engine seized up help kz750

re: 1978 kz750 twin.......so I recently bought some used parts from another kzrider member- and rebuilt the top end of my bike- these were barely used cyls, +1 pistons, rings, and head. The parts looked good. I put it back together- my mechanic tweaked the timing, checked valve clearance etc and the bike ran great for about ten miles.
then i heard a metal on metal sound- cam chain clatter???- and the engine froze- bike shut down.
I can shift through gears but the engine will not kick (I haven't tried turning it in reverse yet.....

So I guess I need to start tearing it donw to see what happened- anyone got suggestions of diagnostics I can perform before I pull the cylinders off?
thanks
Frustrated Jim
1978 kz750

Ah man that sucks!!!

If it won't turn over the first thing I'd think is seized pistons. Did you make sure the rings were positioned correctly when you installed the cylinders? I'd pull it apart and see what pieces are destroyed...that outta tell you what went wrong...

Did you try removing the spark plugs and turning the engine with the kick starter or the 17mm nut on the crankshaft end?

If it truly siezed upafter ten miles, I wonder if the pistons were not getting oil. Ten miles is about how long it takes the internals to get hot and expand into maximum size.

If you heard some clattering I would not think a seizure right away. If you are lucky the starter clutch bolts may have backed out and locked the motor up, not likely considering you just did a top end. Start taking it back apart. Good luck. and you did put oil in it right?

Hate to day it, but...flht is reading my mind. Either no oil or lack of lubrication moving though the motor. The former is a more common problem than you'd think after a rebuild.

In answer to several questions:
1. I have not removing the spark plugs and using a 17 mm yet but that is the first thing I will do. I was so pissed that I just parked the damn thing. I almost took a sledge to it but luckily i caught myself.

2. I put the proper amount of oil in it- does the engine oil lube the transmission or is that separate?

3. I was very careful to stagger the rings accarding to the book. Only thing is these are used rings and I did hone the pistons. The rings are only supposed to have about 900 miles on them. The end gap was about as thin as my thumbnail- my mechanic eyeballed the ring end gap but did not use a guage...

4. My starter clutch was bad when i bought the bike so i always have to kick it- of course I don't think i went anywhere near the starter clutch when working on the top end....

thanks for any ideas- ill post as i go if i tear it apart. I live in atlanta if you know anyone that wants to buy this bike..

what could i have possibly done to restrict oil flow?

jammor wrote:

what could i have possibly done to restrict oil flow?

Oil flow going up top can easily be compromised if any junk falls down into the passages when the cylinder is being removed or replaced, or the base gasket is being scraped off. Also, you need new O rings on both sides to keep the pressure up top and prevent leaks. There are two O rings between the crankcase and cylinder, and four O rings on the two small steel tubes that join the cylinder to the head for oil flow.

My wonder was if the used oil rings and the oil feed holes in the pistons were clear enough to get good oil flow to the cylinder walls.

jammor wrote:

2. I put the proper amount of oil in it- does the engine oil lube the transmission or is that separate?

It's all the same oil.

jammor wrote:

The end gap was about as thin as my thumbnail- my mechanic eyeballed the ring end gap but did not use a guage...

Cold end gap is supposed to be about .010" - .012" (nominal) and about .025" (max) if I recall the spec.

All the O rings I used were used. I used new gaskets- head gasket, etc.....
I cant remember four O rings on the two small steel tubes that join the cylinder to the head for oil flow. I DO remember the large skinny O rings that went between cyl and crankcase. I remember two small O rings about the diameter of a pencil eraser- i think they went on top of the cylinders- this is a twin so wouldn't i just have two of them...?
anyway all the O rings were used....

And I failed to mention the original reason I rebuilt the top end was because the engine seized. Perhaps I simply have a bad oil pump- I think I really failed to consider what lower engine components could be bad...??

I just pulled the plugs and valve cover. put wd 40 in both cylinders and tried to turn the crank forward and backward. I get very minimal play and the feeling im about to break my wrench or severely f*&* something up if i keep trying to turn it. The valves and cam chain LOOK ok- im not sure what im looking for. I suppose the next thing is to pull the head and cyls? can i use a thin wooden dowel in the spark plug holes to try and "unseize the rings"? or is there any point in that?
thanks for any advice in advance
jim