MATE Building Instructions - Page 5



  1. Centre the (Ø15/16") steel stock in the chuck and extend it 70mm from jaws. Face end and centre-drill, maximum Ø2.5mm, for later tailstock support. Finish turn full length to Ø23mm.
  2. Part off at 65mm from faced end. Measure overall length, centre again in chuck and face other end to 63.3mm.
  3. Transfer to a vee block on a surface plate. On the end just faced, scribe a horizontal line above the centre with a surface gauge. Turn material through 90° setting precisely with a square against scribed line. Scribe another horizontal line with the gauge at the same height as before . Repeat again twice. The diagonals of the resulting square are diameters on the 23mm stock, thus the intersection is the exact centre . Carefully centre punch. Set a pair of dividers at exactly 7.15mm, centre on your mark and scribe a line across one of the diameters. Centre-punch the point of intersection, which is the centre of the crankpin.
  4. Offset the stock in a 4-jaw chuck, centreing on the crankpin and extending 10mm from the jaws, ensuring that the stock is lying squarely in them. Centre-drill the crankpin, maximum Ø2.5mm, and bring up the tailstock support.
  5. Set a sharp-nosed knife tool to face and make sure it will be reduce the crankpin down to 6mm without fouling the tail stock Take facing cuts on the crankdisc, maximum 0.2mm at a time, turning to form a crankpin Ø6mm x 4.5mm long.
  6. Check that the tailstock centre is oiled and properly engaged before proceeding. Change to a finishing tool with a .5mm radius nose and capable of taking facing and traversing cuts without re-setting. Face the remaining .5mm from the crankdisc and finish turn the crankpin to a nominal Ø5mm. Polish with #800 paper, oiled and backed with a flat, narrow surface, such as a piece of tool steel, until the crankpin is a smooth-running fit in the big-end of the conrod. Do not radius or chamfer the end of the crankpin. Just break the edge with a stone. Remove from chuck.
  7. Accurately centre the crankdisc in a 4-jaw chuck, inserting 3mm deep only. Bring up the tailstock centre into the previously drilled end. Rough turn the crankshaft journal to Ø10.5mm x 53mm long, then reduce the end to Ø6mm x 17mm long. Be careful as there is not much holding the shaft.
  8. Again, using the .5mm radius tool, finish turn the journal to the nominal Ø9.7mm and face crankdisc to 4.3mm thick. Check fit in crankcase bushing when approaching the final size. Polish with #800 paper to a good finish, following up with Brasso polish. Aim for a smooth-running, shake-,free fit in the bushing.
  9. Face the remaining .5mm from the crankdisc, to form the Ø14mm thrust face. Break the edges on the crankdisc and the thrust face.
  10. Finish turn Ø5mm x 18mm long on the end of the crankshaft and chamfer the end 45 x 1mm wide. Cut thread .8mm pitch x 0.48 depth (check against the nut) to 12mm from the end of shaft. Remove from chuck.
  11. Centre in chuck on crankshaft journal, thrust face against the jaws. Protect finish with paper. Centre-drill and drill hole in shaft Ø5.5mm x 20mm deep. Chamfer hole 45 x 1mm wide and break edge of crankdisc. Take care not to mark the crankpin in any way. Remove from chuck.
  12. Wash shaft and oil. Fit into crankcase and, holding it forward against the thrust face, scribe around the shaft lightly where it extends from the bushing.
  13. Remove from crankcase and centre shaft in chuck on threaded end, inserting to within 1mm of the end of the journal. Protect thread with paper. Check that shaft is running truly and bring up some sort of support into the crankshaft bore, if possible. Set the topslide over to 7 1/2 and turn tapered seat for the prop driver, reducing the diameter until the taper extends as far as the scribed line. Chamfer 45° x .25mm wide on the smaller end of the taper.
  14. The crankshaft is now finished, with the exception of the inlet port, the location of which will be determined during final assembly. Do not alter the topslide setting, as the next part, the prop driver, has to be bored to match the crankshaft taper.


  1. Centre the remaining (Ø19mm) aluminium stock in the chuck and extend 20mm from the jaws. Face end. Centre-drill drill Ø5mm x 20mm deep. Counterbore face 8mm x 10mm deep.
  2. Bore taper. When the bore approaches 9.7mm, check fit with crankshaft taper, holding the shaft, in the crankcase, forward against the thrust face. Cease boring the taper when the two parts fit together allowing .5mm float in the crankshaft. If you have gone too far, face the end off until the correct clearance is achieved.
  3. Re-align to the topslide with the spindle axis and finish turn the stock to Ø19mm x 20mm long. Undercut to finish Ø17mm x 4mm long. Chamfer all edges 45° x .5mm wide. Part off at 7mm from faced end. Put the unfinished prop driver to one side.
  4. Face the end of the stock remaining in the chuck. Taper the face for appearance and chamfer all edges 45° x .5mm wide. Commence parting off at 3mm from the faced end, withdrawing the tool to allow you to chamfer the rear edge before completely parted. File the rear face flat, if necessary and break the rear of the Ø5mm bore by hand, using a centre-drill.
  5. Re-centre the prop driver with the flange against the jaws, using paper to protect the finish. Tighten securely. Set a knurling tool on centre height with the knurl-axis parallel to the prop driver face and the knurl aligned with one side of the face. Plunge the knurl firmly into the face, allowing it to cut at about 250rpm for a few seconds only. The result will be a neatly knurled, non-slip driving face, as good as the best commercial engine! Chamfer the inner and outer edges 45° x .5mm wide.

NUT (7-3)

  1. Use a commercial steel M5 nut, or machine a finct one from hex steel stock as shown.


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