Topic: Strange loud crack coming from engine - If your cases are split, a MUST read.

[GammaFlat]

The last time I rode my CB750 K6 I noticed it was making an occasional random "knock" noise while idling on the side stand.  I stood the bike straight up and it quit.  I didn't like it but tried to imagine it wasn't serious.  

I took the bike for a ride and was satisfied that the noise quit.  My happiness was interrupted by more noise on the back part of my "out and back".  At about 60mph the noise was much louder and serious sounding.  I got back to my place and made a couple small circles and the noise became persistent.  It made this loud "crack" every time I shifted into second gear now.  

I told my tale to many people trying to get ideas of what was wrong.  It was becoming obvious that if I wanted to ride the bike again, I'd have to split the cases.  It was also apparent that my problem wasn't "garden variety".  

Fast forward many months...

The cases are split and the below picture tells at least part of the story.  The dog was trying real hard to engage when it probably shouldn't have.  I didn't realize yet that this picture told the whole tale.  One dog in the trans looked a little messed up but not near as bad as this gear hole.  There were 3 or 4 "bad" gear holes this being the worst.  

I now knew where the noise was coming from but why?...  

Go back to the picture.  The snap ring that holds the gear in place is not in the groove.  The gear was "walking" toward the dog and attempting to engage while the transmission probably had something else in mind.  My bike has 18K miles and the engine didn't look like it had ever been apart.  My guess is that the ring was not installed properly from the factory.

I'm hoping to have my K6 ready for the Indiana portion of "the ride".  Apparently it wasn't my last ride on the K6 after all.  

[scottly]

The snap ring that holds the gear in place is not in the groove.  The gear was "walking" toward the dog and attempting to engage while the transmission probably had something else in mind.  My bike has 18K miles and the engine didn't look like it had ever been apart.  My guess is that the ring was not installed properly from the factory.

Major deja vu!!!
I bought a K7 parts bike with less than 6k in the early '80s. While doing a compression test with the bike on the side-stand, it went into gear all by itself, moved forward folding up the side-stand, and fell over on me, pinning me against the garage wall with one leg folded under me and the other under the bike. It was 95 degrees, gas was leaking all over, I was stuck, and there was no help around...
Long story short, I split the cases, and found TWO snap rings in the trans that were not seated in their grooves; with the bike on the side-stand, a gear that was not supposed to move slid on it's shaft until it mated with the dogs on the gear that was supposed to move. I still have no clue why the snap rings weren't in their grooves. Before I split the cases, I looked for evidence of tampering, but near as I could tell, the bottom end hadn't been apart before, nor the motor out of the frame.

[Hush]

Note to self, K7 may have production line fault, check snap rings that hold gears in place.

[Fritz]

The snap ring that holds the gear in place is not in the groove.

Hi Gamma,

do you remember the orientation of the snap ring? Maybe it has been installed the wrong way around. The "Honda Common Service Manual" has a lot of explanations on how to do things right. Maybe the guy in the factory who assembled your tranny did not read it 40 years ago

[Bob Wessner]

Scotty,

.. it went into gear all by itself, moved forward folding up the side-stand, and fell over on me, pinning me against the garage wall with one leg folded under me and the other under the bike. It was 95 degrees, gas was leaking all over, I was stuck, and there was no help around...

Well, don't keep  us in suspense, how did you get out from under? 

[KB02]

When I first got my F1 ('76 model) it had the same thing happening. When the bike was tilted to the left the knocking got very load and persistent (the gear was falling into the dogs). When the bike was tilted to the right the sound went away (the gear was sliding back where it should have been). It was the snap ring that was not in place, same as yours. I knew that my engine had been apart before, though. I blamed the last guy to dig around in there.

Maybe there was a time between 76 and 77 when there were some bad snap rings?

[chickenman_26]

Hi Gamma,
do you remember the orientation of the snap ring? Maybe it has been installed the wrong way around. The "Honda Common Service Manual" has a lot of explanations on how to do things right. Maybe the guy in the factory who assembled your tranny did not read it 40 years ago

Or maybe it was assembled on a Friday night after happy hour. I think you nailed it, Fritz. I blew the pic way up for a better look, and it does appear the chamfered side is facing away from the gear. I think I'd have a peek at the other snap rings on those gear shafts. Also inspect the shift fork and the dogs on the slider for damage.

