Gurus Needed - CB750F Engine Markings

jordandogtown

jordandogtown

Washed up Weekend Warrior
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I've been working on a 1979 CB750F Supersport build and came across some markings at the rear of the engine. It doesn't look like anything that came from the factory, but I don't see why anyone would have gone through the trouble of stamping anything in this area (although I'm sure we've all seen the great ideas POs have had).

Is this the mark of a holy grail 1980's engine builder? Or did some kid manage to get a few whacks of the hammer in while the motor was out at one point?

106817-280119092549-4894674.jpeg
 
My '82 F doesn't have that ledge on the rear of the engine, but now you mention it I seem to remember seeing some weird letter stamps on my '81 K. It's under wraps at the moment but next time I've got the covers off I'll have a look.
 
irk miller said:
Those B's are to match your crank bearings with the case.
Click to expand...

Are you sure it's not some marking that is placed on the bike when it wins the Isle of Man? Or sets a land speed record? ;D

Oh well, at least it doesn't mean "Belongs in the trash" ... yet
 
jordandogtown said:
Are you sure it's not some marking that is placed on the bike when it wins the Isle of Man? Or sets a land speed record? ;D

Oh well, at least it doesn't mean "Belongs in the trash" ... yet
Click to expand...
Haha. You have B case. They had a couple of bores, so you have to match the right bearing for that case. The info is actually buried in the factory service manual, iirc. The same goes for the rods. There's a chart you have to match the scribed numbers on the rods with the case stamp to get it all right.
 
Man I knew I came to the right place. I've got an old Clymer but knew someone on the internet could shed some light on it a littler quicker (dang millennial thinkin')

Hopefully I won't need to use them anytime soon, but I'll know what to do when I do! Thanks Irk
 
Ha. I'm far from genius. Just happen to have seen this before in my own searches. Plus, I'm a master at google-fu.
 
Also full FSM in pdf files here: http://www.cb750c.com/modules.php?name=Forums&file=viewtopic&t=5
 
irk miller said:
Haha. You have B case. They had a couple of bores, so you have to match the right bearing for that case. The info is actually buried in the factory service manual, iirc. The same goes for the rods. There's a chart you have to match the scribed numbers on the rods with the case stamp to get it all right.
Click to expand...

Are there any noticeable differences between the two, performance/reliability wise? It seems kind of strange that Honda would go through the trouble of manufacturing two different cases if there wasn't a significant benefit for one.

(still grasping at straws to make my bike cooler than it is)
 
It was just Honda's way of matching the bore on the bearings and the cranks. No discernible performance variation. Basically, the same reason if you ever have an issue with half of your case, you never swap out just the bad half. You get matching cases and replace both. I'm just guessing, but I bet that they had an issue in production where they accidentally ran two setups. Essentially, you had a dude on the assembly line or in QC measuring the bores and stamping a number on the case according to his measurement. The bearings are color coded (green, brown, etc) to match the stamps.
 
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