Anyone know anything about flathead 6 engines?

I owned several when I was a kid, and I love the old flathead Mopars.

If it has a deep knock under load, it's probably a main bearing. A lighter knock when not loaded is one or more rods. I've replaced both without removing the engine from the car, though the rope style rear seal is sort of a pain.

Those engines are extremely tough and were used extensively in farm equipment. We had both a combine and a tractor with flathead Mopars and they were as good as any farm equipment made.

The military had thousands of them in trucks like the famous Power Wagons, and if a GI can't break something, it can't be broken. They even used them in flightline tugs. Back in the early 60's, a GI bought a surplus tug in England, modified the snot out of it, and ran it at the drag races. They did a story on it in the Stars and Stripes newspaper, and as a Mopar fan even then, I enjoyed showing my buddies the article.

They are also surprisingly fast. I've driven for miles with the speedometers locked at an indicated 105, and never had one break. The '48 Plymouth I sold a friend was crushed by a tornado, so he decided to see how long the engine would run at full throttle in neutral. He placed a heavy rock on the accelerator and we listened to it scream for over a half-hour before it blew.