Upgrading an old Trek I have an old Trek from the early 80's, the serial number says its either a 610, 613, or 614. A lot of the parts are getting old and I'd like to replace a lot of them.

One part that I an not sure on sizing is the rear derailleur and cassette. It currently has a Sun Tour Vx derailleur with friction shifters on a 6-speed 14-30 cassette. I'd like to replace the wheels and cassette, but not the derailleur as it functions well and is still in good condition.

My question is whether the current derailleur, with some adjustment, would work with a a 7 or 8 speed cassette.

I would buy 700c wheels with a Shimano/SRAM freewheel and a cassette to match, I'm thinking an 11-28 or something similar since I'd like some harder options.

I will be replacing the chain and bottom bracket at the same time along with an identical (but not so worn) front chainring.

Is there anything else I should be aware of in terms of compatibility?

Thanks for the help!
The derailleur specs you have to consider are:


Max sprocket size
Min sprocket size
Total tooth capacity 


If you want to go to a 11-28 cassette from 14-30, max sprocket size is ok. 11 tooth min sprocket may be too small. Capacity is also increased by 1. 

11-28 may work but 12-28 is safer. 

6, 7 and 8 speed systems use the same sprocket spacing and progressively wider stacks of sprockets. The other thing you will need to check is the lateral range if both the derailleur and shifter. It may not be enough to cover 7 sprockets.