I have a what I consider knowledgable source tell me I need 2.5" coilovers on my SAS'd 4680lbs Amigo.

I can see where fundamentally 2.5" is thicker therefore theoretically stronger than 2" of whatever.

The shaft is still 7/8" 2 or 2.5.

Can anyone explain why the 2.5 would be better for crawling over a 2". Is it the weight of the vehicle?

Also, I don't plan on flying over much of anything anymore, do I really need a remote resevoir? Not that this is an option from some companies.

I don't think you need 2.5 unless you can't get stiff enough springs for the 2" but at 4700 Lbs you should be totally fine. I would think around 5500-6000 lbs would be the cut off for a mostly crawling vehicle.

The reason shocks need a resrovoir is this. When the piston moves up into the shock it displaces some fluid, so there needs to be some air space in there. It is much better to have the fluid and the gas seperated by a floating piston to prevent the gas from getting whipped up into the fluid and creating and emulsion. Some shock designs have put the floating piston in the main shock tube but this makes for a very long shock. Most of the high end shocks now put the piston in a resrovoir.

You can also buy emulsion shocks where the gas and the fluid get all whipped up together, with this design the damping coefficient changes a lot more as the shock gets cycled, and more gas gets whipped up into the fluid. So these shocks aren't quite as good from the performance standpoint, but they are still alot better than the twin tube or emulsion shocks you can buy from rancho or monroe or whatever.

I think sway away might have some shock fluid figured out that makes the emulsion shocks work prety good. Maybe AndrewBurroughs will reply, he really knows this stuff.

You will be fine with 2" coil overs as long as your doing mostly rockcrawling, street driving an mild high speed stuff. The biggest advantage of a 2.5" over a 2" is shock capacity. Basically you can valve a 2.0 and 2.5 to be the same but the 2.0 will be working harder thus creating more heat. you will never over heat a 2.0 rockcrawling or street driving, only if you go fast over big bumps for a long time. A 2" coil over is probably twice the shock of a twin tube rancho type an most people dont over heat those by rockcrawling. Other issues are springs, most rock crawlers use light rates and mount the shock near the axle so you shouldnt have any problems finding springs, they make longer, higher rates for the 2.5's, but most of you shouldnt need rates that high. As far as strength goes, the 2.5's have slightly bigger components, but breaking shocks hasnt been an issue for rockcrawlers. In fact, most shock manufactures use the exact same shaft for both size shocks. Gordon is right about the resevoir, while it helps with heat, it's function is mostly to house the dividing piston, I would recommend getting them because the shock works more effectively, more effeciently, and is more predictable. Resevoirs usually cost about $25-$30 extra.