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Could finger get injured being sucked into spark plug hole?

One of the posters here offered up a chastisement for cautioning another poster about that, suggesting getting your finger injured by being sucked into the spark plug hole is impossible. The poster was trying to figure out if the cylinder was at the top of the compression stroke or intake stroke by placing their finger over the spark plug hole. I’ll grant it’s never happened to me, but it still seemed plausible and worth being careful about. I did a little Googling however, and couldn’t find a single report of this sort of injury. It seems like during the power stroke when both valves are closed and the piston is moving down from the tdc you could get quite a suction force at the spark plug hole. Especially if someone were doing that while cranking the engine. So why no injury reports?

I’ve only done this on a non-running small engine and your finger gets blown off the hole. Even on a multi cylinder engine with on plug out and running, I don’t think you’d be able to keep your finger on the hole, let alone have it sucked in. Air is normally not drawn in through the spark plug hole.

Anyone in their right mind would only use their finger while turning the crank by hand.
Not enough speed to cause injury.
You have to move the crank slowly to find TDC.

An open plug hole on a cranking or running engine is going to have pretty violent suction and pressure pulses.
The pressure pulses probably aren’t going to let the finger get close and seal during the suction, unless one were dumb enough to place the finger on the hole then have a “helper” crank the engine.

Without pulling out a pencil to run the exact numbers: If you jammed your finger tight into a spark plug hole at the top of the compression stroke and someone turned the crank 1/2 turn. Your fingertip would be subject to about 1/10 atmosphere (13.2 psi vacuum). That would feel like a 4 pound weight tugging on your fingertip. I expect that would raise a blister, but I doubt it would do any serious damage.

I’ll bet you would not try it a second time.

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Your scenario would have to include some sort of valvetrain malfunction that is keeping the intake valves from opening. During normal operation, the cylinder will be sucking air in through the intake system, which has little or no restriction.

Give the matter a bit more thought. In a 4 stroke cycle, there are two downward strokes in a cycle. You are talking about the intake stroke. I specified a scenario that begins at the top of the compression stroke. No valves are open during the following downward (power) stroke. It would pull a vacuum if there were a finger in the spark plug hole.

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The only worry I would have when plugging the spark plug hole with my finger was getting shocked by the disconnected plug wire.

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OK granted, but remember we’re just cranking the engine over, it’s not starting. Nowhere near enough vacuum to do any harm. That small of a displacement would be hardly noticeable.

In fact, even if it were possible to hold your finger in a spark plug hole of a running engine it couldn’t do anything. Go out to a running engine and take the big vacuum hose off the brake booster and plug it with your thumb. You’ll have 18-20 inches of vacuum and it won’t even tickle.

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You are right. I did not think to compare this to manifold vacuum, which we have all felt. 20" of Hg vacuum is about 10 psi vacuum, which is close to the 13.2 psi vacuum (27" Hg) that I estimated you could make with a single sealed cylinder. 20" Hg does not hurt at all, so I retract my speculation that it might raise a blister.

My only remaining difference with your scenario is that while manifold vacuum is developed only when the engine is running, rotational speed would make almost no difference in the vacuum developed in a single sealed cylinder. The cylinder would pull significant vacuum even if it were being turned with a wrench, unless the valves or rings leak badly.

Why do I think this is one of those threads started by someone saying " Here hold my beer and watch this ".


Yeah that would be my fear. Been there before and don’t want to go back. Goes right up your arm and then some and you can’t let go. (Psst, also be a little careful trimming wallpaper around an outlet with a box knife.)

fyi, here’s a link I found about the possible dangers w/being exposed to vacuum. The force the human tissue would be subjected to depends on the area of the opening. Presumably the size of the openings discussed in this link are much larger than the size of a spark plug hole. But in some cases anyway, according to this article being exposed to a vacuum opening can produce some rather unpleasant consequences.

“The pressure caused by this suction will cause blood and other liquids to flow towards the truck and accumulate there. Vacuum will pull fluids out of the body causing damage to surrounding tissues. If you come in contact with suction, it will cause severe injury or even death. An injury caused by vacuum can be serious. The vacuum action must be stopped as quickly as possible. Seconds matter when the body is subjected to the forces of vacuum.”