how do you seal them?
my pipes are just push together, its been used before and theres no slots in the system to suggest its been clamped, but there is little loops welded on for springs.
how do you make sure the joins don't leak? is exhaust putty the only thing used?

I used exhaust assembly paste for mine, which is different to putty I believe, and it doesn't leak although I had to do it twice.

Our exhaust builder says to use high temp silicon sealant on the sliding joints.

ah yes, assembly paste, thats the one i meant so is that all thats needed? no other sort of seals or anythning?

ah, silicone, just saw that searching on interweb. it says paste is fine on mild steel, but doesn't stick to shiney stainless, its a stainless system i've got, so i'll go with that

coomon on racers not to have anything in the joints. They are a 'decent' slide fit to enable removal and expansion, especialy useful on complex exhaust routings. They are only slack when stone cold, when fired up they blow momentarily and then expand enough to seal.
Usefull to isolate the manifolds from the system which is usually easy to knock if you have an 'off'. Pings the springs off at worst and saves the manifolds (usually the expensive bit!) from damage. All the more important if the system/maifolds are stainless as its not as ductile as mild steel and tends to crack rather than bend.

most suppliers like simspon exh, demon tweeks, Merlin stock them as sets with the weld on tabs or just as spare springs