C72 gearbox problem

pc13cb92 · Post by **pc13cb92** » Sun Aug 05, 2018 2:47 pm

Hi, long term owner of c72 and others, been a watcher/registered on site for many years. competent (!) most of the time at doing most things honda.
However, out today on my recently rebuilt C72 and second gear has just stopped engaging correctly - goes in then feels as though its not engaging correctly and makes horrendous noises. Changes up to 3 and 4 ok, coming back down goes into 2 then makes the noises and doesn't drive, the ok back down into 1.

Question I would like info on, is if I drop the motor out, can I just invert engine and get at gearbox without having to take head and barrels off.

I do have another couple of spare bottom ends so if it needs gears etc changing I should have spares.

Seadog · Post by **Seadog** » Sun Aug 05, 2018 5:35 pm

Yes you can.

pc13cb92 · Post by **pc13cb92** » Sun Aug 05, 2018 5:51 pm

Hi.
thanks for reply, will drop motor out tomorrow.

DJM · Post by **DJM** » Wed Aug 08, 2018 5:04 am

As this seems to have been a sudden failure I'd check for missing/broken gear cotters and set rings (Honda's Parts Book terminology) which could cause your problem.

pc13cb92 · Post by **pc13cb92** » Wed Aug 08, 2018 10:53 am

Hi,
took engine out and stripped it down. no broken bits or worn bush, edges on second gear dogs seemed rounded off, so have swapped for second gear from a donor engine, along with matching gear for other shaft. waitng now for one oil seal to be delivered and then put it all back together. Really glad I didn't have to take head and barrels off.
Will post when it's back on road

pc13cb92 · Post by **pc13cb92** » Fri Aug 17, 2018 3:16 pm

Hi,
dropped motor, stripped and replaced as posted, back together into bike and running, but problem still there. Don't have time to re-investigate as taking bike away in a week. Decided to put complete spare motor in as I know it was running ok when I swapped engines. However decided to check clutch plate before installation and there are only 4 clutch friction discs - thick ones - and it was working.

Parts list picture shows only four friction discs, but says 5 needed. Various posts/sources say various permutations of 5 or 6. Has 4 ever been standard?

Have to decide what to do tonight as its all going back together tomorrow

G-Man · Post by **G-Man** » Sat Aug 18, 2018 1:12 am

Very early bikes had these really thick plates. I have a 1960 C72 with them in.

On your gears problem. Do you think you might have mistaken 3rd for 2nd when the engine was upside-down?

G

pc13cb92 wrote:Hi,
dropped motor, stripped and replaced as posted, back together into bike and running, but problem still there. Don't have time to re-investigate as taking bike away in a week. Decided to put complete spare motor in as I know it was running ok when I swapped engines. However decided to check clutch plate before installation and there are only 4 clutch friction discs - thick ones - and it was working.

Parts list picture shows only four friction discs, but says 5 needed. Various posts/sources say various permutations of 5 or 6. Has 4 ever been standard?

Have to decide what to do tonight as its all going back together tomorrow

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