Stu

[Fritz]

I blew the pic way up for a better look, and it does appear the chamfered side is facing away from the gear.

That's what I was thinking. The specular lights on the edge of the snap ring made me belive that it is facing the wrong way up in the picture. But we don't know if the pic is showing the situation as it was directly after disassembling the transmission.

Cheers
Carsten

[IHWillys]

...Maybe the guy in the factory who assembled your tranny did not read it 40 years ago...

Nah, he shouldn't have to read it, the poka-yoke failed...  hehe.

Ken
(BS Industrial Technology)

[GammaFlat]

The snap ring that holds the gear in place is not in the groove.

Hi Gamma,

do you remember the orientation of the snap ring? Maybe it has been installed the wrong way around. The "Honda Common Service Manual" has a lot of explanations on how to do things right. Maybe the guy in the factory who assembled your tranny did not read it 40 years ago

That is very interesting.  I wondered whether the ring could have "jumped out" of it's groove in any way.  The ring is still on the shaft.  I'll look for a chamfered edge.  This may be a deeper detail of the mystery. 

[GammaFlat]

It was definitely on backwards based on the document previously posted.  Wow.  I would have never known.  Now I wonder if it was never seated or we had one mischievous Honda mechanic with surprises to come for many of us (installing them backwards). 

On one hand, it really seems like it would be difficult for that ring to come out by itself.  On the other hand, it is a spinning mass with centrifugal forces acting on the snap ring, there may be large thrust forces and the chamfer would "help" it climb out of the groove. 

Without thinking about it, I would have said that the chamfer "looks" correct facing outward.  Upon reflection, it makes no sense at all. 

Okay, it's time for everyone to split their cases   Seriously, if your cases are split, it seems to make sense to check them all.  I am not sure I'd go as far as replacing them if you have to remove them.

What are other's thoughts on the "replacement rule".

[GammaFlat]

Update - This gets weirder as it goes.  Thanks for all the input - The document from Fritz was especially helpful.  

The below picture shows "witness marks" (I stole that term from HondaMan) that this ring WAS in the groove and was being tested by thrust.  You can see where the ring was pushing up against the splines.  It finally failed and jumped out.  

HOLY SMOKES.  The orientation of the ring is critical.  As Fritz's document points out, the chamfered edge must not be tested by thrust.  In other words, this ring should go on with the chamfered side facing the gear that wants to push on it.  

[Fritz]

At least, now you know the reason why you need to get new gears ;(
I recommend reading http://www.vf750fd.com/vf750f/Honda_Common_Service_Manual.pdf to anybody who is working on bikes.
Split my case two weeks ago just to change the cam and primary chains. All snap rings in my transmission where installed correctly but I found a different problem that made me replace the counter shaft and a gear...

So, before opening your motor, you'd better check your wallet

[GammaFlat]

So, before opening your motor, you'd better check your wallet

On the good news side, your quote from the book and posting the book may help preserve some member's wallet contents.  Understanding "how a snap ring is supposed to be installed" and "that they should be replaced" will probably benefit my wallet in the long haul. Thanks!   ...and thanks to HondaMan for his book and his help. 

[fmctm1sw]

This thread is scaring the hell out of me.  I'm sure hoping my 550 went back together right...

[scottly]

GammaFlat, the damage in your picture doesn't look all that bad; the gears may not be toast. How do the dogs look on the mating gear? My snap ring was at least 1/4" away from it's groove, IIRC, which would have allowed for full, or near full engagement of the dogs.
Fritz, now that you mention it, I do recall noticing there was a rounded side of the clip, and I think I reversed it, but it was a long time ago. I still can't understand how enough side-load could ever be developed on the gear to force the snap ring out of the groove?   

[scottly]

Scotty,

.. it went into gear all by itself, moved forward folding up the side-stand, and fell over on me, pinning me against the garage wall with one leg folded under me and the other under the bike. It was 95 degrees, gas was leaking all over, I was stuck, and there was no help around...

