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To weight or not to weight, that is the question

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  • Ken Augustine
    A pal came to me once with a 350cc street Ducati single, likely a Scrambler or Sebring, which his slightly build lady friend simply couldn t kick over much
    Message 1 of 14 , Jun 1, 2013
      A pal came to me once with a 350cc street Ducati single, likely a Scrambler or Sebring, which his slightly build lady friend simply couldn't kick over much less reliably start.   He wanted me to turn the top of the piston to remove metal volume on the dome, thus decreasing the compression ratio and cranking resistance.    I said to bring it by and I'd look at it but he had it with him and as it is so easy to disassemble, we started immediately.   I took a tiny bit off the top of the dome but the piston dome interior was highly arched in that area and I couldn't safely remove much metal.   I explained that I had done hardly any compression reduction so he asked me to do whatever else I could.   I recommended a thicker base gasket as Ducati singles do not have head gaskets but stated that anything worth mentioning would be thick and have to be made of aluminum sheet which would take well over an hour with properly measuring then laying out and boring the 6 head bolt plus bore and oil dowel holes.   He then asked if there was anything else I could do so I recommended simply attacking the rest of the dome upper surface with aggressive hand guided rotary burrs in my porting tool.   I cautioned him that the top speed might not suffer badly but that acceleration might.   He replied that his lady friend never ran full throttle under acceleration or speed so he agreed.    Twenty minutes later I was covered in scimitar shaped chips and the piston dome looked like a modest pile of lumpy yogurt or melting ice cream spray painted silver.   He disappeared into his transporter and ten minutes later was blasting around the block on it.   He then reported that he felt no difference in the kicking resistance, acceleration or top speed.   I advised that he could still try making a thicker or add a second stock parchment base gasket but recommended to have his lady friend try it.   A week later after the weekend ride, he returned, saying that she still couldn't kick it through and that it pulled hard but that the vibration was now so bad that she could hardly ride it.   He took it back apart and luckily the full length wrist pin had a straight, fully cylindrical bore so I could make full length sleeves for it to replace the previously removed weight.   These could be pressed in but might move in use or could be slip fit and immaterially rub on the bore like long wrist pin buttons.   The only aluminum tubing I could find had large ID so when the OD was finished, the remaining tube had only a 1 or 2mm wall thickness.   I also built a solid aluminum insert, explaining that it could be tested and iteratively drilled to taste.   He decided just to try it with the solid insert and left.   A week later after the ride he returned again, reporting that his lady friend still couldn't start it but that it now ran more smoothly than ever before and actually ran more smoothly than any other vintage non counterbalanced single cylinder motorcycle than he had ever ridden. 

          The bore is 76mm and I believe the wrist pin is OD is 18mm.   If anyone can measure the wrist pin ID, the weight of the insert can be calculated but I can't imagine that it wasn't at least three to five times as heavy as the weight I removed from the piston dome and can only conclude that the engine was originally over balanced, likely for a high dome racing piston which is heavier.   Live and learn.

      Ken A

        Rod length 103mm
      Piston pin weight 36g
      It'll run happily to 12,000rpm+
      Now we fit an 8g heavier pin.

      Plan A would be to lighten the existing pins - and you now
      have a contact for that now.....

      Plan B would be to use the pins.  The question I would have
      is what is the mass of the piston and what is the mass of the
      "little end" of the conrod ?  i.e. - your total reciprocating mass
      per cylinder.

      I'd guess in the region of 160-170g ?  In which case 8g is going
      to make 5-6% difference in your balance factor - which is
      significant but not a deal breaker - it might even improve the
      vibration levels !

      Cheers                 IAN   

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