John Wolff's Web Museum

Rebuilding the Original-Odhner Model 239 Pinwheel Calculator



Odhner 239 mechanism
Odhner 239 mechanism, 1960s


These notes give a detailed step-by-step procedure for a complete disassembly and reassembly of the Original-Odhner Model 239 mechanical calculator. They are also applicable, with suitable modifications, to most of the Odhner models from the 1920s to the 1970s.

The notes have been prepared and refined during the rebuilding of a number of Odhner machines, and may provide a useful guide to anyone faced with a similar task. The sequence of operations described works well for me, but comes with no guarantees as to its efficiency or appropriateness. No responsibility will be taken for any consequences arising from the use of these notes by others.

These notes are intended to be read in conjunction with three other documents in this section:

Please study this material carefully before commencing your overhaul, and refer frequently as you proceed.

I would appreciate receiving your feedback, comments, or suggestions for improvement via the enquiry form.


Please review the material in the Notes on overhauling a mechanical calculator before proceeding. It is suggested that you print a copy of these notes, and tick off each step as it is completed. An Odhner 239 can be completely disassembled in about an hour, provided that no major problems are encountered.

  1. Covers and carriage
  2. Invert the machine. Remove the four screws and washers in the rubber feet and lift off the bottom cover.
  3. Remove the four screws holding the side covers. Slide the covers downwards at the front, then forward, to disengage the internal clips. Remove the left-hand cover.
  4. If the winding handle will turn, remove the handle stop plug and sleeve (2 special screws from underneath) and remove the cover plate, otherwise leave until later.
  5. Remove the top panel (2 screws each side), and the rear panel (3 screws from underneath).
  6. Pull off the carriage shift buttons and remove the front panel (3 screws from underneath).
  7. Remove the carriage stop plug (2 screws from underneath).
  8. Hold down the carriage release tab and slide the carriage out to the right-hand side. If the carriage fouls on the rotor or other mechanism, leave it and continue.

  9. Main body
  10. Remove the rear panel and check dial mechanism and disassemble. Be careful of the light springs under the detent levers. These are very springy and can easily be lost during handling.
  11. Remove the rotor bearing from the left-hand side plate (3 screws).
  12. Remove 2 screws from underneath into the right-hand side plate. Ease the side plate up off its dowels and remove the side plate and rotor together.
  13. Remove the rotor and disassemble the right-hand side plate.
  14. Remove the left-hand side plate and disassemble the counter reversing mechanism. Be careful of the tiny detent ball and spring inside the right-hand gear (on the short front shaft).
  15. Remove the bell and interlock assembly and the rotor locking lever (under the baseplate).
  16. Remove the carriage shift mechanism and disassemble.

  17. Carriage
  18. Pull off the back-transfer button and remove the carriage covers (4 screws).
  19. Roll the carriage onto its back. Carefully loosen the two screws holding the metal strip under the accumulator until the spring pressure is released, then remove the strip and lift out the carry lever detent springs. Temporarily replace the strip to retain the detent pins.
  20. Carefully loosen the three screws holding the right-hand end plate until the spring pressure is released, then remove the screws.
  21. To remove the counter carry mechanism, first remove the screw and washer holding the accumulator star wheel shaft to the centre pillar. From the rear, push the shaft outwards until it is clear of the carry mechanism, then push out the short pin holding the top of the carry mechanism on the other side. Lift out the carry mechanism and set aside.
  22. Remove the back-transfer stop plate from the end pillar (on your left).
  23. Remove the accumulator star wheel shaft, star wheels, back-transfer assembly (disconnect spring), and the bracket and spacers around the centre pillar.
  24. Remove the 4 screws and narrow retainer plates at the rear of the accumulator register. Remove the two shafts, carry levers, support arms, and spacers.
  25. From the front, fold the register retainers (each end) forward and clear of the registers. Remove the centre bearing retainer plate and screw, then ease both register assemblies up and out together. Disassemble the two registers.
  26. Remove the screw and washer securing the detent lever shaft (at the right of the centre pillar). Carefully push out the detent shaft and remove the detent levers, balls, and springs. The balls are often held by a ring of dirt or oil at the top of the hole, and can spring away unexpectedly when disturbed.
  27. Roll the carriage onto its back, remove the retainer strip, and push out the carry lever detent pins.
  28. Drive out the small pin holding the arm at the outer end of the back-transfer shaft (from underneath) and disassemble.

