Understanding this crankshaft journal table
Discussion
I'm probably being stupid with this but i wanted to double check before I go and get my cranshaft journals reground. I'm measuring the journals on my crankshaft (golf gti mk3 1996) - the image shows the table in the workshop manual listing the crankshaft journal stages but i'm a bit confused as to why there are two '-' numbers?
I'd initially read the numbers above and below the main diameter value as being +/- tolerances, so the basic journal dimension would have a tolerance of +0.022 and can wear down from the standard diameter by -0.042.
Is this the correct interpretation? This is my first engine rebuild so perhaps this is a normal way of listing diameters and will be obvious to all the more experienced people on here.
Cheers!
I'd initially read the numbers above and below the main diameter value as being +/- tolerances, so the basic journal dimension would have a tolerance of +0.022 and can wear down from the standard diameter by -0.042.
Is this the correct interpretation? This is my first engine rebuild so perhaps this is a normal way of listing diameters and will be obvious to all the more experienced people on here.
Cheers!
It isn't wear, they don't wear unless something has gone wrong.
It's grinding tolerance. At the factory they will have ground 1000 cranks per X amount of time, there will have been variations during that time; differences in temp, pressure, wear on the grinding wheels, slight variation in steel, how long Fritz spent on the lapping tongs whilst chatting to Otto etc.
When done they measure them and match the sizes with their factory bearings to get the clearance as good as they can get.
They have lots of different bearing thicknesses available to them which the aftermarket don't do (in my experience of non VAG motors).
Some cranks are colour coded, some letters or numbers dependent on what size as are cylinder bores.
So whilst they are incredibly precise aftermarket isn't.
You can get around this by reading the manual further as it should give you bearing thickness and also by putting the bearings into a torqued up housing and measuring the internal diameter, then grinding the journal to that - with clearance of course.
One of the biggest issues facing the DIYer is understanding tolerances, that's why we had matey boy recently thinking he could set the camber of his car in his own garage to within .1 of a degree and it would matter.
It's grinding tolerance. At the factory they will have ground 1000 cranks per X amount of time, there will have been variations during that time; differences in temp, pressure, wear on the grinding wheels, slight variation in steel, how long Fritz spent on the lapping tongs whilst chatting to Otto etc.
When done they measure them and match the sizes with their factory bearings to get the clearance as good as they can get.
They have lots of different bearing thicknesses available to them which the aftermarket don't do (in my experience of non VAG motors).
Some cranks are colour coded, some letters or numbers dependent on what size as are cylinder bores.
So whilst they are incredibly precise aftermarket isn't.
You can get around this by reading the manual further as it should give you bearing thickness and also by putting the bearings into a torqued up housing and measuring the internal diameter, then grinding the journal to that - with clearance of course.
One of the biggest issues facing the DIYer is understanding tolerances, that's why we had matey boy recently thinking he could set the camber of his car in his own garage to within .1 of a degree and it would matter.
indiatango said:
Haha, well that's just maddeningly unintuitive! Is this a fairly standard thing?
So the maximum tolerated wear is derived from subtracting both minus numbers from the main number?
It's derived from how you specify the fit in the first place. You could be trying to achieve an interference or a clearance for example, depending on application (yes, I know you wouldn't have an interference fit on a crank journal, but this practice is wider than just bearing journals) So the maximum tolerated wear is derived from subtracting both minus numbers from the main number?
The basic number is the nominal size, each of the mating components then has a component tolerance to make sure you achieve your interference or clearance fit as an assembly.
If you wanted a clearance fit, it would make sense that, for example, a nominal 50mm assembly would have the female parts with all the tolerance range positive, and the male part with all the tolerance range negative.
So when you design a part, you call up, say 50mm H7/h6 - because you might know that H7/h6 is the type of fit you want). The female part then gets drawn as 50mm +x / +y and the male as 50mm - x / - y, rather than the female getting drawn as 50.0x +y/-z and the male as 49.9x +y/-z
Have a Google for ISO limits and fits for more background.
Hope that makes sense
Edited by Mave on Tuesday 13th April 00:20
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