Happijac Bed Pin Size

Thanks everyone, 1/4" was the right size, I just underestimated how long I needed them to be.

+1 on that 1/4" at least that was the last one I worked on. If the jack tops (perhaps bottoms) out (I forget which, maybe it's both), it's likely to shear a pin. I understand they've upgraded to 5/16" pins, might pay to call the company and see if the jacks can be modified.
The man I helped with his jack and 1/4" pin didn't want to wait for shipment of the pins so he got some at a hardware store. Problem was they were long. I trimmed them holding in a vise and cutting with a bi-metal hacksaw blade.

1/4"

same as the Tongue coupler pin.

Do you have a pin for reference? You could take it to the store to match. Lacking that, you'll need to measure the hole size for the pin.