  29. Counter carry mechanism
  30. From the rear, remove the circlip and long gear.
  31. Remove the circlip inside the inner end of the star wheel shaft, then remove the shaft and star wheels.
  32. Lift and remove the carry levers.
  33. Push out the lower shaft, leaving the sliding collars and drive gear behind on the toothed key strip. Remove and separate the components.
  34. Remove the spring plates and 7 detent balls from underneath.

  35. Main rotor
  36. Undo the set screw and loosen the round nut at the left-hand end with a suitable C-spanner (or a length of 4mm rod). Unscrew the nut until it is centred on the threaded collar, then push the collar inwards.
  37. Push the inner gear and the adjacent wide C ring inwards into the space created. Remove the split C ring from between the two wide rings.
  38. Push the outer gear inwards and remove the split C ring from the recess in its outer face.
  39. Remove the gears, then remove and disassemble the pinwheels one at a time. It helps to withdraw the round alignment rod as soon as it comes free. Be careful of the internal detent pawl springs.


Please review the general information about cleaning and checking in the Notes on overhauling a mechanical calculator, and proceed as described. Initial washing and cleaning will take about 2 hours.


Please review the material on Reassembly in the Notes on overhauling a mechanical calculator.

Moving parts should be lubricated sparingly as they are assembled. Apply a thin film of oil to steel parts with a (slightly) oily rag to prevent rust, and apply a minimum of light oil to gears, pivots and bearings. The pinwheels and setting rings should not be oiled.

In the notes following, screws listed as (eg) M3x8 are 3mm diameter, standard pitch (0.5mm), and 8mm overall length. Tension springs listed as (eg) 3x10x0.3 are 3mm diameter, 10mm closed coil length, and 0.3mm wire diameter.

Final cleaning, checking, and re-assembly can be completed in about 4 hours if no major problems are encountered.

  1. Base plate
  2. Mount the bell assembly to the base with 3 screws M3x8.
  3. Assemble the carriage shift mechanism. Place the long compression springs over the key stems and tie them shut with a piece of fine wire or string until the shift mechanism is fully assembled. Attach with 4 screws M3x8.
  4. Invert the baseplate and install the interlock lever between the shift mechanism and the interlock plate on the bell assembly. Secure the axle with 2 screws M3x12.

  5. Right-hand side plate
  6. Assemble the winding handle, drive gear, non-return pawl, spring, and wire clip. Check that the long pointed tooth on the pawl swings (approximately) equal distances to either side of the rotor bearing hole, then drive in the tapered pin.
  7. Fit the sliding interlock plate and shaft into the handle stop pillar and fit the circlip. Fit the spring and wire retainer through the hole in the frame.
  8. Temporarily fit the rotor clearing sleeve, plunger and end plug to the handle stop pillar and secure with the two special screws. Ensure that the assembly slides freely.

