Crank measurement.

[G15 Roy]

I had the crank measured today and it was - 60 new shells fitted but still a long way to go.

[TrevorCSR]

A lot of people do not seem to understand what I wrote.
If the crank is standard size and one (or both) journals ‘run' it now measures say minus .038”. Then that wear most likely will be off the ‘top' and nothing off the ‘bottom'. To retain correct stroke you would have to remove metal, mainly from the bottom. Your crank would now HAVE TO BE minus .080”. In a lot of peoples' mind that's then scrap. BUT WHAT IF the grinding machine only takes least metal from top and bottom, it will then clean up at minus .040”! BUT the crank stroke has now lost .080”, 80 thou, 2 millimetres! Your 646cc twin capacity is now down to 630cc. Of course that's presuming both journals have been ground the same. IF not, you've now got the cylinders working differently from each other. AND then, what about the compression ratio? Same as removing .040”, 1 mm from the piston crown.
It would be very interesting to do an experiment by putting a re-ground AMC twin crank in the lathe between big end centres and then clocking the journals.
However, not very many cranks have centre drill holes at the big end journals. AMC twins are quite rare in that respect. Look at most British parallel twins, they don't have them. BSA & Triumph put their sludge trap plugs there. Norton twins do not have them for the same reason. How many car engine cranks have centres? So no good making a grinding machine that can only work from these centres; uneconomical, wouldn't get much use just grinding AMC cranks.
Watch the video carefully. The operator is setting the throw by finding the best point on that journal using a dial gauge. He/she will be looking for the minimum reading on that needle. That is the position the crank will be set for the grinding wheel to take the minimum off. Totally irrelevant to the actual stroke (or big end centres).
TrevorCSR

[zwarts]

Trevor, when I read your post initially, I knew that you had a serious point and one which is not often realised. I didn't respond as I needed to find out some background information. When I saw the drawings for our new 650cc cranks some time ago, from Markus Graf I noticed that the crankpin centres, as featured on the original cranks, were absent. I questioned this as I had always assumed that they were essential for grinding the crankpins. Not so!
Have a look at this video. it will explain how the process should be done and makes an interesting commentary on the dimensional accuracy of production cranks. It's a car crank, but the same principles apply.

[robcurrie]

Trevor, I have run a few bearings in car engines and they wear off all round the journals, not just "off the top". If you do a really good job of it you can also bend the crank slightly which would look like it wore more "off the top" when you check it with a dial guage. You could regrind the bent crank to save it, but it is not recommended.

Rob C

[56G80S]

I got hooked on the videos. Where's the time gone?

And mine's a single so it's a built up crank so no real lessons for me.

Johnny B