Topic: Halloween Horror - my 750 case is cracked at the drive sprocket.

[CycleRanger]

Or "Well I hadn't planned on rebuilding the entire engine just yet."
Or "I always wanted to powder coat the frame....." 

So I was cleaning the grease/dirt oil off the bike today while I had the rear wheel off. The pan gasket needs replaced so there was quite a bit of dirt and oil on the back and underside of the engine. There was a lot of old grease and crap around the drive sprocket.
Imagine (if you can) the feeling in my gut when after a few shots of degreaser and a nylon brush I reveal some PO's JB weld case repair job by the drive sprocket!  Arrggh.

I bought this bike a few months ago and I knew it had thrown a chain at some point in it's life because the drive sprocket cover was missing, the top screw hole for the cover was broken, and the rear hub was slightly chewed.
However, from what I could see at the time that appeared to be the extent of the damage.
( I could also tell after I bought it that it had sat for an extended period of time outdoors. Now I know why.)

I can't blame the guy I got the bike from, I know he didn't do the JB Weld job and didn't know the case was broken and seeing that it had thrown a chain I should have checked the case more carefully before I bought the bike.  *sigh*

Strangely enough I happen to have an almost complete (only missing the side covers) lower end from a '76 750 that I got for $40 a few weeks ago. I didn't really need it (so I thought) but I figured the starter was worth at least that much so I dragged it home.
So If I decide to have the engine rebuilt I already have the case.  Hmm..
But JB Weld is only like $4......

Meanwhile I think I'll start drinking early today..

[Toxic]

You say:
There was a lot of old grease and crap around the drive sprocket.

But was this grime a result of the patch job leaking or just normal  grunge?

I would'nt sweat it unless it's leaking.

Totally degrease the area, test the prior JB weld to make sure it is sound and then I would reapply more JB to make sure the coverage is proper.

I don't think a total tear down is called for.

[750goes]

If it isn't leaking any oil, put it back together and ride it..
just keep an eye on it.

[CycleRanger]

But was this grime a result of the patch job leaking or just normal  grunge?

It appears to me to just be "normal" grunge. I've ridden the bike quite a bit and it never leaked oil from that area.

However, now that I've cleaned that area it appears there is a small hole in the patch.
Thanks for your input.

[blackhog02]

I had the exact same thing happen with my 1st cb750.  I took it to a local welder and he fixed it right up!! 

[CycleRanger]

I had the exact same thing happen with my 1st cb750.  I took it to a local welder and he fixed it right up!! 

You had the aluminum welded up with the engine assembled or you tore it down and then had it welded?
Just wondering.

I did slop some more JB Weld on it last night. I'm going to apply another "coat" tonight and I guess I'll see if it holds....

[the architect]

Sorry to hear about your woes.  I have a similar situation, and I decided to finish cleaning the engine, repaint as planned and reinstall.  I'll keep an eye on it.  I'm the 3rd or 4th owner, so who knows which PO did the deed.  I'll post pics of mine later.  I found some misc shrapnel in the oil pan, too -- related, I'm sure.

So who in Texas can completely and correctly rebuild a '75 CB750 engine?
Meanwhile I think I'll start drinking early today..

I don't know where you are in Texas (I'm in Houston), but I contacted Metric Morotcycles about doing my engine rebuild, a while back.  Pricey, you're right, but I have total faith they'd do an outstanding job.  The only down-side is that you have to supply your own parts.  I can't blame them -- like they told me, they don't really have the time to track down all kinds of parts for 34 year old bikes (mine's a '75).  The place is run by a British guy with a good sense of humor who is more than willing to chat about bikes (or anything, for that matter).  In my case, for a total tear-down and rebuild, I was likely looking at $1k minimum, and I'd be bringing in my own parts -- ouch! A thorough cleaning and paint job was more in my budget.

http://www.metricmotorcycles.com/


EDIT:  This was mine - yuck!



After primer:



Oil pan shrapnel:



Who can identify the pieces?

[the architect]

Thanks for link and the pix.  You can just never tell until it's clean!

I'm in Austin.  If I have to go the rebuild route I'll definitely check out Metric Motorcycles.
 ~$1K is about what I'd expect for a complete correct, guaranteed rebuild.

No problem 

Metric told me he could do a top end only rebuild, if I wanted to save some money -- would be about half the cost, he told me.

[eshumaker]

Sorry to hear about the casing, I discovered last night that I have the same problem. Fortunately I have a master welder in my office and my casing only cracked without shrapnel, so we are going the weld route.

Good luck!

E

[fang]

I just PMed you.  I live in Leander (n. Austin), and I have motors and know how.  Lets get together!

-steve