duff crank

[Andymuck125]

ok this is only my second post but its a good one.....

I have a cb750 f1 i bought it a few months back to 'tidy' up and get on the road for the summer. I was told the engine was rebuilt only 2000 miles ago with new bearings rings chains and tensioners which was nice as it started and sounded sweet as a nut with a megaphone exhaust

the only problem was on the way back from the m.o.t station the engine sounded a bit rattly and snatchy. so i pulled the sump off and had a look. the primary chains and tensioner looked like new but the crank sprocket is badly worn and rounded...... typical

my question is this, is it better to look for a second hand crank or is there an alternative like a cb900 longer stroke crank? i read that from satanic mechanic

i was planning to pull the engine appart at the end of the summer to do the usual big bore, head work, cam, carbs etc so it looks like i might be doing it sooner than i had thought

[Andymuck125]

ok i have mabye sourced a couple of good cranks the next question is, wot sort of power and torque can a standard bottom end take? crank and primary chains?? or is there a way of upgrading it to cope with extra power. 

cheers folks

[BassMan_720]

The standard crank of the CB750 SOHC is well over engineered. It can take a good bit of extra power and torque. I played about with my motor back in the early eighties with some significant improvement in power and with metal removed from the crank to reduce rotating mass. I didn't use the tuned motor for long (the bendy frame and 60s brakes were far to scary at speed) but I suspect that the primary chains would have a much reduced life.

Also, if you are looking to top end mods, watch out for weaknesses in the cam drive. Expect to replace chains and tensionner parts regularly.

Steve