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Taken from the original Suntour manuals-

To convert a conventional derailleur geared bike into an Accushift Plus indexed equipped machine you will require the following essential components:
A Suntour Accushift Plus rear derailleur
A set of Suntour Accushift Plus gear levers and cables
A Suntour AP freewheel or freehub
A Suntour AP transmission chain
Suntour cable stops and guides unless your frame already has these brazed on

All Suntour Accushift Plus components can be mixed 'n' matched so unless you are buying a complete groupset please make the following considerations:
Intended use, touring, racing , mtb etc
Choose a gear mechanism that has a designated capacity within the variation of the teeth on the freewheel and chainrings.
Choose a lever set that matches the number of sprockets on your freewheel.

For accurate and trouble free index shifting certain basic pre fitting requirements must be complied with and the following visual checks should be made on your bike
Rear wheel should align properly in the frame
The chainwheel should be in line with the middle sockets on the freewheel (fit shorter or longer BB axle)
The mounting hanger on the rear dropouts must not be damagedor bent, should have a centre-centre drop dimension of 24 to 28mm and the stop tab should be at an angle of 30 degrees to the perpendicular.
Suntour Blaze and Alpha derailleurs are fitted with a detachable bracket for fitting to dropouts which do not have an integral hanger.

All of these we take for granted these days, everything has to be in exactly the correct place otherwise we know that shifting will be well out. In the early days of index shifting though this was a novel concept with all manner of systems being bodged together!

The manual goes on in much more detail about fitting and then  adjusting the barrel adjuster to get smooth shifting. All this for 6 or 7 gears too! Bear in mind that the shifters were levers mounted in the classic downtube position and still had a d-ring adjuster that meant you could switch to friction mode if your indexing became messed up!