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Tuesday, 29 July 2008

Tech Tips  Those plus and minus numbers at the top of our crankcases. 

As part of an ongoing thread on the Slash 2 Group (on the internet), the subject of crankcase shrinkage, expansion and deformation over time came up again recently. We are discussing the crankcases made for the models between the first R51/3 through the end of production of the R69US.           

There seem to be two questions here. 1. Over time, does the crankcase expand, shrink or deform?  2. If the above is true, how can we determine which replacement gears to order?

Mark Huggett writes:

“The cases do not shrink....they deform over time, the cases  grow! We produce the gear sets for BMW so we have probably done more research in this matter than any body else.”    “(Loose gear fit)... mechanically does not matter at all. It just means that the motor is not as quiet as correct matching gears. If the gears are too tight, then you will have problems.”            “The replacement number that you choose depends upon:

1. Theory: the same number as what is stamped on the housing

2. Reality: Measurement of the out of tolerance of the distance between the centers of camshaft and crank shaft at a controlled temperature of 22,5°C  (72.5°F)”  

Hello Mark,           

In order to measure the out of tolerance factor, would this measurement be made with the crankshaft and camshaft installed less gears, calculating the distance between centers?  Could you enlighten us as to the meaning of the numbers inscribed on both the cam gears and the cases, such as -3, or +7, or any of the other numbers?

It is understood that no two cases were made exactly alike, and the cam gears came in a variety of diameters to properly fit corresponding cases.

I would like to know for example, how much difference would there be between a 0 case and a -10, or a +10?

It is also accepted that the shape and size of our engine cases change over the years. If we order gear sets from you, what information would you need?



Ron Cichowski  writes:

“Mark, I keep seeing this timing gear issue reappear every so often. Having spent some time in a machine shop I know it would be no real difficulty to accurately measure the center to center distance between the crank shaft and cam shaft.” 

“The whole problem seems to be the fact that no one has published a gear set center to center distance chart to establish what gear set numbers are for a specific center to center distance. If this information was available then it would be a simple matter to match gears to engine.” 

“Does the number on the gear actually indicate an arbitrary center to center distance? If so what is the center to center distance associated with eachgear set number?”


Craig Vechorik writes:

“I took up this subject of gears, and sizes versus 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 and 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 cam shaft and crank shaft is exactly 90mm. The tolerances are measured in hundredths of a mm. That is a ‘-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 quantitive 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 gets 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.”


There’s More!

Duane Ausherman writes:

“...the case doesn't shrink or grow.  It only changes shape.The reason we say shrink or grow is because we are really only concerned with the distance between the crank and cam. That is the part that shrinks or grows.

I fully realize that what I am about to say contradicts Mark's observations. I can only comment on my experience, however it is long ago, but in this case it is quite vivid. This is only worth what you pay for it.

My experience is limited to the few dozen that we worked on. As soon as I discovered that I could send the lower ends to Roy Reynolds, we stopped doing the work on most of them.

I never saw a case that would take the original size gear set. All took a set with a smaller size. I think that the compression of the cylinders is constantly trying to shove the top end out. For the top end to get shoved out, the case gets shorter, which happens over millions of cycles.

This was confirmed by the highest quality BMW rebuilder of that era, Roy Reynolds of Salt Lake City, Utah. The official business name was Reynolds Machine Tool. They specialized in rebuilding lathes, mills and other machine tools for machine shops around the west. I think that they were especially qualified to do what for them was very simple BMW machine work. The motorcycle business was purely a hobby for them. They were originally an NSU dealer and took up BMW around the late 50s.

Roy was my most important mentor in the mechanics of BMW and I certainly appreciated his willingness to assist me with any issue. He bailed me out many times with his sage advice and experience. He is alive and well in SLC and I have visited him a few times recently. We really enjoy discussing how much we have forgotten about BMW and hoping the other can fill in.

It was his family business that manufactured the back rest for me for the /5. We had only a verbal contract on charges and costs. I never questioned the honesty and integrity of anyone from that family on financial matters. I never dealt with anyone in the motorcycle business that was more honest. I feel it a great privilege to count them among my friends.

Back to the case and timing gears. In the early days the breather plate had only one pin to drive it. That pin commonly fell out and got into the timing gears. Later BMW used two pins and they still fell out and reeked havoc with the gears. The final solution was to have them cast onto the plate.The issue was no longer.

Even when nothing of any size gets into the gears, they commonly show wear in the form of small amounts getting chipped away. Here we have a chicken or egg first issue. Either as the gears wear, the shafts get closer and the amount of play stays small, or, the case change causes the gears to wear because the shafts get closer. How can we know? It matters little really. The affect is that they stay quiet as a result.

Where the valve pushes back against the valve train, the timing gears are sort of forced backwards. If there is any play it will be heard at a low idle as a clicking sound. It is at this point that the maximum gear wear occurs. One must check the gear lash at that point in the rotation to see the max amount.

As was commented, one set of -3 isn't made of gears of the same size as another -3. So, we are dealing with two variables, the case change and unknown gear sizes. We never found one single case with miles on it that would take the gear size that was stamped on the case. In my opinion, it was too bad that the contractor who bored the case stamped the size into the case. That number was only useful for the person who selected the gear set at the factory. A removable note would have been better. Many owners have ordered the same size gears, only to find them to be too tight.

Roy Reynolds told me that he kept a large assortment of sets in stock. He would first try a set of  3 or 4 sizes smaller.  His record was a case that took 7 sizes smaller. Imagine if you had that happen and your case was stamped a -9 in the first place? You would be in trouble.  The only recourse would be to mix and match to get a "small" crank gear and a "small" cam gear.

Often my shop didn't have the right size set in stock. We would start with the closest size we had. Then we would usually have to go to the "used box" and change only one of the gears until we found a nice tight fit. Too tight isn't good, as while the gears will wear in, think about the crud getting caught in the slingers. A "loose set" will only make some rattle noise at idle and no other downside.

While we would all like a new set that fits perfectly, I am not sure that it is so important in the long run. I have installed a lot of worn sets and had no trouble at all.

The only time we would dress a set of gears is when the pin got caught and made some nasty high places.

I have tried to address the many issues related to the timing gears and hope I didn't forget anything. 


So how does this information help us figure out what gear sets to buy? The numbers stamped on the cases don’t apply because of movement and fatigue. It has been suggested that measurements be taken to determine the center of each bore in order to get the information needed. If you are not accustomed to precision measuring, it might be best to take your engine case to a machine shop and have them make these measurements. Make sure to look over their shoulders. You will need metric measurements.

Brian Caro contributed the following information:

“This is based on Vech's analysis of the numbering system. Average center to center of cam and crank (one must measure the full range of movement within the bearing play)”  

89.90mm 3.5393"  -10  
89.91mm 3.5397"  -9  
89.92mm 3.5401"  -8
89.93mm 3.5405"  -7
89.94mm 3.5409"  -6
89.95mm 3.5413"  -5
89.96mm 3.5417"  -4
89.97mm 3.5421"  -3
89.98mm 3.5425"  -2
89.99mm 3.5429"  -1
90.00mm 3.5433"   0
90.01mm 3.5437" +1
90.02mm 3.5441" +2
90.03mm 3.5445" +3
90.04mm 3.5449" +4
90.05mm 3.5452" +5
90.06mm 3.5457" +6
90.07mm 3.5461" +7
90.08mm 3.5465" +8
90.09mm 3.5469" +9
90.10mm 3.5472" +10



Last Updated ( Tuesday, 29 July 2008 )
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