Motobecane78

Trek 400D recently purchased recently is all original - Shimano S-424 w/ SIS + Light Action RD + 6-sp FW.
Learned this early SIS model had “design issues”, i.e. the small tabs for SIS break off. I want to minimize costs and do not mind dt or friction-only shifters.
Several questions:
1) does shifter work as friction-only with these tabs broken off ?
2) if replacing all, how is the chrome boss cover removed? Is it just glued on, or just stuck and I need to pry off w/ muscle?
3) What Shimano models would work on this boss (is it a standard for all dt or just Shimano) ?
4) On ebay old shifters can cost more than I paid for whole bike….

rccardr 

SHould work fine with friction only, just turn the arrow on the shifter to friction instead of SIS. 1050 series 105 shifters are an excellent replacement option and inexpensive. Loose Screws may have them NOS for $30 +/-. Chrome boss cover should pop right off, if not grip firmly with pliers and gently remove (prying is not a suggested methodology). Any late 80's Shimano model shifter should work with that boss, although many of them are 7 speeds or more.

Bianchigirll 

I am not so sure. Something tells that without the 'tabs' to hole the inner body in place it won't work in friction. You have the frame and shifter give it a try.

If you don't mind friction any older DT shifter will work.

Motobecane78

Thanks. I may also convert to 7 x 3 gears. If so, then may go to a simple friction shifter with Power Ratchet similar to older Sun Tour. Rivbike sells one for $44. Will all these fit the Trek frame braze on?

HillRider

Yes, they will if you have the common two bosses, one on each side of the downtube. If you have a single boss on top of the toptube, you will have much more difficulty finding replacement shifters.