank was using 144bcd. op just needs a new chainring for whatever bcd he has, prob 130. might need 2 tho, as modern CRs are often phased- or whatever they call it- to their partner ring, i.e. made to go w/ a 53a or 53b depending on whether the inner is a 39 or 42


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## Dave Hickey (Jan 27, 2002)

My RX-100 cranks are 130 bcd. My vote is replace the rings


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## drdiaboloco (Apr 10, 2010)

pmf said:


> I don't think I'd run out and buy anything real expensive since this was the group below 105 at the time.


The only difference between RX-100 and 105 of the time is the finish on the crankarms. The RX-100 was clearcoated and the 105 was painted silver. I have both and they look the same, measure the same, and weigh the same (within 5 grams). The rings are identical in every way, down to the engravings and "Shimano HG" sticker on the big ring.

Actually there is a very small difference between the two in that, when installed, the chainline is off by 1mm, with the 105 being further away from the seat tube than the RX-100.

The clearcoat of the RX-100 doesn't last, I'll wager the OP has a very nasty-looking crank on his hands...


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## pmf (Feb 23, 2004)

Kerry Irons said:


> Most cranks that came with a 42 small ring will not fit a 39. They had a 144mm bolt circle diameter vs. the current standard 135 or 130. You can pretty easily tell by looking at the small chain ring: if the chain ring bolts are just about touching the chain with the 42, that's all the smaller that will fit.


I used to swap 39's and 42's all the time. Its a standard 130 mm diameter ring. 

I had a Cannondale 3.0 with RX-100. As someone saiid, it was the same as 105, but with a different finish. Yeah, his cranks might not look pretty, but they probably still work fine. Man, that was one rough riding bike.


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## ohvrolla (Aug 2, 2009)

I had a 53/42 and wanted to go to a 52/39 and it was cheaper to buy a Octalink Shimano 105 crankset than to purchase chainrings. Just robbed the chainrings off of it and put the arms up for safe keeping.


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## gr8blues (Nov 20, 2009)

I put a 38 on my 80's 600 crank, no problem. This is about the easiest bike job you can do, it will cost about $18 and all you need is an allen wrench. Do this first then see how you like it any other option will cost a lot more.


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## drdiaboloco (Apr 10, 2010)

pmf said:


> Yeah, his cranks might not look pretty, but they probably still work fine.


Oh of course they WORK fine, it was just an observation. I replaced my RX-100 with NOS 105 kit of the same vintage just because I was appalled by the condition of the crank arms. Of course the rings were exactly the same so this wouldn't help the OP's desire to change that, but as ohvrolla mentioned, it would probably be cheaper to pick up a complete crank and pilfer the rings than to buy new rings.


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## pmf (Feb 23, 2004)

ohvrolla said:


> I had a 53/42 and wanted to go to a 52/39 and it was cheaper to buy a Octalink Shimano 105 crankset than to purchase chainrings. Just robbed the chainrings off of it and put the arms up for safe keeping.


He only really needs to replace one chainring. A 39 is $25 and a 38 is $35 at Excel. I find it hard to believe you're going to find an entire crank set for $25. Sounds like he wants to do it himself, as cheaply as possible and isn't an experienced mechanic. So I'd vote for changing the ring.


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## gr8blues (Nov 20, 2009)

Here is a better price on a 38 http://www.jensonusa.com/store/product/CG408A06-Sugino+Standard+Chainring.aspx I have been using this for a few months on my 600 crank and it shifts fine. I went for the 38 because I can use every tooth I can get here in the mountains and can't afford a compact.


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