Long story short then background...

When I try and start my Ford 641 it sounds like the starter is just spinning with no engine turn over. Identical to this:
https://youtu.be/Rgn6zPn6ZJY

Background:

A few months ago I purchased this tractor. It started right up but ran a little rough. I did a tune up (plugs, wires, ignition coil, new fluids and filter). After tune up she fired right up and I ran her a little. However after a week I noticed gas in the oil.

So... I rebuilt the carb and she fired right up (I also know now not to use to much choke). I needed to change the oil again due to gas in the oik and it had a minor oil pan leak so i figured I would replace the oil pan gasket. After waiting 2 weeks for the part to get in and finding the time today I did it. Because she had sat for so long I didn't want to fire her up to move before changing the gasket and recycle the gas oil.

Put her in neutral and towed her 40 feet so I could work on her. Replaced gasket just fine. Went to start her up and the YouTube video is what it sounds like... Like I said prior to this no issues starting.... Any ideas?

Royse gave you good advice. Follow it!! don't be like some others who come here for advise and then argue every point.

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This may also be due to a leaky fuel cut off valve - Always cut off the gas if it's going to sit for more than an hour.

I am just wondering why it would happen right after changing the oil pan gasket... or if it was just a coincidence. Maybe oil splashed up there? - It is doubtful the two events are related.

Royce gave you good advice - Pull the starter and look for oil/grease on the starter shaft or starter helix.

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I don't believe splashing could get there in this case.

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It's always on a trickle charger and connections are good. I did check that. I am just wondering why it would happen right after changing the oil pan gasket... or if it was just a coincidence. Maybe oil splashed up there?

When mine broke, it didn't even make a noise first, so its possible.

One other thing you might try before pulling the starter is to

fully charge the battery and make sure all the connections are

clean and tight. It sounds like its spinning fast enough to me,

but it certainly wouldn't hurt to make sure its in top shape.

If its cold in your part of the world, oil on the shaft can make the

bendix sticky enough that the starter will spin but the bendix won't

slide and engage as it should.
You could also try a 12V boost straight to the starter, not jumping
to the battery. But as I said, it sounds like its spinning fast enough
to me.

Would it just break without any pre warning? Also, it's a 6v system.

Sounds like the bendix is stuck on the starter shaft so it doesn't

engage, or the bendix is broken. Common on 12V conversions.

I would pull the starter and have a look.

If the bendix isn't broken, clean the shaft really well and leave

it clean and dry. No oil or anything on the shaft.

If it is broken, you should easily notice the teeth missing.

Link to video