Topic: Here's another question about 650 cam swaps! Ready?

[newguyoldbike]

For starters, this forum has taught me a ton, so thanks to all. Next, I am relatively new to motorbikes, especially to building motorbikes. Last year a good friend of mine graciously allowed me to ride his bored out 750 at will, and then gave me a 71 cb500 (that was in a million different boxes and jars) that was just wasting away in his basement. After a whole lotta research on this website I started to break the machine down totally and clean, and clean, and clean every part I could get to. This process took me to splitting the cases, and getting so deep into my project that I thought I would never get it all back together. This is where I will probably irritate the purists, but stick with me: I found a fantastic deal on a 650 cylinder head, and my research has led me to believe I can get a bit more push from some swapping, so I jumped on it. I will be putting a 650 camshaft in there also. So my question is, will I be doing myself a dis-service if I don't replace the 500 pistons with the 650 pistons? The piston heads are different so there is a larger gap between the piston and cylinder head. Also, with an electronic ignition will I need to do anything crazy to adjust the timing for all of this? I have the help of a friend who has built plenty of bikes, but some of his ideas disagree with some things I've read in these forums..
I still have the cb500 cylinder head, so if it makes more sense I will just put the original head back on with the 650 camshaft, but I'd really like to figure out how to work this 650 cylinder head swap. Thanks!

[trueblue]

If you use the 500 or 550 pistons with the 650 head you will be going backwards massively.  It will drop the compression ratio quite a bit, because the 650 has a domed piston and the 500 or 550 have flat topped pistons

[newguyoldbike]

Thanks trueblue. I had a sinking feeling that I was gonna have to go that route. So, is it conceivable that a cb500 crankshaft can deal with the extra weight of the cb650 pistols? Or would I have to switch that as well?

[MiGhost]

It has been done. Check out the 550-650 big bore build up that paulages did.
http://forums.sohc4.net/index.php?topic=40414.0

To save your self a lot of hassle from changing the sleeves. Get a 650 cylinder with the pistons. You might not get much more improvement over simply swapping in the 650 cam to the 500 head though.

[dave500]

just put a complete 650 motor in the bike.

[trueblue]

It has been done. Check out the 550-650 big bore build up that paulages did.
http://forums.sohc4.net/index.php?topic=40414.0

To save your self a lot of hassle from changing the sleeves. Get a 650 cylinder with the pistons. You might not get much more improvement over simply swapping in the 650 cam to the 500 head though.

The problem with using the barrels off the 650 is they are taller to allow for the longer stroke of the 650.  You would need to shorten them somehow, talk to a good machinist.  They shoud be able to point you in the right direction. 

Thanks trueblue. I had a sinking feeling that I was gonna have to go that route. So, is it conceivable that a cb500 crankshaft can deal with the extra weight of the cb650 pistols? Or would I have to switch that as well?

The 500 crank won't have an issue with the heavier pistons.  The conrods are the weaker link, but even they should be fine