Topic: CB650 Engine Rebuild Question

[StreetSpirit]

I have one for the experts...

I'm looking at the inside of a 79 cb650 that was purchased cheap because the motor smoked and had low compression.  Pulled it down to the block and it seems like somebody has been in this engine before...

Its all a bit too clean, the gaskets were new, and it looks like somebody has taken a hone to the cylinders based on the fact that some of the carbon has been rubbed away above the rings.

With the mic/bore guage, I'm measuring about .0015" above stock bore at max diameter with .0005 egg shape/taper.  No damage otherwise, clean shiny crosshatch etc, and the bore doesn't exceed the service limit.  My theory is that the smoking and low comp was caused by an over zealous hone combined with re-using the original rings.

Now I guess the question is, do I just slap new rings on the original pistons and call it done?  Is there something else I need to worry about?

[Vinhead1957]

Could there be another reason for smoke and low compression?   Bent valve?  Was it low across all cylinders.  Ideally, you are there already, why not new rings pistons with a little bore action.   If you don't know, why not make sure and you will know.

[trueblue]

It depends on how much wear there is on the pistons, the 650's have a maximum piston to bore clearance of .004".  If you are within that that then you should be good to go.  If you are getting close to the limit I would consider an oversize.  Give the bore a decent hone, don't forget this will remove some material, fit new rings and put it back together.  While you have it apart don't forget to check the valves and guides, and replace the stem seals .

[StreetSpirit]

Well I'm getting some measurements that don't make a great deal of sense but it could be user error, bad instruments or a combination of the two.

With a freshly zeroed micrometer, 10mm from the bottom of the skirt and perpendicular to the wrist pin per the service manual, I'm measuring the pistons at around 2.3545, which is .0005 bigger than the stated stock piston.

Using the same mic with the .0005 dial bore gauge, I'm getting a max bore diameter of 2.3558 so a piston to cylinder clearance of .0013 which appears to be too low. 

Now the spot I'm measuring on these pistons has no visible wear, and further up on the thrust face there is some clear rubbing.  Should I be worried about the low clearance here?

[trueblue]

No, that will work fine. Maximum clearance is .004".