How to turn a regular fiddle into a Hardanger fiddle:
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I have converted several cheap fiddles to 8-string "Hardanger 
fiddles" quite simply. The process can easily be accomplished 
with a weekend's tinkering. The following "instructions" make 
the project appear far more complicated than it really is! 'Tis 
really quite straightforward & simple.
 
- Start with an inexpensive fiddle -- although the end product
  does indeed sound quite nice, I certainly would not want to 
  "butcher" a good fiddle.
  
- Remove the fingerboard.  These are generally (but not always)
  held on with weak glue. Find the weakest part of the
  glue-joint & encourage the fingerboard off with the careful 
  help of a very thin knife and some wetness.
  
- Once the fingerboard is off, rout or scrape the channel
  through which the sympathetic strings run down the entire
  length of the neck surface, and likewise the underside of the
  fingerboard. The channel in the neck should be about 3/32"
  deep and wide enough so that the sides (unchanneled part) of
  the neck are left perhaps 3/32" wide. The resultant channel 
  in the neck should be narrower at the nut end & wider 
  "upstring". Treat the fingerboard channel in similar manner. 
  For both fingerboard channel & neck channel, it need not be 
  as deep towards the nut end, as the strings don't vibrate as 
  much here as they do further on. I prefer to scrape these 
  channels using a common flat wood chisle held perpendicular -
  takes a bit of time, but works nicely.
  
- This description of how to make an 8 string tuning block is 
  going to sound crude -- it is, but it works very nicely. Saw 
  off the scroll & back 3/4 of the peg-box just "nutwards" of the 
  E-string tuning-peg holes (ouch!). Angle the saw slightly by 
  tipping it counterclockwise before sawing (when viewed from the 
  side & with scroll to the left). From a 1/2" thick plank of 
  flat wood (hardwood or softwood) fashion a new tuning block (to 
  accomodate 8 inexpensive mandolin tuning gears) & be sure to 
  include a "tongue", which when fussed with & fitted carefully, 
  will plug into the amputated 1/4 of the peg-box. Once a good 
  fit is obtained, secure new tuning block with glue & small 
  wood-screws. Beautiful eh! Like I said -- use a cheap fiddle!
    
- Fashion a simple "nut" for the sympathetic strings.  I use a
  short length of fret-wire with grooves for strings. Works fine.  
  This nut is positioned just below main string nut & a tad
  "tuning-gearwards".
  
- Bottom side of main string nut must be modified (tunneled) to 
  allow sympathetic strings to pass through unimpeded.
  
- I have had best luck using a "non-standard" home-made
  bridge in which I cut/file an oval "slot" roughly 3/4" long
  by 1/8" high through which the sympathetic strings pass.  When
  calculating the position of this slot, take into account the
  desired elevation of the sympathetic strings  as well as
  their spacing. I have had good luck making this bridge out of
  1/8" plywood!
  
- It's a simple matter to modify the existing tailpiece to
  accomodate 4 more strings.     
  
 
I use a light-gauge set of standard mandolin strings for the
sympathetics.  They work nicely.   
 
My tuning differs from traditional Hardanger tuning,  but in my
honest opinion, sounds at least as good!   Living in the
relatively Scandanavian north country, I have heard a good 
number of beautifully constructed real Hardanger fiddles & can 
honestly attest to this last statement. All who have 
heard/played my humble creations have concurred. I simply tune 
the sympathetic strings to the key in which I am playing -- 
Being as I mainly play in the key of D, my sympathetics are 
tuned DADA (high to low) I should caution that to get any 
sympathetic string to sing optimally requires very careful and 
exact tuning of the sympathetics to the main playing string's
key.  
  
Dennis Havlena - W8MI
Mackinac Straits - northern Michigan 
My web-page is at    http://edcen.ehhs.cmich.edu/~dhavlena/
 
PS #1  
No doubt one could be concerned that the extra 4 strings might 
exert enough additional pressure to either buckle the soundbox 
or at least quickly warp the neck -- I have not had the
slightest problem in this regard. I myself was initially
concerned with this & in one experiment scraped a channel in
the neck into which was glued & pinned a steel reinforcing rod. 
This proved unnecessary. 
PS #2
My interest in Hardanger fiddles was kindled by a very 
inspiring fiddler/craftsman - the late True Dabill of Pine 
River, Minnesota, who made a myriad of very folksy but
absolutely BEAUTIFUL sounding Hardanger fiddles out of plywood 
and 2x4s, using stove bolts for tuners & galvanized milk bucket 
sections for tailpieces! He sold these for $50! Is anyone else 
out there in internet-land familiar with this kind and 
ingenious gentleman's works??

Thanks to Eric Reiswig for providing me with a copy of the above
article which I (like a dummy) had neglected to save!