The International Cessna^® 170 Association

A moment while I grab my napkin and pencil...

300 cubic inches is 0.17 cubic feet and would be moved each full cycle of the motor. (suck, squeeze, bang, blow)
A 4 cycle engine means 1/4th the RPM is blow or in this case, suck.
(2700RPM/4)*0.17CFM = 114 cubic feet per minute

A 5 horse ShopVac pulls about 175CFM
https://www.shopvac.com/specifications/quiet_series.asp


So, plausible.
Somebody call Mythbusters to try it out.

Once it passes the carb and into one of the two intakes, that number would be cut in half.
57CFM

Once it passes into a cylinder's intake, that number would be cut in thirds.
19CFM

If my math and assumptions are right, it seems less likely.

Actually....

Half the cylinders suck on any given revolution, so you get 150 cubic inches of suck per revolution (it takes TWO revolutions to go through the complete suck-squeeze-bang-blow cycle). 2700 rpm times 150 cubic inches comes to 405,000 cubic inches per minute. At 1728 cubic inches per cubic foot it comes to just short of 235 cfm. That's a pretty good vacuum cleaner.

To carry the math a little further:
I'm not sure how big the carb throat on an O-300 is but, if it were two inches, that 235cfm would be passing through at nearly 180 ft/second, or 100+ knots. Sounds like a good reason to have a screen over the carb heat intake duct...

robrien wrote:Attached are images of the plugs. Bottom plug lost part of the insulator. I'm replacing the cylinder.

Thanks for posting the spark plug pictures. I was expecting more damage, I guess I am used to seeing the damage a broken of valve head does. There must be enough piston top to cylinder head clearance for the nut. I find it hard to believe a nut, even as small as this one, would pass between the valve and valve seat.

And that's why I failed Power Tech in high school

That would make my numbers off by 2 less rounding errors (0.17CF vs 0.1736 CF gives you 229CFM vs 234CFM)
But that would still be halved and again thirded before the nut could make it past the valves.
39CFM at the cylinder.


Plausible

Many years ago I used to work in a Ford truck engine overhaul shop. We had this customer come in and wanted his engine and drive train overhauled . He raved how good the engine was ,so smooth and economical . It was very worn but it had done some hard miles. It was bored out 10 thou and new Pistons fitted . The old Pistons were going to be thrown away . One of the sharp eyed over haulers noticed one of the Pistons had a raised bump in the top of the piston. He scratched around and to every body's surprised a silver coloured coin had embedded itself flat on the piston . It had been there a long time . The owner said this was the first overhaul since new . This coin was the size of a silver dollar . And the owner was bragging how smooth the engine ran . The head internally had a couple of small marks on it . So how did it get there in the first place? And for so long ?

I saw an oil sump once that had the early style intake flow divider attached with AN3 bolts and castellated nuts. Perhaps this was similar, and it came loose.......

bagarre wrote:And that's why I failed Power Tech in high school

That would make my numbers off by 2 less rounding errors (0.17CF vs 0.1736 CF gives you 229CFM vs 234CFM)
But that would still be halved and again thirded before the nut could make it past the valves.
39CFM at the cylinder.


Plausible

Likely even. My guess is that it's the velocity that moves the trash, not just the CFM. Lower CFM at the cylinder, but a smaller cross section could result in similar velocity. When my engine swallowed a valve in 2005, debris from the #5 piston fouled the plugs in #1, but left #3 clean. Hard to say what happens after you get past the carburetor...

n3833v wrote:At one time intake could have been open and someone dropped a nut and rolled into bottom of intake and couldn't find so figured it went on the floor. It can happen but I look till I find to confirm just as a surgeon has to confirm all instruments used in an operation.

John

I once spent four hours searching for a washer I dropped while working behind the panel of my 210. Oh, man, that was bad.

In the Air Army Corps down at Brooks Field, I once had a guy workng for me who inadvertantly dropped a nmber ten steel plain washer into the blower section of an engine. He spent almost a whole day fishing with a small magnet on the end of a piece of brass safety wire, and finally fished it out. If he hadn't got it out, the engine would have to be sent back to the overhaul depot since we were not allowed to pull the blower case at the field level.

A 3/16th castle nut is small enough to be dropped thru a spark plug hole.

T. C. Downey wrote:A 3/16th castle nut is small enough to be dropped thru a spark plug hole.

That's the kind of thing most would only realize through direct experience...