I see a lot of inquiries regarding spring fitment and parts interchangeability between Traxxas shocks, as well as shock upgrade questions, so here's some info that you may find helpful when looking to better your shock setup. When someone is considering buying Big Bores, but already have the stock ultras, they are more likely to just need need new springs and/or oil in their ultra shocks, or maybe a rebuild kit, and here's why:
A common misconception is that Trx big bores are an upgrade to the Trx ultras. The fact is that the ultras can be upgraded to be superior to the big bores, with the exception of the aluminum shock body. This is because...
TRAXXAS BIG BORE SHOCK BODIES ARE SMALLER IN DIAMETER (OUTER AND INNER) THAN ULTRA SHOCKS, AND ALWAYS HAVE BEEN.
Big Bores got their name before Ultra Shocks were even created, and back then the normal Traxxas aluminum shocks were smaller in diameter than big bores, therefore the big bores were an upgrade. But for anyone who already has ultras and is looking for something to absorb landings better, you'd be better off getting aluminum caps, better springs, new oil and TiN shafts for the ultras.
Ultra shock (body only)
Length: 1.777" (45.1mm)
Outer Diameter: 0.521" (13.2mm)
Big bore (body only)
Length: 1.78" (45.2mm)
Outer diameter: 0.49" (12.4mm)
Inner diameter: 0.40" t10.2mm)
The "Big Bore" shocks actually have smaller pistons too. RPM's instructions for their two-stage pistons for ultra shocks, say they will fit Big Bore shocks if they're sanded down. < Yeah, cuz that'll be an accurate fit, right?
If you are into the really big launches, and are tired of hearing the chassis slap the ground on every landing, then youll probably want to look into stepping up to 8th scale buggy shocks. I think the guys are calling it the Losi treatment. Lol. I'll let these other members add their info and experience with the Losi buggy shocks, as I have no experience with them...yet.
Oh, and sorry for bumming out the BB guys who had though they were upgrading when they replaced the ultras. No harm intended.