Topic: cb650 crank in a cb550

[Mitchell]

What has to been don to run a 650 crank in a 550? I got a cb 650 with a good bottum end and was thinking about puting the crank in my 550 for a little more stroke. I want to use my 550 topend and pistons since its in such good shape an i dont want to loose my kicker either. Im guessing i would need the 650 cam sprocket, tensioner, and chain. What about the rods? Would the be any clearance issues?

[bwaller]

Use a longer stroke 650 crank and you'll need the taller 650 cylinder....and on & on. Check out Paulages thread on the subject.

[CBGhia]

Use a longer stroke 650 crank and you'll need the taller 650 cylinder....and on & on. Check out Paulages thread on the subject.

Yep, crank will cause the pistons to hit the head when using the 550 cylinders.  And if you use 650 cylinders, you may as well use the 650 head, as it is light years ahead of the 550. 

[calj737]

Use a longer stroke 650 crank and you'll need the taller 650 cylinder....and on & on. Check out Paulages thread on the subject.

Would that still be true if he used 550 rods and pistons, Brent? Do 550 rods even mate to a 650 crank?

[CBGhia]

Use a longer stroke 650 crank and you'll need the taller 650 cylinder....and on & on. Check out Paulages thread on the subject.

Would that still be true if he used 550 rods and pistons, Brent? Do 550 rods even mate to a 650 crank?

It would still be true because the longer stroke would cause the piston top to extend past the cylinder because of the longer stroke.

[Mitchell]

Well i would want it to look like a 550 too, so i would wanna keep the 550 head on it. If you were to run 650 barrels with 650 pistons wouldnt the dome on the 650 pistons be to much for it to fit in the combustion chamber of the 550 head? Im not sure how different the rods between the 2

[bwaller]

The rods will interchange. The 650 pistons will not work with the small 550 combustion chamber. The 650 bore is under 60mm so one could bore & use 60 or 61mm pistons. It would be necessary to investigate piston compression height using the 650 cylinder & the 550 head. Not a problem, just don't bolt everything together without measuring.

[Mitchell]

Thanks guys! Another question what about the ignition plate? Would i have to use the 650 ignition parts too? Or is there a good aftermarket setup that would work

[bwaller]

Again check paulages thread, http://forums.sohc4.net/index.php/topic,40414.msg416213.html#msg416213 as I recall he went through some step by step mounting a Dyna.

I believe one of the guys on here makes an ignition for the 650, Pamco maybe.

[RAFster122s]

There was some machining involved in the end of the crank for the stator I think... I forget exactly the issue but in either the early iteration of the 650 top end or the later 650 hybrid Paulage did some reworking of a tapered end IF I recall correctly. 

But, It would pay you to copy the thread to a Word document and print it and study everything in it...
Or, request a moderator (with a small donation) to pull the text from Paulage's thread on the hybrid build.
Then, you will need to copy the images into that document onto your computer in order to insert them in their proper place. 
Yeah, it is some work...but, you need to study it long and hard or have deep pockets and a good machinist to do some work for you at several steps along the way.  It is hard to say if you can still get everything, but dropping a rebuilt 650 motor in the chassis and doing without the kickstart (as the 650 didn't have one) is the way to go with less investment.  Doing it right will save you major headaches. Trying to modify it drastically from Paulage's hybrid might get you into uncharted waters. Hopefully you can swim well, should you choose to do so. 

There is an overbore kit to take the 650 motor to 674 that I think a few guys on the forum have done (or are doing) to drop the 650 motor into the bike with the electronic ignition from the 650.  M-units are a nice way to adapt the bike with a solid solution and the remote fob for security makes it easier to keep it from being hot-wired and taken that way.  Calj737 is very experienced with the M-unit and I believe he has documented the wiring when using it on a 550 build...the 650 changes would be very similar to stay with some of the stock ignition system components.
 