Well, don't keep  us in suspense, how did you get out from under? 

Sorry...
My left leg was folded under me, with the left end of the handlebars (clubmans that I had temporarily installed, with no rubber grip on the left side) resting on my leg just above the knee, firmly pinning me to the floor. I managed to get the handle bar off my bleeding leg (wearing cut-offs at the time), rolled a bit to the right to get my leg out from under me, then somehow worked my right leg out from under the bike. I got the bike back up on the kickstand, which was a Herculean task, but with all the gas, I had no choice. I knew I was going into shock. I made it back into the house, wiped the blood off, and fainted onto the couch. The bleeding wound wasn't severe, but 500+ pounds of Honda focused on a 7/8" tube was quite painful. My lower thigh and knee were quite swollen for about a week.

[GammaFlat]

GammaFlat, the damage in your picture doesn't look all that bad; the gears may not be toast. How do the dogs look on the mating gear? My snap ring was at least 1/4" away from it's groove, IIRC, which would have allowed for full, or near full engagement of the dogs.
Fritz, now that you mention it, I do recall noticing there was a rounded side of the clip, and I think I reversed it, but it was a long time ago. I still can't understand how enough side-load could ever be developed on the gear to force the snap ring out of the groove?   

Only one of the three dogs was damaged and it wasn't bad (see below).  Oddly, three of the six mating holes were damaged in varying degrees.  I think the picture in this thread is the worst one of the three.   

I don't know what to say in terms of sufficient side load to cause the "jump" of a snap ring except to note that all evidence points to the notion that it happened.  I will be certain to examine any snap rings I have access to while inside an engine but I won't be tearing down an engine "just in case".  I think I'll also replace any that I have to remove (per Fritz's book).

[scottly]

I don't see that much damage; if it were me, I would be tempted to just put it back together. This is a stock motor, correct?

[GammaFlat]

I don't see that much damage; if it were me, I would be tempted to just put it back together. This is a stock motor, correct?

I'm lucky enough to have a spare trans so I may replace both gears.  What does back cutting cost? 

[scottly]

I'm lucky enough to have a spare trans so I may replace both gears.  What does back cutting cost? 

Don't know: I did it myself on my 836 with a Dremel, and it was worse than your gears. It used to pop out of gear at the worst times, like WFO@8K RPM. In fact, the popping out of gear was one of the reasons I bought the K7 in the first place. I later learned about back-cutting the dogs, which I did on the 836 K1 gear-box. The last time the K7 moved under it's own power was when it fell on me....

[scottly]

Only one of the three dogs was damaged and it wasn't bad (see below).  Oddly, three of the six mating holes were damaged in varying degrees.  I think the picture in this thread is the worst one of the three.   

See the witness marks at the bottom and top edge of the dog? These are the only points of contact, at least in this pic. I would grind a slight bit off the worn area at the bottom at the most.

[chickenman_26]

Update - This gets weirder as it goes.  Thanks for all the input - The document from Fritz was especially helpful. 

Here's another one for you. Suzuki had some problems after transmission repairs from mechanics not understanding circlip installation, so they issued this bulletin in '78.

[GammaFlat]

Here's the summary I'm gleaning from all of this. 
When inspecting or re-installing snap rings:

• chamfered edge toward the thrust^{(I, II)} or another way to put it, sharp edge out - in this case chamfer towards gear
• use new snap rings after removal^(I) - I'm sure lots of folks would argue/ignore this point
• after installation, rotate the snap ring in it's groove to ensure it's seated^(I)
• protect the ends of the clip by rotating clip until clip ends are covered by splines^(II)

^I Honda Common Service Manual http://www.vf750fd.com/vf750f/Honda_Common_Service_Manual.pdf Honda Motor Co., LTD 1988
^II Suzuki Service Bulletin dated June 23, 1978

Thanks to chickenman_26 and Fritz for the documents. 

[rklystron]

Awesome read. I never would have guess this. My cases are split and I will take a look to be sure they are installed correctly. Thank you all for this post.