  9. Left-hand side plate
  10. Check the gears in the reversing mechanism for damage. The small pressed-metal gears rivetted to the end plates may be badly worn, or may have bent or broken teeth if the machine has been forced. Repair as necessary.
  11. Place the side plate flat on the bench with the inner face upwards.
  12. Stand the two 5x28 pillars in the rear holes in the side plate, flanged ends down.
  13. Place the bright plate with two gears over the pillars, long arm forward, gears on top. Fasten with 2 M3x12, 1 M3x7, and 3 flat washers from outside.
  14. Check that the T-bar and plunger move freely in the reversing mechanism body.
  15. Fit the tiny compression spring 2.2x5x0.5 and 3/32" ball into the hole in the right-hand end of the reversing body shaft, then fit the small gear with the cam side inwards and the notch over the ball. Insert a suitable tool (eg the back of an Exacto knife blade) between the cam and the gear, press the ball down into the hole, and push the gear fully home. The ball must rest in the detent notch in the gear, and not in the gap between the gear and the cam. The ball will be clearly visible from the outside when correctly fitted.
  16. Fit the larger cam and gear to the left-hand end, then the large gear with the counter operating finger. The cam and the operating finger both face towards the central body.
  17. Stand the large end of the assembly in the bearing in the left-hand plate. Fit the resetting arm into the small pivot hole. Fit two long spacers over the pillars, then fit the top plate. Insert the third pin and spacer and fasten with 3 screws M3x7 and flat washers.
  18. Attach the spring 2.8x4x0.3 to the rear of the resetting plate.
  19. Fit the side plate assembly to the base with 2 screws M4x14. Firm up but do not fully tighten.

  20. Pinwheel rotor assembly
  21. Fit the driving gear and flange assembly to the plain end of the shaft, then fit the thick C ring to the single groove.
  22. Stand the shaft upright in a suitable support (eg disc 13) and assemble the pinwheels and setting rings in order, starting with number 1. There are two sets of wheels numbered 2 to 5, and there are two numbers on each (2-10, 3-9, etc). Use the numbers next to the installed carry pin. There are two sets of setting rings numbered 1 to 5, which match the first number on the pinwheel. The second No.1 ring goes on pinwheel 6.
  23. Fit the alignment rod into the holes, then the end disc with the concave side out.
  24. Repair the threads on the inner locking collar where they have been damaged by the set screw. Screw the two collars together and fit to the shaft with the shouldered end and the set screw hole towards the outside.
  25. Place the small gear, two wide C rings, and large gear onto the shaft.
  26. Fit the first split C ring into the outermost groove on the shaft. Slide the large gear outwards so that the split ring is held in the recess in its outer face. Fit the second split ring between the two wide rings. Tighten the round nut.
  27. Check the operation of all the pinwheels.
  28. Fit the toothed locking bar and ring from the driving end and check again. When all is well, re-tighten the round nut and the set screw.

  29. Pinwheel rotor installation
  30. Check the fit of the two rotor bearings into their mountings. Ease any scrapes or nicks around the bearing or the housings in the side plates. The thinner bearing goes on the right-hand side. Remove the bearings.
  31. Counter reversing timing Check that the right-hand counter reversing gear is still in its detent position. Rotate the gears so that the timing marks on their cams align with the V-notches in the fronts of the adjacent sub-frame plates (see illustration opposite).
  32. Set the handle in its stop and the rotor setting rings to zero (fully rearward).
  33. Hold the rotor with the setting levers slightly rearward of top. Assemble the right-hand side plate to the rotor so that the rotor locking ring engages with the slot in the large pin at the top rear of the sliding handle interlock plate.
  34. Insert the left-hand end of the rotor into the bearing housing (no bearings yet) and lower the right-hand plate onto its dowels. Fasten with 2 screws M4x14, firm up but do not fully tighten.
  35. Lift the rotor clear of the driving gear and turn it until the interlock bail (on the top plate at the front of the bell assembly) aligns with the cut-out in the rotor end disc. Engage the driving gears and insert the right-hand bearing.
  36. Engage the gears on the left-hand side, check the timing and correct as necessary, then insert the bearing.
  37. Check the reversing operation. Carefully make a forward turn and watch as the counter drive pawl passes through the cut-out in the rotor shaft. The pawl will foul on the shaft if the assembly is incorrect, and may cause damage if turned vigorously. Check that the T-bar has moved to the left, and the timing marks have returned to their aligned positions. Lift the locking bail again and check that it resets the T-bar to the centre position. Make a backward turn, and check that the T-bar moves to the right and resets.
  38. When all is correct, fit the bearing covers with 6 screws M3x8. Adjust the end stops temporarily to align the gears - final adjustment will be made later.

