Mismatching FD gears?

[CHANGELING]

Ok I think I read somewhere once that it was a big no no to only change one of your final drive gears ( the big ring gear) regardless of whether you set up the backlash, tooth engagement, shims etc Is this right or can you do it?

I realise the wear patterns will be different on the two gears but surely they will find a happy median before total destruction occurs ? Or am I full of s...t.

 

[Colban]

Well the Max BMW parts website doesn't give individual part nos for either gear but gives p/n 3311-231-1186 for the gearset.
I'm not qualified to say whether two "mismatched" gears would work together.
I think i'd be looking for a secondhand final drive.

 

[uncle dick]

Colban is right - find a good secondhand fd

 

[ciderman gs]

i think the fd gears are stamped as pairs so you would have to replace them as a pair if you where to do this then you would have to reshim the drive to the new set of gears

 

[bumpmuncher]

having been through a simmilar problem to yours. and having spoken to various people about it. the reason you dont use missmatched gears is that it makes it very dificult (if not impossible) to set the backlash correctly and if you do it will get out of spec very quickly nessecetating a full strip to reshim it and try and reset the backlash for another few thousand miles. there is the possibility also if the backlash is set too loose that the gears will not mesh properly and lock FD not to mention the posibility of bits of broken teeth floating about waiting to lock the rear end.
if the repair is for your wifes secret lover then go for it but if it is for yourself get a matching gear set or GOOD 2ND HAND FD

 

[CHANGELING]

I paid for a "good" secondhand final drive and when it arrived it has obviously been in use for a while in an "unloved bike" to say the least and compared to my final drive looks ugly, besides when I popped it apart to clean up the insides, check the big bearing etc I found excessive backlash and lateral play on the pinion shaft which is visible to the naked eye so now I have to budget a way to have one drive rebuilt out of two using all the best parts + new bearings shims etc. hence the reason for my post.

 

[nick.ct]

When BMW or any other maker is assembling the final drive units, dont they just take a crown wheel from the crown wheel cupboard and a pinion from the pinion cupboard, and then shim the FD?

If that is so, whats the difference between that and doing the same at home? Assuming both parts are in good condition.

Just wondering as I have always been told that you must used a matched pair too.

 

[ciderman gs]

you might find that the lateral play comes from the needle roller at the end of the input shaft this will also give excessive backlash i would decide which drive is the best and replace all the bearings and then take things from there

 

[MattW]

When BMW or any other maker is assembling the final drive units, dont they just take a crown wheel from the crown wheel cupboard and a pinion from the pinion cupboard, and then shim the FD?

If that is so, whats the difference between that and doing the same at home? Assuming both parts are in good condition.

Just wondering as I have always been told that you must used a matched pair too.

Thinking aloud...
The gearset in a used FD will have 'worn together' - i.e. the contact patches on the gears in the set will 'match' (and I'd guess will slowly wear / flatten with each other).
If you then introduced a new gear to an old partner I'd imagine it'd be difficult to shim the tooth contact properly using engineers blue and working out the contact patch position as they would have slightly different profiles (new vs. old)? I could also imagine accelerated wear (and shimming fairly quickly going out of spec) for the same reasons?

I might be (am probably) completely up the wrong tree though