Topic: Restoring my k0 cb750

[Cragdog]

I have a 1970 cb750 k0 and I rode it for a long time before I had knocked the lower case on a cement bench riding off-road around our local camprgrpunds and it's been out of commission ever since. I hit the alternator area and probably bent the crankshaft and ruined my cylinders, but that's what I need to find out! This is my first time opening up an engine but I've tinkered with bikes for a few years now. I have the chilton manual for the bike too. I'm posting here to show some pictures and ask for help when I, inevitably, run into snags! I have taken the valve cover off and am going to try and get a look at what the cylinders look like tonight, but we'll see what happens...

[70CB750]

You can download shop manual here:

http://forums.sohc4.net/index.php?topic=17788.0

better than clymor, IMHO.

Also, there is The Book    - http://www.lulu.com/us/en/shop/mark-paris/my-cb750-book-hardcover-edition/hardcover/product-16107855.html

Mark Paris is local guru, there is a coupon for his book every so often published here:

http://forums.sohc4.net/index.php?topic=65293.0

The book is a hands on how to for what you are going to do plus more.

Good Luck!

[edwardmorris]

+1 on the shop manual and Hondaman's book, first timer myself, these two books complement each other very well.

[Cragdog]

You can download shop manual here:

http://forums.sohc4.net/index.php?topic=17788.0

better than clymor, IMHO.

Also, there is The Book    - http://www.lulu.com/us/en/shop/mark-paris/my-cb750-book-hardcover-edition/hardcover/product-16107855.html

Mark Paris is local guru, there is a coupon for his book every so often published here:

http://forums.sohc4.net/index.php?topic=65293.0

The book is a hands on how to for what you are going to do plus more.

Good Luck!

Thanks for the Links! I really want to purchase Mark's book, but I'm in a bit of a bind with funds at the moment, but hopefully soon I will be able to order a copy. I'm still trying to post pictures and having a tough time with it!

[Cragdog]

Okay, so the files were too large and it doesn't accept .jpeg. Let me try these:

[70CB750]

Is the crankshaft actually bent?

[Cragdog]

Is the crankshaft actually bent?

I haven't been able to determine that yet. I can turn the engine over with the kicker but I can see the magneto wobbling when I do. Once I split the cases I'll have a better idea, but like I said the nuts and bolts to get into the heads are so tight. I'm gonna try and get in there tonight if I can.

[scottly]

Post some pictures of the actual damage; from what you've described, the cylinders and top end should not be affected. Do not do any more top end dis-assembly until you know what is really wrong.

[Cragdog]

I don't have the cover anymore so it's hard to get an idea of what's damaged internally without splitting the cases. I have the heads off now so I'm getting closer to figuring out the issue, but when I turn the kick starter I can see there is a wobble on the alternator side of the crank.

[Cragdog]

I got the bolt securing the rotor to the crank, but I don't have the special tool to remove he rotor.. Any ideas???

[scottly]

The rotor itself may be bent, not the crankshaft. At this point, do yourself a favor and buy the proper rotor puller. Once the rotor is off, you can measure any "wobble" in the crankshaft with a dial indicator; this should have been done before any top end dis-assembly... 

[Cragdog]

Yeah, I'm gonna order that part and crack the cases anyway. It's not been any inconvenience to me opening the top end, I was curious to see what the insides were looking like anyway. This will also give me a chance to clean everything as well. I'm just happy I've finally been putting time into this bike and less time into my shadow.

[scottly]

Pull the rotor and check the end of the crank BEFORE spitting the cases!!!

[754]

Check the crank run out before splitting cases.
 If it has runout ,inspect around the dyno side main bearing area for cracks.
I smacked a rotor worse that that but it was around a 90 mph impact.
 Did it split your dyno cover open? Got any pics of it, I would like to see that.

[Cragdog]

Unfortunately the wreck happened a while back and I don't have any pictures or the case because my gf stole my bike out from under me and took it to a local mechanic who was going to instal the new case but his diagnosis was that there was more damage to the motor than worth a rebuild. So when I came back home from a Colorado trip I learned about all this. I guess they threw out my old case thinking they were gonna sell the new one to my gf and fix my bike, but when I got back I told them I didn't want to pay for the $200 case and I'd take the bike home and figure it out myself. I was working on getting my shadow running for a while so the cb has been sitting in my garage and now that I fixed my shadow and have a running bike I'm now focusing on the cb again. Long story short, i think the mechanics made it worse sounding than it actually is and i like to work on my own machines myself even though I still have a ton to learn.