  39. Rear panel and check dial
  40. Check that all the detent pawls are free to move. Carefully fit the springs into their holes and fold the detents over.
  41. Fit the narrow star wheels onto their shaft and attach with 2 screws M3x10.
  42. Fit the numeral wheels onto their shaft with the flat washer at the left. Replace any shims on the right. Fasten with 2 screws M3x12.
  43. Assemble the wide star wheels and the long shaft onto the slotted locating plate and fasten with 2 screws M3x8. Check that the wheels turn freely.
  44. Place the star wheel assembly in position, fasten the retainer strips with 2 screws M3x8, and fit the shouldered screw at the bottom left. Check that the plate slides freely between the two stops.
  45. Place the handle in its stop and the setting rings at zero. Turn the check dials downwards until they stop at zero.
  46. From the rear, place the panel in position on its dowels. Look through the rectangular cutout and engage the sliding plate with the tab on the handle interlock plate. Fasten with 4 screws M3x8.
  47. Check that the dial operates correctly. If the setting rings are locked, push the locking bar slightly inwards to release them. (The locking mechanism can not be finally adjusted until after the rotor has been aligned with the carriage).

  48. Carriage preliminaries
  49. The die-casting alloy can move with time, so it is advisable to check the carriage alignments as follows before commencing assembly.
  50. Insert the carriage into the base and check that it moves smoothly from end to end. Stiff spots will leave tell-tale marks on the bearing surfaces, which can be eased with a fine file or scraper.
  51. Fit the carriage positioning rack (teeth down) at the front of the counter register and fasten with 2 screws M3x8. Check that the shift controls operate correctly.
  52. Check the fit of the three register shaft bearings in their housings. Remove any nicks, scrapes, or spurious die-cast metal to ensure a smooth fit.
  53. Check that the detent shaft is straight, and that it can be inserted smoothly into place at the front of the carriage. Correct as necessary.
  54. From the rear, fit the lower 4x70mm shaft, the two star wheel support arms, and the two spacer washers at the right of the arms.
  55. Feed in the upper 4x93mm shaft from the right. If the shaft is tight, ease the holes or file the supporting surface until the fit is smooth but not loose.
  56. Temorarily fit the 4 shaft retainers and screws M3x8.
  57. Check that the star wheel shaft is straight. Check that it passes smoothly through the outer pillar and the intermediate supports, and seats properly in the centre pillar. If not correct, try interchanging the intermediate supports. If necessary, carefully bend the centre arms until the shaft passes smoothly through all four supports.
  58. When all alignments are correct, loosen the 4 retainers and remove the upper shaft.
  59. Examine the bracket that goes around the centre pillar. The two points on the front ride on the register cams to lift the interlock bail during clearing. If the counter side is badly worn there may be problems later with the resetting the reversing mechanism.

  60. Carriage registers
  61. Assemble the clearing components, numeral wheels, and cams onto the register shafts in the same order as removed.
  62. Place the compression springs and detent balls into their holes in the carriage frame.
  63. Feed in the detent lever shaft from the right-hand side. Fit the register retainer arm (big end down), back-transfer latch, accumulator detents (short arm vertical at the front), counter detents, and the left-hand retainer arm. Secure the shaft with 1 screw M3x7 and flat washer at the right of the centre pillar.
  64. Fit both register shafts into the centre bearing (groove to right). Align the flats on the end bearings and ease the whole assembly down into the carriage frame. Keep the back-transfer lifting arm (at rear) pointing upwards.
  65. Fit the centre bearing retainer and attach the right-hand end plate (4 screws M3x8).
  66. Turn all the dials by hand and check the detent and clearing operations.
  67. Attach the spring 4x7.5x0.5 from the back-transfer arm (rear) to the latch lever (front). Check that the back-transfer handle raises, latches, and drops the lifting arm at the rear.

