Topic: Rotor bolt question

[Jbird]

Happy sunday to all. I just aquired 2 CB 750s, a 72 and a 76. The 72 will be pickled for a while. The 76 I'm gonna make rideable for a few weeks then I'll tear it down for any mechanicals and some sorely needed cosmetics.

    I got the 76 running, it sounds solid but I encountered a problem. The bolt holding the alternator stator to the crankshaft came loose, the stator itself loosened from the crankshaft, I found this out when the starter would spin but the engine wouldn't.
    I put everything back together and tightened the bolt with an impact wrench, everything seemed OK for quite a few cranking sessions then the same thing happened again. I searched through our archives and the only things I found were problems removing the stator. I reassembled everything but this time I cleaned and dried everything and used Locktite on the crank taper, the bolt threads and I got a bit more agressive with the impact. I've only cranked it a few times since then because, now that it's life sustaining juices are flowing freely again it fires up after just a couple of seconds. My biggest concern is having that bolt come loose at highway rpms, it could make quite a mess.

   Has anyone ever encountered this problem before???    Jbird

[scunny]

a torque wrench will be the better tool to use

[kghost]

If memory serves ...the bolt has a plastic insert

Acts as a lock......

[Clyde]

I assemble dry and use a torque wrench.
Make sure there are no marks/burrs/dents on the taper.
Clyde

[bryanj]

Original 750's had a problem with this and cure was:-

LIGHTLY grind in the rotor taper to crank with a small amount of fine grinding paste WARNING make sure you remove ALL reidue

Torque up the bolt to recommended torque

[Jbird]

Thanks guys, I'll check to see if it's loose again when I replace the, now leaking, alt cover gasket, I'll probably pull the rotor anyway just to lap it in to the crank taper.

  Torque wrench??? you mean that long breaker bar thingy with all the numbers on it, I thought it was broke, it clicks everytime I use it to tighten up hose clamps and such.

When you've used the same torque wrench and impact wrench for thirty some odd years you can get pretty close with the impact before you use the torque wrench. I do have my torque wrench recalibrated every few years. The last time I checked, a torque wrench calibrating machine was somewhere around 50K USD.

   Thanks again Jbird

[Bodi]

The rated bolt torque avoids stripping the threads in the crank or the bolt: you're not going to crush the crank end or bust the rotor open. The only thing holding the rotor from turning is friction in the taper, I've thought a key would be nice more than once. Lapping the two together is the best fix, and making sure both parts are clean and dry is vital. I have had to lap them together after crankshaft work, the machine shop wear and tear damages the taper slightly and slippage can result. Bikes with the starter driving the rotor have trouble because the starter rotation "unscrews" the holding bolt. On a running engine slipping occurs on power pulses which tends to tighten the bolt so the taper locks up tighter.

[bryanj]

Why pay to have your wrench calibrated use math and a quality spring balance or if really fussy accurate weights

[Jbird]

74chopper, thats what I was thinking too until I read Bodi's post.

bryanj, My tool rep gets it recalibrated for me really, really cheep, he sends it in as a warranty thing.   Jbird

[Chris Liston]

    I got the 76 running, it sounds solid but I encountered a problem. The bolt holding the alternator stator to the crankshaft came loose, the stator itself loosened from the crankshaft, I found this out when the starter would spin but the engine wouldn't.
   

Curious, what made you check this?  I've got the same issue on my starter.  It spins but doesn't spin the motor.  TwoTired said its probably the roller springs on the starting gear are stuck.  This is a 550 of course and your 750 is probably different.

[Jbird]

I pulled the alternator cover off to see what was up with the starter drive gears, when I touched the rotor it spun on the crank with very little effort. I checked the bolt and it was quite a few threads loose.  I don't know if the 550 is the same.    Jbird

[bryanj]

550 is diferent as starter gear is "inside" the crankcase