Topic: Another broken chain hole in the case, thread (with pictures)

[sparkyman]

So, my 750K8 that I got (not running) for $300 has been a great learning experience so far. You don't learn anything if everything goes smoothly, so I'm learning a lot!

Backstory is that the bike didn't run and the engine was seized...... and had no chain. If I was smarter, I would have taken that as a sign. But no.

So I have motor out, took the head off, then the cylinder off and removed and bagged all the parts, pounded out the rusted piston, ready for the machine shop. But I still couldn't turn the crankshaft, so I thought I must have transmission issues, but that's ok, because I plan on going through everything anyway.

I go to take off the front sprocket, and I find a chunk of aluminum stuck in the gears, so they couldn't turn! Take out the two broken pieces and everything moves.

Here it is with the two pieces that picked out with needle nose pliers:



So, if I understand correctly, when I split the cases and remove all of the moving parts, I can get this welded? Some of the threads I read were a few years old, so I'm just wondering if this is still the best way forward?

[Jerry Rxman Griffin aka MuthaF'er]

You can TRY to have it welded. Just not sure what the outcome may be. Worth a try if you can locate a competent welder.

[sdefoor]

Wow. I posed the same question about mine today and was told it was in pretty good shape as far as the chain case breakage. Now i see why they said that. here's a pic of what I am looking at.

[sparkyman]

Wow. I posed the same question about mine today and was told it was in pretty good shape as far as the chain case breakage. Now i see why they said that. here's a pic of what I am looking at.

Yeah, no gaping hole in yours! 

[sdefoor]

Well I feel like I got a deal with mine then. Cause it was $300 with the bike pretty much intact. Minus the crack of course and some serious wiring issues.

[jamesw]

That's gotta the be the largest hole I've seen from chain slap damage.

I'd just source out a new engine.

[uksparky]

Put it back together using JB weld....did the same thing with my FZ1 few years back

[754]

I am in BC, i may have a set of black 77 or 78 cases available In the next 2 weeks.

[danyo]

I'm parting k8 engine if anybody from us needs cases just let me know.

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[HondaMan]

That K7 hole is going to be easier to fix than the other one...
I am just finishing up my K0 engine, which had a chain pitched thru it some 35 years ago, and was welded back up. It worked fine for many years until the last owner took it apart and left it outside for 25 years. With no cover...

[sparkyman]

I'm checking with a couple of local welders to see if they think they can do a good job. Otherwise, I'll be in contact with someone for a new case.

[Rocky2010]

Not good, remember there are chain guards for the crankcase that can be purchased to hopefully stop that from happening.
Never used one but surely it would prevent the breakage. In the early days it was the battery overflow tube which leaked acid directly onto the chain causing some of the chains breaking until they change the positioning of the Battery over flow tube.

http://www.yamiya750.com/index.php?main_page=product_info&cPath=246_21_250_60&products_id=1725

[754]

 It easy to get bottom case stripped for welding.
When you take lower off, the engine should be upside down with topend intact. With a block under back of the case.
 At this point you can strip the transmission and clutch, leave crank in and alternator rotor on.
 Wrap the stuff left,to keep stuff out.
Then if the welder will work with it that way,far less dissasembly.
 And dont let them take the weld all the way down on the case split, they often get low spots.

[sparkyman]

It easy to get bottom case stripped for welding.
When you take lower off, the engine should be upside down with topend intact. With a block under back of the case.
 At this point you can strip the transmission and clutch, leave crank in and alternator rotor on.
 Wrap the stuff left,to keep stuff out.
Then if the welder will work with it that way,far less dissasembly.
 And dont let them take the weld all the way down on the case split, they often get low spots.

Well, actually the hole straddles both the top and bottom case, so I'll have to have both sides welded, then mated well. It's a learning experience! 

[MRieck]

It looks like the output shaft seal boss is cracked too.

[Stev-o]

I'd just source out a new engine.

+1.  I bought an entire bottom end for $100 after I blew a 3" hole in the Big Bang bike.

[PeWe]

A lot of crack that does not make you high, opposite, low mode.
I also see a crack over the outgoing shaft seal. The case is die-casted which means no strange casting marks?

Are all these cracked CB750 cases the result of the non use of a decent drive chain? I'm not surprised if cases cracked in the 70's and early 80's due to the weak chains.

Non o-ring chains were abandoned here in the first half of the 80's by all bikers I knew. The UK bike riders might have used their std chains.
The std 530 chains for CB750 back then were a joke, dangerous too. I had one chain were a lot of the rollers disappeared and chain jumped in low speed when the sides went together.

O-ring and X-ring chains today is something totally different. The O-ring chain I used in the 80's stretched some which I noticed when touring and I had to bring tools with me for release wheel nut and adjust the slack. Could snap if the slack was too much and sudden WOT.

The X-ring chain I have on now has stretched very little after 10.000 km, mostly rather hard riding and more power than before. I think it will be OK for 10.000 km more.
A good expensive and durable chain will be cheap in the long run.

If my K6 case will crack I'll search for a repair by a good welder. The engine number is a part of the bike from the very beginning.

[sparkyman]

I'd just source out a new engine.

+1.  I bought an entire bottom end for $100 after I blew a 3" hole in the Big Bang bike.

Resurrecting this thread as I'm going to get back to working on it tonight.

I think my best option is going to see if I can get a whole new bottom end, since I don't know how much damage might have occurred when the chain blew a hole in the cases and pieces were jammed into the gears inside. Since this is what made the bike not move in the first place, it was probably jammed in there pretty good.

I don't know how this place works for buying/selling. Is my best option to place an ad in the wanted section here? I looked at Kijiji and saw a good motor for $450 but he won't ship to Winnipeg. Too bad.

I think I just need the whole bottom end at this point. Suggestions?

[Stev-o]

Yes, start a thread in the parts wanted section on this forum.  And keep watching your local Kijiji.

Here's a pic of my "Blow out"....

[danyo]

I have k8 engine for sale. One piston is rusted to cylinders. So far can't separate it, soaking it in kroil. Engine has 6000 miles so I believe bottom end is in good condition. I can clearly see honning cross hatch on 3 cylinders. I have no idea how expensive will be shipping to Canada but if you can't find anything just let me know. I'm willing to sell only bottom end (if will be able to remove cylinders) to keep weight down.

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[sparkyman]

I have k8 engine for sale. One piston is rusted to cylinders. So far can't separate it, soaking it in kroil. Engine has 6000 miles so I believe bottom end is in good condition. I can clearly see honning cross hatch on 3 cylinders. I have no idea how expensive will be shipping to Canada but if you can't find anything just let me know. I'm willing to sell only bottom end (if will be able to remove cylinders) to keep weight down.

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I would take you up on that depending on what it would cost (purchase and shipping). We can take this to private message if you like.

I had a heck of a time separating my cylinders from the bottom end due to a rusted piston. I soaked the pistons for weeks, then hung the whole thing from a chain hoist by a couple of points on the cylinder and let it hang and used a little persuasion from a rubber mallet. That didn't really work so I left if hanging about an inch off the table and pounded the piston with a hammer and a wood 2x2. If finally came loose. What a pain.

[754]

The reason I suggested shipping cases only is they are light and easy to ship.
And it's not a bad idea to put fresh main bearings in... and a good checked transmission.
 I have the black cases here now, but Have to wash and scrub to closely check the chain area. .
To open the cases and visually check everything, takes several hours, so that adds to the cost of inspected good parts. Could probably sell a whole lower end but it can contain surprises.
And the difference in shipping cost will be a lot higher..
 Just a few things go consider.