Motorcycle Repair: 78 honda CB 400, honda cb 400, journal bearings

Question
Hello Michael,

     my name is Jon and i have a problem with my 1978 honda CB 400.  i was riding home last night when there was some sort of explosion that sounded like metal hitting metal.  my bike started running really rough so i tried to give it more gas and that almost stopped it from running.  i pulled over and turned it off and then tried starting it up again.  i kept it in first to try an limp home but as soon as i gave it alittle bit more gas, there was an awful sound of metal grinding and crashing.  it stalled again and would not start up again.  my oil light was now on at this time.  so i ended up leaving it a a grocery store last night.  do you know whats wrong?  any help would be great.  thanks for listening

Answer
Jon,
  You probably suffered catastophic failiure most likely whithin your engine and not in the transmission.  To rule out transmission failiure coast the bike with the cluch in (disengauged) and put it in 2nd gear.  If it rolls when you do and turns the engine so you feel drag when you let it out (engauge) your clutch is fine and most probably your gearbox as well.
 In order to see the extent of the damage you will first need to remove the head and check the condition of your cylinder walls. If they are scored and/or the pistons have sustained damage you will need to rehone the block and install new pistons and rings.  This is the best senario, just "top end" damage however, the fact that you witnessed the oil pressure drop (assuming the oil was not low) makes me think you have burned out your journal bearings in your crankshaft.  These are the bearings that sit where the piston's connecting rod attaches to the crankshaft.  If they wear significantly due to lack of oil or excessive wear from age the fit won't be as tight.  This can make a good deal of noise before total failiure and your oil pressure controlled by the tight fit will now be unobsturcted and drop very low if not all together.
 Replacing the bearings, the crankshaft(if it is worn too) and the oil pump (it may have been the cause) would require you to split the case, very labor intensive.  If you have to go this route you would need to rebuild the top end too and possibly replace some of the gearbox bearings and possibly the cluch too while you have the case apart.  You don't wan't to have to do this again.
 Sorry for the bad news, you now have to ask yourself how much you love the bike before making this kind of commitment (geez, sounds like a girl!).  If you hold no emotional attachment I'd say chuck it, it's a basket case.

Good luck,
Mike