"Sucking oil past the rings"

I've still got a '56 Panhead Harley that I bought from a friend in '74. One of the HD service manuals for those old bikes said to close the throttle at cruising speed occasionally to lubricate the valve guides. Didn't mention the rings, but the cyl. wall to piston clearance is about the same as the valve stem to guide clearance. Should pull a little past the rings too, I'd imagine.

Garrett Fulton

The pistons have the same up and down travel, and the same amount of oil per revolution squirts out from the rod. The only things that I can think of that change from the perspective of the motor when you let off the throttle are:

- crankcase vacuum (and blow-by)

- rate of crankshaft rotation (and associated centripetal forces)

- if the spark timing varies, the ignition force on the connecting rods

I'm puzzled why "sucking oil past the rings" would be desirable. If it is sucked past the rings, it will burn. A worn out engine allows oil to be sucked past the rings, and this results in carboning, oil consumption, and blue smoke. A properly functioning oil ring should leave just enough oil on the cylinder wall to lubricate the rings as they travel down and then back up, but not more or the rings will scrape the excess into the combustion chamber as they travel up.

If anything I would think hitting the throttle hard would cause MORE oil to get past the oil ring because of the blowby pushing it away from the cylinder wall. Letting off the throttle in this case would burn less oil but I don't see how it would aid in lubrication.

Maybe he is confused with 'lugging' the motor by opening the throttle past the point where it produces a RPM increase. That used to be hard on bearings in older motors but I don't know if it is much of an issue anymore.

blowby will force oil back in the crankcase. High vacuum in the cylinders on the intake stroke is what pulls the oil past the rings. Not sure if that really works or not as it usually comes down the intake valve guides first. But it's a good practice to vary the loading on the rings anyway during break in.

nate

Sounds like something you tell a 16 year old to trick him into slowing down. How old were you when he told you this?

Ted

Gotta agree with ted but to a different degree. Seems like todays technology would prevent this from being effective. Years ago when rings & pistons weren't what they are today, this probably would ring true but I'm thinking in the world of today rings are made better & of higher quality to seal things up nicer, not to mention the improvements in oil rings (wipers). I'm also thinking that the air cooled bike motor from that era would, indeed, have the piston clearance a little looser than a water cooled motor in a car. Jus' theorizin.

Too young to drive! We were driving somewhere and I asked him why he did this. Might have been mid-late '60s.

I suspect it does. I've seen cars with bad rings really smoke when you backed off, as engine wound down. Smoke was worst on trailing throttle than when accelerating.

Upon reflection you may be right. I remember having a new head put on an old Valiant and it did smoke a little bit on overrun subsequently so I know that in that case it wasn't valve guides. However I suspect that valve guides are a problem, as I've ridden in cars that would lay down a cloud after taking a hard corner but at no other time, which makes me suspect oil pooling in the heads and subsequently being sucked into the intake passages.

nate

My belief is that the answer is no to both 1 and 2. There is no throttle plate to close for diesels and they seem to get by just fine (even turbocharged diesels). While it is true that closing the throttle while the engine is turning at high rpm will increase the vaccum in the intake tract and cylinders, which may result in pulling more oil past the rings and intake valve seals, I don't believe this is beneficial. I'd put this down as an urban legend or old wives tale.

Ed

I agree totally.

Sounds like an old husbands tale to me.

Pistons and rings need very, very little oil. When an engine runs out of oil it's the bearings that go first; they're much more highly highly loaded than the rings and need constant oil supply. A microscopic amount of oil on a cylinder wall is more than enough, and any excess will only get burned and coke up the engine. Too much oil, as many know, will cause the rings to "hydroplane" and lose their effectiveness. Worn bearings often contribute to that condition by allowing too much oil through large bearing clearances.

Dan

By "lose their effectiveness", I assume you mean a temporary loss of sealing?

Worn bearings often contribute to that

Why would this be so? I'm afraid I don't understand the connection.

That's what's meant.

Overlarge bearing clearances = more oil squirting and splashing onto the cylinder walls.