  68. Accumulator carry mechanism
  69. From the rear, feed in the upper 4x93mm shaft from the right and fit the carry levers, starting with the bell lever (with the extra tail) next to the centre pillar. Tighten the 4 retainer plates.
  70. Assemble the back-transfer gears onto their shaft, hubs first. Attach the end bracket and drive in the pin. Place the assembly in position, with the end of the shaft engaged with the fork in the lifting arm.
  71. Feed in the star wheel shaft through the end pillar, then through the register retainer arm, back-transfer arm, star wheels, inner back-transfer arm, 3.5mm apacer, and the cam follower bracket around centre pillar. Place the 0.5mm washer between the pillar and the counter side of the bracket.
  72. Fit the M3x7 screw and flat washer into the rear of centre pillar to hold the accumulator star wheel shaft, but do not tighten.
  73. Check that the register dials and carry levers all move freely. It may be necessary to remove the star wheels and ease the intermediate supports sideways to equalise the clearances along the shaft.
  74. Fit the back-transfer stop to the rear of the right-hand side with 2 screws M3x7. Check that the back-transfer gears rise, latch, and reset correctly.
  75. Roll the carriage onto its back and drop in the carry lever detent pins. The wedge at the bottom of the pins must be aligned horizontally, ie, along the carriage axis. Insert the springs and fit the retainer strip with 2 screws M3x7. Check that the edges of the screw heads do not extend beyond the level of the base, and correct if necessary.
  76. Check that the carry lever detents all operate smoothly. Note that there is no detent on the bell lever.

  77. Counter carry mechanism
  78. Line up the seven sliding collars left to right on the bench, with the keyways at 12 o'clock and the operating fingers at gradually increasing angles. The fingers on the rightmost collar will be "back-handed".
  79. Hold the key strip with the wide tooth (with the hole) towards the right. Place the drive gear immediately to the left of the wide tooth, then add the collars in order (right to left) with the fingers on the right.
  80. Position the frame with the rear closest (long arms away). Put the collar assembly in position (left-hand end first) with the gear resting in the slot in the base. Insert the plain end of the shaft from the left-hand side, and assist the collars onto the shaft. The last teeth on the key strip are positioned either side of the right-hand frame. Check that both rows of fingers are aligned in a uniform progression.
  81. Be careful handling the assembly prior to installation as the mainshaft can easily fall out.
  82. From the front, hold each carry lever with the long end up and the open side away. Pass the long end up through the slot in the top plate, fit the open side into the V-groove in the collar, then drop the bottom end into the slot in the base.
  83. From the rear, start the plain end of the star wheel shaft from the left-hand side. Fit the 1.8mm spacer, the star wheels with their hubs to right, and the 8.5mm spacer at the right. Fit the circlip at the inside of the left-hand side.
  84. Fit the long gear and circlip.
  85. From underneath, fit the seven 3/32" detent balls, spring plates, flat strip, and 2 short screws M3x4.
  86. From the rear of the carriage, push back the accumulator star wheel shaft until it is clear of the counter side. Check that the curved register retainer is in position, lift the central cam follower bracket, and insert the carry lever assembly. Hook the long forward arms over the detent shaft at the front.
  87. Push the accumulator star wheel shaft inwards to lock the carry assembly, then insert the shouldered pin from the outer side. Tighten the retaining screw on the star wheel shaft.

  88. Installing the carriage
  89. Counter carry timing Examine the narrow gear at the outer end of the counter carry mechanism and locate the tooth marked with a dot on its tip. Align this dot with the V mark on the adjacent frame (as illustrated).
  90. Support the front edge of the machine on a small block so that the rotor interlock lever (under the left-hand side) is clear of the bench.
  91. Set the winding handle in its stop.
  92. Lift the back-transfer interlock arm (under the front right-hand side of the rotor) to its upper position.
  93. Slide the carriage in from the right-hand side until it stops against the shift mechanism. Press the carriage release lever and continue carefully. Look in from the left-hand side and assist the long gear in the carriage to engage with the driving gear in the main body. Re-check the alignment marks and repeat as necessary.
  94. Temporarily fit the carriage stop plug (under).

