Fan Belt Issue

[spacebiscuit]

I have recently purchased a '70 bus which I am restoring. The other day just after starting the engine the fab belt came off. Speaking to a friend he suggested it maybe related to the clutch, I don't remember the exact detail (big end?) but it can be tested by pushing the clutch and watching for movement in the engine compartment.

Any opinions on this and possible remedies?

Thanks,

J.

[Wildthings]

There should be very little fore aft movement in the crankshaft. If you bar the crank forward and and then watch the crank while someone pushes in on the clutch you should not be able to see any movement. Even with some movement, say 1/16" you should not have any problems. If the movement is much greater than this then your engine is getting near the end of the line.

I would be looking to verify that the belt was running true and not jumping all over the place and check that the pulleys are not bent. The belt tightness needs to be in the correct range as well. The wrong match of parts can cause the pulleys to not align correctly with each other.

[airschooled]

Hi J.

Next time you reinstall the belt, look at it from 90° to the side and sight to see if the pulleys are perfectly parallel. There are a few different backing plates that work with different-sized generators in the 68-70 model year cars. One or two parts swapped around could surely offset a pulley by a half inch or more.

After that, carefully rotate each pulley, especially the crankshaft pulley, and look for notches or Vs in the contact areas. After this, start the engine cold without a belt and feel the crank pulley edges with a thin shop glove (non-rubber.) You don't want to feel any vibration in or out, or up and down.

--

Like Wildthings said, the crankshaft end play couldn't be significant enough to cause this issue without other major issues. You CAN test for end play by shoving the crank pulley straight forward, setting a dial indicator to 0 and having a helper step on the clutch pedal. Stock engines should have .003" to .005" of end play using this method. I've driven buses that were decent drivers with slightly low oil pressure with .100" of end play. They were rattley and thrashy-sounding, but their belts all stayed on.

Good luck!
Robbie
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[spacebiscuit]

Thanks guys - it seems your suggestion tallies with the advice that I have also received.

What is the solution if there is a lot play, is it an engine re-build?