Datsun Roadster Forum-311s.org

BUMP! I'm still chasing this down and suspected that I either have a minor hole in the piston or a busted/bad ring. The compression numbers have always been somewhat close but that cylinder just does not pull a vacuum. This weekend I pulled out the plug on that cylinder and poured a bit of oil in. Using an endoscope I was able to watch bubbles stream from around the cylinder when I pressurized the crankcase with my air compressor at 50 psi through the PCV opening in the valve cover. I definitely have a ring issue which is disappointing. Unfortunately I just don't have time to tear the motor down at this point so it looks like the car will be sitting for the time being.

Well, perhaps, but did you compare your results across all cylinders? Rings have a ring gap. Even my total seal rings will leak some with no ring gaps, so they will bubble, no? You may certainly have a problem with rings or even a cracked piston... there was impact. I'd pull the pan and pull that piston.

Thanks for the reply Greg. I can see the piston and it looks intact (from the top anyway). There should not be the volume of bubbles coming out at this low of pressure (in my subjective opinion). I think I have some sort of broken ring/ring issue. Is it possible to pull only one piston out by dropping the pan? I thought there was too much going on underneath to make this possible. On a related note...is it even possible to reinstall a piston from the bottom?

I'm not completely sure on pulling a piston with the crank in place. In my minds eye, I think you can. I think the crank will be far enough away for the rod to slip past with the crank rotated to the very bottom, but others will have to pipe in. The next question though would be, can one successfully re-ring that piston and expect them to seat without at least honing it? I guess if the best you can do is pull that piston to see if it's a broken ring etc, then you could decide from there. Ultimately though, it pretty much sounds like you're gonna have to disassemble it again if you're gonna fix it. Bummer man.

Did you do a leak down? That would confirm rings, or valves. I know you said you didn't have the equipment, but all you need is a compression tester hose and a compressor. You take the Schrader valve out of the screw in compression tester hose (you can use a pair of tweezers to unscrew it), put a male end on your compressor hose so you can plug into the compression tester hose, and you're set. First, rotate the engine until #3 is at tdc or confirm from the cam that all valves are closed. Put the car in gear and set the brake hard, or have someone apply the brakes or it can run over your foot. Then put like 50lbs to it via the compression tester hose and listen to where the air is coming out... valve cover (rings) , intake (intake valve) or exhaust (exhaust valve). If the engine moves, make sure the valves stay closed. You don't need to know how much in a percentage of lost air, just what's happening. Compare to #2 in the same cam and piston position. Is it the same, or a lot less? At least you'll know what's what. Who knows, maybe it's something else... this is a simple test that I think will tell you one way or another.

Oh wait, after reading this I realized the compression tester hose probably has a male end... should be able to plug right into the compressor hose.

Might be time for a professional assessment. It is great that you are so knowledgeable and can do a lot of your own work, but at this point a pro might help you assess a bit better after all your hard work up to now.
Good luck
Linda