I will second the notion of having the pistons at the top..... the valve won't be able to escape completely at least. Even if it is down in the guide, a pointy magnet will get in there and allow you to pull it back up. Another option would be to stuff some cotton rope in the spark plug hole, turn the engine so the rope compresses, and holds the valve close. I have had engines that would turn when air pressure was applied...... that was exciting.....

Got the room to use one of these type of valve spring compressor?

 

Yeah, I'll do it. Just didn't want to add to the tediousness of the job if it wasn't going to do any good... meaning if the valves could still fall completely in at TDC.

That tool would probably work on some of the easily accessible valves. At least it's open, so you can get to the keepers. Wish I had some extra keepers now. Gonna have to be real careful. We're under quarantine now, so I don't see me running to get anything else.

So the engine turned on you? Do you think that's because all the plugs were out? With no plugs in the cylinders, there would be zero compression, so I could see the air pressure in one cylinder being enough to put things in motion. I should probably pull the plugs as I do the corresponding springs, vs pulling them all out at the same time.

 

I used to replace valve seals on engines with clothes line rope. That thick cotton rope you hang clothes on. With the piston about half way up, feed several feet of rope in, rotate the engine to compress the rope at the top of the cylinder, then go to town on the keepers. It won't bleed off and the valves won't budge. When you get done, rotate back a little, pull the rope out and go to the next cylinder. I'd get a new rope though and don't let momma hang clean clothes on it afterward. Not unless you have a comfy couch to sleep on. That trick was real handy for SBC valve seals.

 

I suspect I didn't have the piston at exactly top dead center, so, air pressure was able to force it down. With the piston at TDC, the rod has no leverage on the crank, so, pressure can't move it.

Yep, that's exactly what I was talkin' about.

 

Yep, engine will turn on you 95% of the time. On Duramax's they make a tool that locks the flywheel ring gear to the transmission case because the harmonic balancer has to come off to do the water pump, and the crank bolt is about 250 ft-lb. Would be nice if they made that for our engines. With a clutch, you could latch the parking brake and put it in 5th gear. The rope is a good idea.

 

What about putting a breaker bar or ratchet with long extension pipe on the crank bolt and locking it against something?

 

That's how you knock the balancer bolt loose on said duramax- lock a breaker bar to the frame and ring the starter

Given the 6 sided bolt and 4 sided socket, odds are pretty good that one could find a good spot near TDC to lock it.

 

I'll use a half inch ratchet wrench. That's what I've been turning the motor with anyway.

 

I'm going to answer my own question here. When the piston is at TDC, the valves will not fall into the cylinder. They will drop roughly an inch or less, then hit the piston with plenty of stem still protruding. I guess I should have known, since this is an interference engine, so the piston can touch the valves. To find TDC, I stuck a long screwdriver through the spark plug hole so it was sitting on the piston. I then rotated the engine, watching the screwdriver rise to max level, stopping just before it started the descent.

I ended up using air. Didn't have any suitable rope and as long as I was at TDC, I didn't have any worry of the valves falling if the air bled off.