Crank measurement.

Information relating to the Matchless G12 or AJS Model 31 650cc twin
G15 Roy
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Crank measurement.

Post by G15 Roy »

Can any one tell me the measurement for -60 on a 650 Twin crank.
Roy
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dave16mct
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Re: Crank measurement.

Post by dave16mct »

Standard is 1.62575 - 1.62525. So -60 thou (-0.060") = 1.5675 - 1.5625.
Hope thats right! :D
Cheers Dave.
G15 Roy
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Re: Crank measurement.

Post by G15 Roy »

I have a measurement of 01.5900 is that still -60. :)
Roy
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dave16mct
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Re: Crank measurement.

Post by dave16mct »

Hi Roy,
-40 is 1.58575 to 1.58525,
so it looks like you might get -40 with a very light grind. (oooh!) :rofl:
Cheers Dave.
PS a mate of mine is desperate for -10 centre mains.
TrevorCSR
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Re: Crank measurement.

Post by TrevorCSR »

I learnt a long time ago that if a crank has 'run' it will wear the top of the big end, i.e. against the compression. So a crank journal might read just over minus 40 thou, BUT most of that is from the 'top'. Now the crank re-grinding machines are very clever, they will 'even' this, so it takes the minimum off top and bottom. So you will get it ground to minus 40, BUT the throw (stroke) has altered. To put this into context, IF only one big end has 'run' you will end up with a crank having un-equal throws (stroke). What will that do to the balance and general running of the motor? I had it with a six cylinder car engine, and one throw was around 1 mm shorter than the other five. Needless to say I never installed that crank. BUT on a six hardly likely to have made much of a difference, BUT a twin?
Apparently only the factory machines could grind to a measured stroke. AND of course anyone who has a similar machine, though they must be very expensive.
John Jarrett
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Re: Crank measurement.

Post by John Jarrett »

Trevor,
I must be missing something as I am struggling to understand your post and the alteration to the stroke on regrinding.
Surely whatever the wear on the big ends, the centres in the crank will always be used for regrinding meaning that the stroke is always the same as the centre never changes?
I am happy to be educated on where this logic is wrong.
Kind regards,
John
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dave16mct
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Re: Crank measurement.

Post by dave16mct »

Blimey Trevor I never thought of that. I wouldn't have thought it would make much difference to our old clunkers though, they are 360 degree cranks afterall, and were only talking about a thou or two (thats less than a midge's isn't it??)
I was a woodworker who worked to sixteenths not thou's!! :beer:
Cheers, Dave.
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Stuoyb
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Re: Crank measurement.

Post by Stuoyb »

I agree with John, stroke is measured from the centre of the crank pin.
The relationship between crankpin & conn. rod is constant despite the removal of material from the worn journal.
The larger gap between rod & pin is taken up by thicker (undersize bearings).
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clive
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Re: Crank measurement.

Post by clive »

Stuoyb wrote:I agree with John, stroke is measured from the centre of the crank pin.
The relationship between crankpin & conn. rod is constant despite the removal of material from the worn journal.
The larger gap between rod & pin is taken up by thicker (undersize bearings).
If I understand correctly what is being suggested is that when the crank "runs" the wear is greater on the compression side of the journal. The regrind machinery works to minimise the amount removed whilst leaving the journal circular rather than as worn (oval), and so will tend to make the circle off centre from the original centre of the crankpin. As the maximum regrind is only 60 thou as far as I can see the maximum change in stroke, if all the wear was only at the point of compression and only affected one of the two journals, would be 30 thou and as the pistons are moving together I doubt it would have much effect.
clive
if it ain't broke don't fix
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Stuoyb
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Re: Crank measurement.

Post by Stuoyb »

The crank grinding lathes that I've seen all use the crankshaft ends as datum points so that it is centred.
The lathe then turn the crankpin to be re-cut around the fixed grinding wheel so that the crankpin is always ground to the centre line, not around the point of greatest wear.

Last edited by Stuoyb on Mon Apr 11, 2016 5:21 pm, edited 1 time in total.
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