[scottly]

Long story short, i think the mechanics made it worse sounding than it actually is and i like to work on my own machines myself even though I still have a ton to learn.

The mechanics may have made it sound worse than it actually was, but you may have made it worse by your eagerness. If indeed the rotor is bent, and not the crank, you could have repaired the damage with the motor in the frame and without touching the top end. You now have a major job ahead of you just to get back to where you started...   

[Old Scrambler]

Crapdog..............you getting very good advice from Scott and Frank. I have a K1 motor with a bent crank snout (the rotor end). Mine also has a slightly crushed bearing race so both cases are junk.  Yes, try to get the 'run-out' measured before you split the cases........

[754]

What happened to you when it hit the bench?
When my rotor got hit, my foot got crushed, broke at least 8 bones..

 Crank has to be checked for runout in the cases not outside the cases.

[Cragdog]

Crapdog..............

Not funny if you were trying to make a joke.

Long story short, i think the mechanics made it worse sounding than it actually is and i like to work on my own machines myself even though I still have a ton to learn.

The mechanics may have made it sound worse than it actually was, but you may have made it worse by your eagerness. If indeed the rotor is bent, and not the crank, you could have repaired the damage with the motor in the frame and without touching the top end. You now have a major job ahead of you just to get back to where you started...   

Well I was going off the advice of the mechanic who had looked at my motor last, he told me that my crank was bent and because of that my cylinders would most likely be gouged and need a whole rebuild. This is the reason I began with the top end because I wanted to see if this truly was the case. I'm going to wait until I get that part to remove the rotor before i do anything else. I appreciate all of your guys help/input on this. Once I do this I will measure any run-out in the crank and let you know my findings.

What happened to you when it hit the bench?
When my rotor got hit, my foot got crushed, broke at least 8 bones..

 Crank has to be checked for runout in the cases not outside the cases.

Fortunately for me the impact was all taken right on the rotor and it nearly stopped my in my tracks so my foot was saved! I had to return to the campground alone with nothing but a truck, and a sheet of plywood to get it back home. Man that was an interesting test of my will...

[Cragdog]

Can anyone explain to me how exactly to use the special tool? My clymer (vague as always) simply says to use the special tool to remove the rotor, from what I've seen online, it sounds like you just thread the tool into the rotor and smack it with a hammer a couple times and the rotor just falls off??? Is this correct? Also, does anyone have any qualms about just going to the hardware store and buying a 20mm hex bolt instead of trying to get one from honda or ebay???

[edwardmorris]

Can anyone explain to me how exactly to use the special tool? My clymer (vague as always) simply says to use the special tool to remove the rotor, from what I've seen online, it sounds like you just thread the tool into the rotor and smack it with a hammer a couple times and the rotor just falls off??? Is this correct? Also, does anyone have any qualms about just going to the hardware store and buying a 20mm hex bolt instead of trying to get one from honda or ebay???

Lots of help on the topic http://tinyurl.com/lxzd7zb

[scottly]

The puller threads into the rotor and gets tightened up against the end of the crank; if the rotor won't come off after tightening as much as you can, then you TAP on the puller to pop it loose. A hardware store bolt may work, but be aware that the end is not relieved and most likely not square, and may dig into the end of the crank.
I don't understand how bending the end of the crank could gouge the cylinders?? If the crank is indeed bent, the case is probably damaged as well, and it may be easier and cheaper to just buy a whole motor.   

[Cragdog]

I don't understand how bending the end of the crank could gouge the cylinders??

I'm not really sure what the mechanic was thinking. He painted a really gloomy picture of my motor to me. His notion of the damage was that because the crank was bent (his opinion, not mine) in the impact, then the forces transferred through the connecting rods and into the pistons which could've gouged the cylinders.

If the crank is indeed bent, the case is probably damaged as well, and it may be easier and cheaper to just buy a whole motor.   

And that's exactly what he told me to do, part out my motor and buy a new one. BUT, because I've always been fond of this bike and the fact that it is a low engine number I didn't feel right parting it out if I could fix it myself. I decided not to listen to him (he wasn't getting paid for the job anyway and I don't know how much time he actually spent diagnosing this "bent crank", he thought he was just replacing a case..) and post about it here instead. Hopefully I can get the rotor off next couple days and find out for sure if my crank is damaged. I'd be super stoked if all I had to replace was my alternator.

[754]

He end of the puller must be square or it can damage a good crank. I torque itup a bit, hit the end tighten a bit more if it will, hit again.

[Old Scrambler]

My apologies.............old eyes and bifocals...........