Changing from 5 to 6 cog freewheel on 1986 Stumpjumper-Sport Refining a prior posted question.  15 speed 1986 Specialized Stumpjumper-Sport has original 5 cog freewheel, 14-30. Original Analog shifters.  Biggest cog needs replacement.   Individual cogs are no longer available.   5 cogs is plenty for my needs, but 5 cog freewheels seem pretty rare these days.  Shimano makes two versions of a 6 cog freewheel that are sold through Walmart.com for $10 - $15. 14-28 (MF-TZ20), or 14-34 (MF-ZH30).  Prices seems pretty good.     What are the issues involved in transitioning from a 5 cog to one of these 6 coggers?  


Would it fit in the same axel space?  
6 cog chain compatibility spec states 3/32".  Does that imply a Shimano Hyperglide chain? 
If so, would SH chain be compatible with existing front chainwheel cogs? 
Any derailleur/cog clearance issues switching from 30 to 34 cog?
(axle width) Maybe, there is a pretty good chance your bike is already at 130mm O.L.D. spacing. Road bikes would be at 120 or 126mm from that era, but a mid 80's MTB has a reasonable chance of being at 130mm. 


http://sheldonbrown.com/gloss_n-o.html#old 

2-3. All chains for geared bikes are 3/32 interior dimension, the difference between 7-11 speed is the outside width. You should buy a new chain if you buy a new freewheel, any quality 8spd chain is fine and should work on your current chainwheels. 


For an MTB derailler 30 -> 34 should not be a problem. You will likely need to make adjustments to the B screw to make sure the cage clears the largest gear. 


http://sheldonbrown.com/gloss_ba-n.html#btension