Plus and Minus Cam Gear Sets PDF Print E-mail
Written by BMWVMCA NEWS   
Tuesday, 05 August 2008


Plus and Minus Cam Gear Sets

I took up this subject of gears, and sizes versus numbers on the gears, with the people that MAKE the gears in Europe.  It turns out I was wrong.

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 measured 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 the 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 millimeter smaller, “+4” is 4 hundredths of a millimeter 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 tolerance 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 quantitative measurement of the gear size. Imagine an extreme case of two motors with a “-7” housing.  The one has a larger aluminum gear and a small steel gear, the other has a smaller aluminum 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 aluminum alloys. As they get hot, they expand. As they cool, they 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.


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