 

[calj737]

Dyna, Pamco or C5 ignitions are all electronic and suitable. There's even Electrex out of the UK that make a very nice CDI unit that can run without a battery 

Sounds like (from Brent) the 650 crank with 650 jugs and head would be required. At that point, you might as well use the 650 block (yeah, I heard you want the kick starter...). But a 650/715 modded motor like Paulages did would be the Cat's Pajamas in a really nice lightweight packaged 500 frame. Grab the Dresda swing arm for sale from Ricky Racer and you're on your way to street beast with vintage soul   

[Mitchell]

Thanks for all the input you all have given me lots to chew on. My thing with the 650 hybrid and using the 650 head is why not just us the whole 650 motor and drop it in instead of doing all that work to have a motor that looks like a 650, i dont want that i want it to apear as a 550 with a kicker.  I used a displacement calculator online and if i used a 550 bigbore 61.5 piston and 650 crank it should be 663cc. I wonder if the wrist pins on the 550 piston and 650 piston are in the same place. Time to get to reading paulages thread

[SOHC4 Cafe Racer Fan]

I suppose anything is possible with enough machining work.

Along with DaveBaubier, I am one of those guys dropping a 650 engine (with ported head, cam and 674cc Dynoman piston kit).  I thought of going the Paulage's route but that would have required some serious machining finesse.  I don't really give two cents about losing the kickstarter. 

[Retro Rocket]

I suppose anything is possible with enough machining work.

Along with DaveBaubier, I am one of those guys dropping a 650 engine (with ported head, cam and 674cc Dynoman piston kit).  I thought of going the Paulage's route but that would have required some serious machining finesse.  I don't really give two cents about losing the kickstarter.

Yep, the 650 swap would be cheaper and will probably still be faster than the hybrid , the 650 is a better engine overall...

[Mitchell]

Thanks guys. Next chance i get i going to tear the 650 motor down and check out the internals to see what kinda shape there in. Im thinking sleeper 550 i want it to look like a 550 thats why i wanna keep the kiker and 550 head on her. I guess i will find out soon enough but what about the rods between the two emgines, are they the same length?

[RAFster122s]

 Mitchell, the 550 head is far more inefficient than the 650's hemi head design. The '79-'80 650 have the earlier carb design while later uses the CV carbs. So, you cannot as far as I know benefit from the 550 head on 650 cylinders and crank.

[Mitchell]

I guess the biggest benefit would be the better flowing 650 head, makes since. Well this project is far from getting started and i still need to pull the 650 motor down and see if its in good shape on the inside. And if it comes down to it i do have 2 650 heads i could rebuild and use

[bwaller]

Certainly the 650 head has improved ports with larger valves. However as a result the combustion chamber is much deeper and this in turn requires a piston with a large dome, not as efficient for other reasons.

A 550 head can work with a 650 crank & cylinders, it just depends what you're looking for. A CB550 cylinder head can be developed to handle up to 75 horsepower with 60 or 61mm bores in a race engine, that's double the stock output. An engine with 50 to 60hp in a nimble lightweight motorcycle for the street can be tons of fun.

[Mitchell]

It seems like if nothing else the longer stroke and bigger bore would result in more low end torque which would be good for the street like you say. I already cleaned up the castings in the 550 head, portmatched the manifolds with a set of 750 carbs mounted up with v stacks, and put in a 650 cam. Got to get tag and insurance so i can start riding it and dial the carbs in.

[CBGhia]

I guess the biggest benefit would be the better flowing 650 head, makes since. Well this project is far from getting started and i still need to pull the 650 motor down and see if its in good shape on the inside. And if it comes down to it i do have 2 650 heads i could rebuild and use

If you want to get super crazy, I remember someone modified a 550 rocker cover to fit on the 650 head.  I remember it being a lot of work, though.

[SOHC4 Cafe Racer Fan]

An engine with 50 to 60hp in a nimble lightweight motorcycle for the street can be tons of fun.

Yes, indeed!

[akabek]

So what did you end up doing?

[Mitchell]

I still havent got around to starting the project, my 750 is about to get some much needed attention so this will be on the back burner for awhile.