Is that necessary to do while replacing the cover, or just recommended if it's worn out, etc?

My seat was 5-6 years old and looked fine. If you wanted it to be more comfortable I would add some foam to it but otherwise I'd leave it alone.

I'm recovering passenger for aesthetics. While I'm at it I figured I'd add some comfort. Where's that thread on the dimensions and the writeup (how to)?

I'm not doing anything to the foam on mine. The seat is in great shape...I just want it to match the bike.

Well, I recovered my rear seat this weekend. Here are the pics:

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I stretched the cover over the front first, made sure it was straight, and stapled it down...then did the same to the back side.
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After I got the front and back stapled down I rechecked the seam on the front...it was a little off so I just pulled the staples out and fixed the alignment
Then I moved on to the sides and did them one at a time...
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Once I got all sides stapled down with a few staples and it looked good, I went back around and put alot of staples all the way around.
Here is the finished product:
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Here it is on the bike...the lighting down in my garage is horrible...kinda makes the seat look a little purple in the pics, I think the fiber pattern does something weird to my cameras flash...I will get a few pics of it out in the sun later.
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Thats all...I may get a rear passenger handle/strap to match.

When I bought my set on Ebay, it came with a passenger handle. Maybe give the person you bought it from an email asking if they have a solution.

Who did you buy it from? In the pictures that I bought it from it doesnt show a strap.

Excellent job. You inspired me to do my own. I have a fabric store nearby to get the vinyl from, just need dimensions.

I may need to get a strap made on someone's machine. I know a few places I can take it and get that done.

http://www.customseatcreations.com Originally purchased from them on Ebay, looks like they've moved on to different ecommerce websites since.

Looks good, man.

Nice work.