I am just getting started in fixing bikes, and seem to have the most difficulty in figuring out what is compatible with what.

For example, I have an older (not exactly sure how old) Scattante 670 with a newer, but still old, 9-speed Ultegra groupset. I was looking to replace the RD, but I have no idea what I could replace it with. I assume I don't need to find the exact same derailleur, but would most 9-speed RDs work, or do I need to stick with Shimano parts to match other parts for some reason? Also, I was looking to get shorter cranks, but wasn't sure how to match the crankset with the bottom bracket. Do I need to disassemble it all and use calipers to measure everything since there are very few markings of size and measurement on most parts?

Any tips on finding compatible parts rather than buying parts and realizign they're wrong would be great.

"In General" most Shimano 6-10s road and 6-9s Mt RD are compatible. It's about cable pull "per click"
Shimano has compatibility charts but they don't often cover multiple generations of a model.
Other sources are out there.

https://www.celebrazio.net/bicycling...atibility.html
​​​​​​https://bike.bikegremlin.com/1278/bi...compatibility/

Shimano, Campagnolo and SRAM have a lot of technical documents on their website. Shimano keeps them all here.... https://si.shimano.com/#/ They have compatibility tables that show their current and recently discontinued stuff. And archives that go way back. It does take some effort to get use to how they work.

Experience will be your best teacher. Learn from your mistakes. Avoid mix and match components if it's not necessary. Keep road groups together and mountain bike groups together. Though some do cross over with no issue.

The BikeGremlin site attached above does a reasonable job of explaining RD compatibility. That should provide the basics of what to look for.

So, any Shimano road RD up through 10 speed (except old Dura ace 740? series or Tiagra 470? series) should work. If looking for brand new, Shimano still offers the RD-5701 in short & mid length cage.

As for your crankset question, if you have the 9 speed Ultegra crank, your options are limited if you want to keep the bottom bracket. The BB for these use a hollow splined spindle called Octalink, (commonly called Octalink V1). Only 9 speed Dura ace (770? series), Ultegra (650?) & 105 (550?) cranks fit this BB for road use.

Other cranks would need the proper BB.

Here is Shimano's published compatibility chart from 2005. Not included are earlier 550? series cranks. Also doesn't include newer model RDs that may be compatible.

Tiagra 4700 won't work.Tiagra 4600 and Sora 3500 will work. Microshift R-series (Road) will work.

Pick your crank and THEN use the correct BB that fits. Older cranks tend to use longer spindles, so your odds of matching up aren't good.
Picking a crank to fit your current BB is basically backwards. Kind of like finding a tire and then buying something it fits.

Tiagra 4600

4700 wasn’t a 9-speed RD anyway, right?

Here's a chart listing specs for most derailleurs. Find your current one on the left column yours is probably row 289 or 290, the very top line lists columns of single, doulble and triple chainring limits and the line below that lists various data to show what the specs of every derailleur listed. Anything that doesn't have a "pull ratio" of 1.7 is a no-go. Check the 'min and max low' for cog size limits. Microshift are good quality for what you need only because the 6500 Ultegra groupset has been out of production for over 20 years and Shimano changed the pull ratio for almost all their road groups after going to 11 speed in around 2013 so buying used can be risky. Doesn't matter if the derailleur came with a different speed groupset as long as the pull ratio is 1.7. https://docs.google.com/spreadsheets...#gid=847158527

Who created that??

A forum member here posted that a few years ago and mentioned he had contributed to it so not sure if it was mostly his creation, but I wish I had saved his name as I thank him almost every week for this info. If you are the person who created this and are reading this thread

How short a crank length are you looking for? If you want to go from 175mm to 170mm, you can easily find a 170mm 9 speed Ultegra crankset on Ebay. If you want to go down to 165mm, it's out there but quite a bit more rare. As for anything shorter than 165 -that generation of Shimano cranks were never made in lengths shorter than 165.

4600 and 4700 are both 10-speed. 4600 uses the old 10-speed pull rates. 4700 is a 10-speed application of the 11-speed versions of 105, Ultegra, and Dura-Ace.

The intended 'speed' in itself is not what is important, the actuation ratio that it designed for is. We can though, determine the actuation ratio of most Shimano RDs by knowing the 'nominal speed' and road/MTB group that it came from (the 740? & 470? being exception)

See the BikeGremln site attached above for a little detail.

The BF member may be alexdi.

On an old thread, he responded and said that he would make the correction to the google doc based on my post. See attached.

Great find, that's him. Did a little searching and found an old thread from 2018 where I thanked him for posting that link around that time. He sounds extremely knowledgeable on the subject and probably created the chart. Thanks

I brought up 4700 to differentiate it from 4600.

Yeah saw the other post. I didn’t realise 4600 was also 10sp

i have that sheet on my desktop.

to the author(s)

4700 10sp. and GXR 10sp. will work with 11sp road shifters too, IIRC.

half the time, or more, i just bolt up a der. and try it.
most SRAM, Shimano, and Campy don't mix well. Shimano compatible Road front and MTB front don't mix, etc.

most of my builds are 10 sp. and less... as are ALL my personal bikes.
Campy introduced a 13 sp. group recently... too much is never enough, eh?
https://www.cyclingweekly.com/review...st-a-knock-out