620 pistons in unstyled a

If using a mid 40's 6 3/4 stroke crank ( instead of unstyled 6 1/2 stroke crank ) with factory jd unstyled head . I was interested in using 620 pistons . Is this a good idea ? I have done the 6 3/4 crank with mid 40's pistons in unstyled a and moved oil ring to bottom compression ring Grove ( after modifying the ring Grove ) anyone done the 620 piston thing and have advice ? Also wanted to mention that I am using stock rods .

The 620 pistons are going to be way too short compression height to do what you want. My notes say 4.425" CD for a 620 piston, and an early A M&W is 5.545". Are you wanting pulling tractor compression or plow tractor compression?

Pulling compression , maybe lengthen the rods then ? Or just use mid a pistons you think ?

The A pistons would work better than the 620, but if this is strictly a pulling tractor and you put the compression up where it needs to be the A piston doesn't really work that well either.

The problem is that on an unstyled block the spark plug is a long ways down from the deck of the block (almost 2" from bottom of SP hole to deck on virgin length block). So when you start pushing the piston past the spark plug hole you have to notch the piston. I ran some numbers real quick and to put the compression where in my opinion it needs to be(for the sake of discussion, I assumed 5.625" bore), using a M&W early A piston, you would need to lengthen the rod not quite an inch. That puts you way past the existing notch in the piston and into atleast the 1st 2 if not 3 compression ring grooves. And a 620 piston it gets a lot more ugly because the rings are a lot closer to the top of the piston because that piston was made for the spark plug in the cylinder head, so there is less room/material yet to make a spark plug notch.

If this tractor were something you were going to plow with, take on tractor drives and parade on 87 octane pump gas, the early A pistons (assuming aluminum M&W type and not OEM cast iron) would work well. Since it sounds like though that this is strictly a puller, in my opinion you need a custom piston to configure the dome/compression height/ring location to fit what you need.

Thanks for your help sounds like custom pistons will be needed.

Another thing is the block has a step in it. If you bore it .125 over it removed the step .