  95. Final adjustments
  96. Support the front edge of the machine on a small block so that the rotor interlock lever (under the left-hand side) is clear of the bench.
  97. Check the operation of the carriage shift mechanism, and ensure that the carriage is held securely in each position. Set the carriage fully to the left.
  98. Set the first and last rotor discs to 9, then turn the handle until the extended pins reach the star wheels. Adjust the lateral position of the rotor to provide the maximum engagement. Check that the counter drive pawl is centred over the corresponding star wheel. Temporarily fit the top cover and check that the setting rings do not rub on the sides of the slots. Straighten any bent or damaged slots, and fine-tune the rotor position as necessary. Tighten the lock nuts when a satisfactory compromise has been reached.
  99. Check that the setting rings are locked when the handle is pulled outwards, and that they become free when the handle is in its stop. Problems may be due to bending of the locking bar and ring, or to lateral misalignment of the check dial mechanism at the rear. Correct as necessary to obtain reliable operation.
  100. Check the operation of the rotor clearing mechanism. Hold the sleeve on the handle stop pillar inwards with your thumb, pull out the handle, and turn forwards until it stops (about 1/4 turn). Return the handle to its stop. It may be necessary to adjust the lateral position of the sliding gear assembly on the rear panel by bending the interlocking tabs.
  101. Set the rotor to zero and the carriage fully to the left. Rotate the counter clearing handle and watch the movement of the interlock plate under the left-hand end of the rotor. The two teeth on the right-hand end of the plate should rise freely between the setting rings, and fall back at the end of the turn. Modify the teeth as necessary to correct any interference - do not upset the rotor position. The easiest method to get access is to remove the carriage, remove the four M4 screws from underneath into the side plates, then lift off the sides, back, and rotor complete. Adjust the teeth as necesary, check that the plate is flat and straight, and reassemble.
  102. Give the handle a forward turn. Rotate the counter clearing handle and watch the movement of the slotted resetting arm under the counter reversing mechanism. The arm must rise far enough to reset the pointed T-bar to its central position. Check again for a backward turn. This movement is frequently inadequate, due to wear on the cam follower (the bracket around the central pillar on the carriage). Correct by building up the top surface of the bracket, or extending the operating arm on the interlock plate. A piece of 1 or 2mm material can be soldered, brazed, or screwed into position and filed down to give a satisfactory operation. (Excessive build-up will cause binding on the cam).
  103. Check the operation of the back-transfer interlock. Give the handle a forward turn, then check that the back-transfer lever can not be engaged. Reset the interlock by clearing the rotor using the sleeve on the handle stop, then check that the back-transfer gearset rises, latches, and releases correctly. The interlock is operated by an arm attached to the handle interlock plate (under the right-hand end of the rotor), which may be bent as a result of forcing or incorrect removal of the carriage. Correct as necessary.
  104. Check that ripple carries operate smoothly in both directions on both registers, then verify the test calculations.

  105. Finishing off
  106. Assemble the perspex windows and decimal indicators in the covers.
  107. Remove the carriage and install its covers with 4 trim screws M3x8.
  108. Fit the back, front, and top covers (in order) to the main body. Replace the carriage shift buttons and back-transfer button.
  109. Remove the handle stop plug and clearing sleeve. Fit the side covers, sliding them rearwards then upwards to engage the internal clips. Tighten the clips first if necessary. Fasten with 4 screws M3x7. Replace the clearing sleeve and handle stop plug.
  110. Reinstall the carriage. From underneath, re-fit the carriage stop plug and tighten the M4 screws into the side plates.
  111. Fit the bottom cover and rubber feet. Satisfactory replacement feet can be cut from 4mm neoprene sheet with a 1.5" wad punch.

Original text and images Copyright © John Wolff 2009-12.
Use at own risk; beware of errors; suggestions for improvement welcome.
Last Updated: 27 May 2012

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