Topic: CB650 and 550 top end swaps

[purf_man]

So I scored a 650 motor (top end was unbolted as it had been seized but PO had managed to free it up) and my current 550 motor needed to be tore down and cleaned (left it in my shop with plug out....junk got in it but that is a whole different topic)

So I figured I would take on doing a big bore and crank swap on my 550 cases.  I have the full 650 motor (less carbs and exhaust)

Where is a good source for a ~61mm 750 piston for swapping?  all I can seem to find are 750 big bore kits.....same goes for sleeves.

What would be a good cam to swap in?   I have a spare 550 cam for a core so figured I would get it reground so I can keep my 550 tach drive bits.

Since I will have a leftover motor worth of parts I figured I would toss the stock 550 parts on the 650 cases.  Since this shouldn't require any major parts and no major mods (tensioner notch is it?)  I think the 650case/550 rotating assembly motor might become my build project break in motor.  I might toss the 650 cam in it just to keep it as a cheap "performance" build.

So the plan:
"650+" motor:
550 cases and covers (clutch, sprocket, alternator)
650 sump
650 cylinder, head, rocker cover
650 crank
big bore sleeves and pistons
550 cam (reground...."street" profile)
550 lower end gasket
650 top end gaskets (big bore head and base gasket)
Carbs?  spacing is not the same it seems so I at least need a new carb plate
650 primary drive gear and clutch
550 pressure plate and transmission gears


"the little motor"
650 cases
550 top end and crank
550 clutch/primary drive with 650 pressure plate
650 cam

I already have a 550 gasket kit on hand so I guess a 650 one needs to be ordered.  looks like my rings got comfortable with my old 550 sleeves so I might run a hone in it and toss in some new rings.  Or can I have the 550 sleeves bored for the 650 pistons I won't be using now?

The ultimate goal here is to build one solid street motor (for a weekend warrior type bike....maybe a fair weather ride to work) and then build the leftovers into a motor I can run in the short term while building the stroker motor.  The leftover motor will be mostly for shorting out the bike before I take it all apart to have the frame and other bits powder coated and final paint done...that and give me a first go at building one of these SOHC motors (I have built several car engines but this will be the first for me on a bike build)

[trueblue]

If you haven't already read this thread, it covers pretty much everything you need to know.

http://forums.sohc4.net/index.php?topic=40414.0

[goldarrow]

I got a bit
Lost along the way reading your post.   But all I can say Best and Easiest upgrade you can do to 550 motor is put in 650 cam.

Good luck.

[purf_man]

ok I did some more reading on the topic this afternoon....and re-read that article again.

I guess I had assumed stock 750 pistons and sleeves were used but at the very end it was mentioned that 65mm pistons were used.

So it looks like I will be going to a 65mm bore on the hot motor.

Guess the real question now is should I just go with a cast piston or put in a forged?  I would like to keep this bike on 91-92 octane so will need a compression ratio and cam that will live with that.

[trueblue]

Cast pistons are lighter, but have less room to play with when it comes to machining them.  Final compression ratio comes down to how all your mods work together, not piston type.