Guys,

I took up this subject of gears, and sizes versis numbers on the gears, with 
the people that MAKE the gears in Europe.   Turns out I was wrong also...   
It seems that gears are not precisely sized, with corresponding numbers, as I 
had thought.  The numbers denote deviation from design standard, and the 
crank can cam gear are matched to EACH OTHER. This may account for the fact 
that the two of the same numbered used gears that I have mesured have been 
different in diameter.  I thought it was my poor measurement techniques or 
wear, but this is what I was told...

  The tolerance markings on the gears having absolutely nothing to do with 
the individual size of the gears. The "0" dimension between centers of
  cam shaft and crank shaft is exactly 90mm. The tolerances are measured in 
hundredths of a mm. That is "-10" is 1 tenth of a milimetre smaller,
  " +4" is 4 hundredths of a millimetre larger. 
  
  The different size wheels cannot be purposely produced, even by modern day 
methods. They are all from the same production, and measured according
  to out of tolerence by measuring the distance between the flank of 6 teeth 
at a 90 degree angle. This can only be carried out electronically.
  The gears are sorted according to deviation from the "0" or exactly 
38,495mm.  

  
  The steel crankshaft gears have a far lower deviation than the Aluminum 
gears for the cam shaft.
  
  
  The gear sets (one crank gear and one cam gear) are matched to each other 
according to end deviation e.g. "-7", and not  according to deviation of the 
individual gears by them selves as this would just not be possible.   This is 
where the misconception of sizes comes from, and it is the most difficult for 
people to understand. That means that a +6 cam shaft gear on one engine could 
possibly  be used as a "-2" or a "+4" on another engine. You must forget 
about viewing a size on one gear wheel, but always look on the set as a 
whole.  The case is marked with the deviation number of the gears that were 
installed in it when new. the number on the gear is not a quantative 
measurement of the gear size. Imagine an extreme case of two motors with a 
"-7" housing.  The one has a larger alu gear and a small steel gear, the 
other has a smaller alu gear but a larger steel gear. The overall deviance 
from specs of BOTH of the engines is "-7".


Also, in regards to cases and the contraction with age: 
The case grows and get bigger when warm. This is normal for all aluminium 
alloys. As they get hot, the expand. As they cool, the
contract. This cycle carries on thousands of times over the motors life span. 
Over time, this constant expansion and contraction results in material
 fatigue and the alloy eventually loses its "memory" and will no longer 
contract to the dimension it once was when first cast.

Vech
  



Bench Mark Works
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