OK, this is turning into another bad day. I have an old 6-spd that I wanted to replace the freewheel for a higher T one. While I was on the road, I ordered a cheap Sun Race 13-30T 7-spd cassette(this is being edited, I meant freewheel). It wasn't until I got home home that I looked to see the RD is a 'short-cage'. It's a Shimano 600 SIS and I've got a bad feeling that it's not going to reach a 30T. Let me down gentle.

The cage length just determines the amount of 'slack' chain the derailleur is capable of taking up when you're not in the large-large combo. To determine the capacity you need, add up the teeth on the largest chainring and largest sprocket, then subtract the teeth on the smallest chainring and smallest sprocket. You say it's a 6-speed, do you mean the whole drivetrain and that there's only a single front chainring or are you just talking about the rear wheel? If there's a single chainring, your required capacity with this cassette is 30-13=17T: well within the range of the short cage derailleur.

Does not compute

What does not compute? This is your standard mid-80's Shimano 600 road double with 52/42 Biopace crankset and a 6-spd 21T freewheel that I wanted to replace with a larger cogged one for climbing. I was looking for a 6-spd about 28T freewheel, but found this 30T 7-spd instead. I see no probably with using a 7-spd freewheel in DT friction mode, or am I wrong?

You mentioned in your OP that you were planning on swapping a 6-speedfreewheel with a 7-speed cassette. If this is the case, then it won't work as a cassette will not fit a hub designed for a screw-on freewheel. If it was just a terminology error, then it should work fine.

I meant 7-spd freewheel, not cassette. So, your saying that there is no issue using a 600 short cage with a 30T 'freewheel'?

3 things: Does the RD have enough 'throw' to reach all 7 cogs (probably yes). Does it have enough capacity to handle 52/30 and 42/13 (probably yes, but make sure to get the chain length right and don't crosschain) and finally, can it handle a 30T max cog (probably out of spec but I'd bet it will work).

You're most likely exceeding the rated limits of the part but those are often fairly conservative so it will probably work. Good luck!

I have used the SunRace 13-30 freewheel with Shimano short cage 105 and Exage 500EX RDs with no problems.

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YeeeHaaa! This is going to be a good day. Thanks all.