Setting up a Crankshaft and bearings on a CB 400
A little Q & A from Classic Motorcycle Mechanics to help you get through setting up the crankshaft and bearings on a CB 400. Click the image to enlarge the piece so you can see all the info better, or just read through the text below.
Can you help me with setting up a crankshaft and bearings in a 1975 CB 400F?
I have some new shells sealed in genuine, original Honda wrappings. The part number printed on the boxes matches with a Honda parts list so I can tell the bearings are supposed to be ‘yellow’.
However, despite all the manuals mentioning coloured marks on the back of shells, which indicate their sizes, I can’t find them, even on brand new shells. I’ve also stripped a crank from a second motor. All I can find are part numbers like ‘D5A-A’ and ‘D4 J-A’ imprinted on the
back of the shells for adjacent big-end bearings on the same crank. Again, no coloured marks have survived to help me, if they ever existed.To complicate matters even further, both these different identification marks are imprinted over ‘STD’, which I take to mean ‘standard’, but standard what? There is no ‘standard’ size for these old Hondas.
It’s supposed to be a case of measuring the size of the crankcase in-line boring, measuring the diameter of the crankshaft journals and then selecting the correct size shells by colour to get the correct running clearance. Same technique for con rods too. Unfortunately, it is just practically impossible to do this. It would also be impractically expensive to buy all the different size shells and then selectively assemble. Can/should the crank/big ends be assembled and then Plastigauged for reassurance? Again, nowhere can I find what the running clearance of the bearings should be. I am completely baffled. Help!
John Clement, Victoria, Australia
You’re quite correct in that the shells were originally colour coded to match the machining tolerances. Think of it this way: The crank journals and bearing housings in the cases and con rods would have been machined on the production line to a target size. After machining, they were then measured to see how close they were to target. Bearing shells were then selected from one of three sizes (thick, medium and thin) to bring the bearing clearances to bang on target. This practice is called ‘grading’ and is commonly used to ensure a precision fit (BMW use a similar process to match pistons to cylinder bores, for instance). The three bearing shell sizes were marked with a lick of paint on the edge to identify the grade, though all three shells were stamped ‘STD’ on the back – which is, in fact, quite meaningless as no ‘oversize’ shells were ever supplied for regrinds. The numbers stamped on the crank webs and crankcases are codes, which identify the precise measurement of the bearing housings, and can be used to select the graded shells on the production line.
However (and this is where it all goes pear shaped), the numbers will only tell you what size the journals were when the crank was new – not after 30 years and half a million miles – so they aren’t useful now. Truth is the difference between graded bearing shell thicknesses is so tiny as to be academic, and standard practice when reconditioning an old engine is to fit the thickest shells to compensate for wear. These are usually coded black.
For a more accurate check, use Plastigauge. You should be getting a reading of 0.041 to 0.071 if the bearing clearance is still within factory tolerances. RG.
