cant keep drive gear tight on crankshaft 1972 cz400

[jimg1au]

hi
i have tried 3 times to get this motor right. thought i had it this time but while doing a plug chop i spun the drive gear again.i have lapped the sleeve and the gear in twice now.it just wont stay put.the crank is not in the best shape as it looks as if this is not the first(3rd for me) time this has happened.i want to race this thing on sunday but it wont go now.yesterday had it running sweet in the driveway.easy to start and it sits and idles well.revs clean.last rev a little longer and tested the kill switch.took it out today and it wont turn the engine over just spinning on the crank.any ideas to get this gear to stay
cheers
jim

[hotdog1975]

forgive my ignorance..never worked on a CZ...is the gear on a taper & nothing else..?? if it's not splined or on a woodruff key could a woodruff key or square key be retro-fitted..?? or not..?? just trying to understand your problem.

[jimg1au]

its on a taper only.there is a sleve between the taper om the crank and the taper on the drive gear.ca change the design in 1973 for taper and woodruff key.i think i might have fixed it today after 4 hours work just on this part.fingers crossed
jim

[pancho]

 jim have you tried lapping the drive gear first with a piece of bar shaped into a mandril with the exact same taper machined onto it, then machine a female version of the mandril and lap the taper on the shaft. when you've done that lightly lap the drive gear onto the shaft and check for high spots, if there are some, lap it heavily then re-machine the taper on the "female mandrell" and try it again.if it looks o.k. lap the drive gear direct onto the shaft, clean off all the lapping paste thoroughly with carby cleaner and do it up bloody tight. DONT be tompted to put in a drive key because taper drives NEVER drive through the key. cheers and have fun sunday, wally.

[jimg1au]

something like that hope it works.will find out friday morning

[hotdog1975]

hope it works....could you fit the later model crank...??

[jimg1au]

a later modle crank will fit i will need the drive gear as well
cheers
jim

[Tim37]

The early zed crank runs a special sleeve with grooves in it between the male crank taper and the female gear taper . If it is allowed to spin, [usually due to the nut not being tight enough ]
it removes matierial from all three parts ,if  mild somtimes a new sleeve may fix it .

 Yours has spun three times , best  get the crank and gear metalsprayed and reground and fit a new sleeve . Bodging about never works, the gear will loosen at the most anoying time ie- before your first race or when your leading .

The only way to do a bodge is by assembling with hydrochloric acid but you will never get the thing apart again ! I had a motor like this had to cut a groove right though the primary gear to remove ..unfun ! Good luck Tim

[jimg1au]

i fixed it and so far it has held together,if it spins again the motor will have to be split and i will have to do as you say tim
cheers
jim