Topic: Need Help IDing some CB750 Parts

[jaguar]

So i bought this CB750 motor that im almost sure has a yoshi 811/812? kit in it.
The rods are what has my puzzled though

They are labeled RSC.  isnt that honda racing?  cant find any info on google about them
The look thinner then stock and are polished.


I know its a crappy pic, ill get more and better tonight.



They say POP Y  thats yoshi right?
why do they look like the valve pocket are two valves cut over each other?

I also have an "odd" cam that ill get info on.

Thanks so much for any help

[Bill/BentON Racing]

All bad stuff,way old,no good,send to me,I'll help you out and get rid of it.We'll hide it in another engine! You have Yosh 812 kit,RSC racing rods,nice,still need to check them,cam  pics? Fly cuts are for different so you can use pistons in which ever cylinder.Good luck,you get any other goodies,check trans,cam chain,tensioner etc.Bill

[jaguar]

What are rsc rods?

[Bill/BentON Racing]

Racing Service Center,I think.Honda's high performance factory stuff/dept.The early name,then HRC,HPO,high performance options.Yosh sold a lot of RSC stuff.Other names maybe,but factory race parts,#'s from Honda ended in 971 for CR750 stuff.Bill

[jaguar]

OK so these are yoshi pistons.  what rings do they use?  seem to remember something about cb350 rings?







So whats this cam?


This doesnt really look stock....


Mild port cleaning


not sure what year the motor is




Looks like the crank is modified in some way


So about those rods, how are they compared to other rods?

[Sam Green Racing]

Jag, they would be similar to what were in the Daytona bikes or the ones offered in the CR kits.
Would they be as good as modern forged rods? I doubt it.

Sam.

[Bill/BentON Racing]

Any marks on end of camshaft?

[jaguar]

Any marks on end of camshaft?

Ill have to look when I get home

[MRieck]

That crank has been pork chopped

[jaguar]

That crank has been pork chopped

Thats what ive come to find out.

what is better or worse about a pork chopped crank vs a "normal" lightened crank?

[MCRider]

That crank has been pork chopped

Thats what ive come to find out.

what is better or worse about a pork chopped crank vs a "normal" lightened crank?

Can't answer your question directly but a seriously lightened crank may only be really beneficial in a serious roadracer. The idea being to reduce the gyroscopic resistance to left/right transitions.

For a streetbike and straightline performance, you (we) may find a little weight is a good thing. No weight makes for a very twitchy throttle which is a pain in traffic. And for launching purposes, unless you've got the guts to hold the throttle open, a light crank with low throttle won't carry much oomph leaving a stop light.

I love all your vintage parts you've uncovered. As someone said you may still want ot have the rods (and crank) tested for cracks. And by the time you do that you may be better off buying some new modern parts. That's what i found building my motor.  You could make up for the serious light crank with a heavier rotor perhaps.

[jaguar]

That crank has been pork chopped

Thats what ive come to find out.

what is better or worse about a pork chopped crank vs a "normal" lightened crank?

Can't answer your question directly but a seriously lightened crank may only be really beneficial in a serious roadracer. The idea being to reduce the gyroscopic resistance to left/right transitions.

For a streetbike and straightline performance, you (we) may find a little weight is a good thing. No weight makes for a very twitchy throttle which is a pain in traffic. And for launching purposes, unless you've got the guts to hold the throttle open, a light crank with low throttle won't carry much oomph leaving a stop light.

I love all your vintage parts you've uncovered. As someone said you may still want ot have the rods (and crank) tested for cracks. And by the time you do that you may be better off buying some new modern parts. That's what i found building my motor.  You could make up for the serious light crank with a heavier rotor perhaps.

The motor is running a stock rotor.
I assume that this was a race motor that was converted to street use.  It has stock points and a stock rotor.

never really thought about testing for cracks.  I know a guy that might be able to have the parts magnafluxed for me.

I like the idea of real vintage parts for this build and its fun to find them.

[MCRider]

I paid to have some rocker arms magnafluxed. Not cheap,  But if you can get it done cheap, that's the ticket.

Got the big rotor, good.

Love the old parts. I've got some 1970s vintage Yosh 2 ring slipper pistons i used back in the day and hope to use them again.

Not to be a total buzzkill, but what the heck...

If you use a heavy rotor with a light crank on a hot motor, you run the risk of twisting the end of the crank off, from high RPM shifting.  I've seen that twice. Both were customer bikes that were hotted up, but they didn't lighten the rotor.  It is a benign event. You may hear a "ping". The bike continues to run fine, in fact a little better as it is no longer hampered by any rotor at all. Then after a while, the battery goes dead.   

[Hinomaru]

what is better or worse about a pork chopped crank vs a "normal" lightened crank?

A pork chop crankshaft is machined beyond the normal lightened crank as even more weight is removed in the process. Normally used for maximum acceleration in pure race applications or a street sleeper. Here’s a pic from cyclex that shows a standard CB750 crank compared to their pork chop crank:

[mec]

If you use a heavy rotor with a light crank on a hot motor, you run the risk of twisting the end of the crank off, from high RPM shifting.  I've seen that twice. Both were customer bikes that were hotted up, but they didn't lighten the rotor.  It is a benign event. You may hear a "ping". The bike continues to run fine, in fact a little better as it is no longer hampered by any rotor at all. Then after a while, the battery goes dead.   

and the breakdown of the oil pressure will free another HPs, cause the oil pump does have to work against less withstand

mec

[MCRider]

If you use a heavy rotor with a light crank on a hot motor, you run the risk of twisting the end of the crank off, from high RPM shifting.  I've seen that twice. Both were customer bikes that were hotted up, but they didn't lighten the rotor.  It is a benign event. You may hear a "ping". The bike continues to run fine, in fact a little better as it is no longer hampered by any rotor at all. Then after a while, the battery goes dead.   

and the breakdown of the oil pressure will free another HPs, cause the oil pump does have to work against less withstand

mec

Tee Hee!  The first one i saw the customer came in because his battery wouldn't stay charged. The electric start had quit working too. It would kick start though. A test indicated no alternator production. Me, the shop flunky, got to pull the alternator cover. When i did it felt awful heavy. The rotor with the tail of the crank still in it was nestled into the stator, snug. The crank was sheared off flush with the cases.

[jaguar]

Great....something else to think about....

[Bill/BentON Racing]

Your engine is 1974 CB750K. Bill

[scottly]

If you use a heavy rotor with a light crank on a hot motor, you run the risk of twisting the end of the crank off, from high RPM shifting.  I've seen that twice. Both were customer bikes that were hotted up, but they didn't lighten the rotor.  It is a benign event. You may hear a "ping". The bike continues to run fine, in fact a little better as it is no longer hampered by any rotor at all. Then after a while, the battery goes dead.   

and the breakdown of the oil pressure will free another HPs, cause the oil pump does have to work against less withstand

mec

The early, heavy rotor was also known to break stock cranks in hot motors. Honda used a lighter rotor starting with the F1, I believe. I know the K7 had the lighter rotor.