Topic: Suspicious engine-case repair found from thrown chain...

[markreimer]

The things you find on these old bikes..

I've owned this cb750K4 for one season. Last night I was removing the sprockets to replace with new ones plus a new chain. While cleaning up all the road grime and sludge from around the front sprocket, low and behold! I found that the engine case had been repaired by a PO from a thrown chain. I've done enough trolling through these forums to know that this isn't exactly uncommon, and that a lot of people find their engines repaired with J&B, etc.. So I was happy to see that at least this one was welded. However, if you look at the photo, it looks like the repair doesn't provide enough clearance for the chain as it seems that a chain has scored the weld pretty bad.

I tried running a chain through and it seems like there is enough clearance, but I also know that the sprocket has some play in it, so my question is... Is this safe enough to leave? Am I at a bad risk of premature chain wear leading to another snapped chain? I've ridden the bike about 8-9 thousand KM since owning it, and I didn't find any noticeable wear on the outside of my old chain, but still.. this makes me a nervous.

What is behind this section of the engine? It's impossible for me to know how thick the weld is, but could I presumably grind this down to allow for an extra mm or two clearance?

I also noticed how amazingly close the chain is to the engine case as it runs to the rear sprocket. Do any of you know offhand if your chains have more than a mm or so between the chain and the case?

As always, thanks so much for your help!
Mark

[lone*X]

Is the front sprocket the stock size?  18 tooth I think.  If it has been increased in number of teeth it would reduce the clearance.  Dropping what ever size you currently have by a tooth would increase your clearance a little and would give you better acceleration but a a slightly reduced top end. 

If the weld isn't leaking I wouldn't mess with it.

[bryanj]

17 tooth is standard, and the chain will "slap" on the sprocket if its loose when you shut the throttle and can hit the case. The damage to the repair looks old , if it was me I would fit new chain and 17 tooth front sprocket and ride it keeping the chain well lubed and adjusted right

[markreimer]

17 tooth is standard, and the chain will "slap" on the sprocket if its loose when you shut the throttle and can hit the case. The damage to the repair looks old , if it was me I would fit new chain and 17 tooth front sprocket and ride it keeping the chain well lubed and adjusted right

Bryanj - I was under the understanding that 18t was the standard. That is what was on my bike, and that is what I bought to replace it. That said, I like the idea of dropping to a 17t as I'll be doing a lot more riding with a passenger this season, not to mention the added clearance benefit. I don't mind loosing some top end speed... I don't tent to fly past 120km/h very often.

[754]

 18 is stock since 71 or 72..have seen 19 teeth on the front..
 If you run a 17 you should be fine, but I would clean the weld up a bit