Originally Posted by Onegun
Someone mentioned indexing. If indexing was even used on 6 speed freewheels, I wasn't aware of it, nor was it necessary. So again, no worries.
Suntour made a 6 speed indexed system in the late '80s. I have one on an old MTB. I imagine they made barend and downtube 6 speed index shifters as well.
Ebay is your friend to find NOS and barely used Suntour freewheels, in 5-7 speed variations. Prices are from cheap to moderately expensive, but there always seems to be a good supply. I've had great luck with the Suntour freewheels. Other then needing a less then superior 2 or 4 notch removal tool, I find them to be very reliable.
If you have to replace the a Suntour freewheel and maintain your current 6 speed indexing system, you need to first determine if it is an "Ultra" (narrow spaced cogs to replace a 5 speed freewheel) or a "standard" spaced freewheel. Generally speaking for single bikes, "Ultras" were used on 120mm spaced dropouts and "standards" on 126mm spaced dropouts. I'm not certain what the equivalent is on an older tandem.
I hope this is helpful. Best of luck on your decision. What a great way to let your wife enjoy the thrill of bike riding!