Topic: CB750K3 engine question.

[jpmire11]

So I picked up a 73 cb750. I just recently replaced the top end gaskets and piston rings due to the piston rings being fried and causing the bike to foul the two right plugs and smoke blue. Now that I have put it back together I have run into a little bit of a problem. I was attempting to start the bike ( kick it 10 times then try the electric start) I did this process about 3 times when the engine locked up. It sounded like metal hitting metal. So I assumed I didn't time it properly. So here is where it gets interesting. I pulled the engine back out. Pulled the valve cover off, rocker pins and rocker arms. The cam sprocket has two bolts on it. One of the bolts is on the bottom side of the cam. I can't get the bottom bolt out because it is hitting the head. I once again tried to turn the engine over using the kick start but still no movement. I was under the impression that I should be able to spin the engine once the rocker arms where off? Do I have a bigger problem here than I originally thought? Could something have fallen into the bottom end and caused it to seize?

[gane]

j, Immediate thoughts... since bike wasn't running when crank stopped spinning & removing rockers still provides "lock-up" w/kick start, my  "best guess" is that either something has fallen into the cam chain/crank gear, & is jammed or a valve has "dropped/stuck/broken" &  is hitting piston.  Before destroying cam chain, (in order to remove cam/sprocket) try rotating crank backwards, & see if rotation is possible. debis in cam chain would allow motion & cam & sprocket removal.with possibilty  of clearing via oil pan removal.  a dropped /broken valve typicaly allows @ 30-60 degrees of rotation at crank before locking again. sorry, G

[MCRider]

Cam sprocket bolt is hitting the head? This is hard to imagine. Pictures? Is the other bolt accounted for?

Do you mean its hitting the head when you try to remove it from the sprocket/cam? Or that it is heading the head in the rotation of the cam.

A dropped or broken valve would be visible upon inspection just looking at the spring stacks. The stack would be loose, or otherwise kinda weird.  With all the rockers out the spring stacks should all stand proud at full extension and be tight, meaning all the valves are seated.

With all the rockers removed the engine should turn fairly easily. Maybe broke a ring and a bit of it has lodged between the piston and the cylinder wall.

[jpmire11]

Cam sprocket bolt is hitting the head? This is hard to imagine. Pictures? Is the other bolt accounted for?

Do you mean its hitting the head when you try to remove it from the sprocket/cam? Or that it is heading the head in the rotation of the cam.

A dropped or broken valve would be visible upon inspection just looking at the spring stacks. The stack would be loose, or otherwise kinda weird.  With all the rockers out the spring stacks should all stand proud at full extension and be tight, meaning all the valves are seated.

With all the rockers removed the engine should turn fairly easily. Maybe broke a ring and a bit of it has lodged between the piston and the cylinder wall.

The bolt won't come all the way out because it is hitting the head. I got the other one out already. I have tried to rotate the engine both ways and got no luck. I am starting to think that a broken piston ring my buddy so said he got all the pieces out, might have fallen in the crank. So any suggestions on how to remove the bolt if I can't get it spun at all???

[MCRider]

Its really difficult for anything that falls from the top to the bottom to lodge in the crank in any way that would keep it from turning. More likely it just falls into the sump, harmless. The moving parts of the crank have such close tolerances that nothing of any bulk can get in. Debris measured in thousandths can get into the bearings for sure, but were talking more about muck and not hardware.

If it fell down from the cylinders it would continue dropping down, and not go sideways to the cam sproket or primary chains. If something dropped doen the chain tunnel I suppose it could get in the sprocket, but a back and forth motion would free it I'd think.

As i mentioned, I hate to say, you may have broken one of the new rings in assembly and a tiny bit of it has turned sideways and is lodged between the piston and the cylinder walls, bringing everything to a halt.

Drop the sump and see what you can.

But anything can happen I suppose. Sorry for your troubles.

As to your question, first resist the temptation to turn the crank hard with the nut on the ignition. It can only take so much.  Rather, remove the alternator cover and wrench on that big rotor bolt. If I were ready for whatever, I'd put a breaker bar on that bolt and turn the crank to where you can get the cam sprocket bolt out. That means further scratching the cylinder walls, if I'm right, or messing with the crank if you're right.

Otherwise, cut the chain. But learn your options, chain replacement usually involves crank removal, unless you want to use a masterlink on the cam chain. Its done but its not SOP. And you  may need to get to the crank to satisfy its clean anyway.

[Duke McDukiedook]

Yeah, sounds like a knackered ring to moi.

[jpmire11]

I don't understand how a ring could break putting the pistons in the cylinders. But I will try the breaker bar on the alternator side next time I get a chance to mess with it. But if I do pull the head off and the pistons still don't move, that means something is jammed in the gears right?

[MCRider]

It is not uncommon to break a ring putting the cylinders on. As the cylinder drops down over the rings, a ring may jump out of whatever compressor you are using, come out of its groove just enough that it hangs up on the botttom pof the cyilinder, breaks, and then the cylinder slides on down over the broken ring. The part of the piston where the rings are is smaller than the skirt of the piston, so it has more clearance between itself and the cylinder wall. In that clearance a bit of ring or other debris can get lodged.

All of these are low probabillity events. Jamming in the gears can be ruled out by utting it in neutral. Jaming in a chain sprocket is harder to rule out, but also a low probability event. I've never actually heard of anything getting stuck in a crank sprocket, cam or primary, that couldn't be dislodged by turning the engine backwards.

I have heard of and seen broken rings and damage to cyinder walls.

A tip:if you go to pull the dyno cover off, there will be a bit of oil in there. Tilt the engine to the right to get it to drain into the sump. Be sure the sump drain plug is still in. Just trying to avoid a mess on your bench.

You've got a weird problem and i certainly won't be surprised if it turns out to be something other than the ring. I'm just thinking out loud.

[1974cb550k]

Had a similar prob on cb400f. Could not remove lower cam sprocket bolt as motor locked up and would not turn. I had to cut the cam chain and remove the head. good luck

[MCRider]

Had a similar prob on cb400f. Could not remove lower cam sprocket bolt as motor locked up and would not turn. I had to cut the cam chain and remove the head. good luck

Why was your motor locked up?