What's this thing on header?
 

https://i.ibb.co/QnV4jqF/fa20-3-2.jpg

I found this interesting header from Toda Racing, but what are the things circled in red? Springs?

They have a joint there if you look carefully. That's a weird design. I don't see how it could possibly not leak at that joint

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The springs hold the pipes to each other. Looks like a motorcycle pipe

It`s for movement of the pipe as it expands and shrinks with temperature changes. The joint helps prevent stress fractures/cracks and the springs obviously keep it together.

^^this^^ The stock header and many others use a flexible bellows for the same purpose.

This is pretty common with motorcycle exhaust designs, the pipes have a pretty tight slip fit and the springs keep them pulled together snug. I've had a couple bikes with several joints like this and it was never an issue.

Thanks. So pipes are not welded there, but just slipped in?
Wouldn't this lead to leaks?

One would think. It may work fine on motorcycles but I'm gonna assume their O2 sensor is upstream. Otherwise you'd get a bad reading

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Leak? Whaaa?

Now the opposite question: headers like JDL doesn't have anything against thermal stretching, wouldn't this be an issue too?

Looking to buy the Toda Racing header?

Most very earliest aftermarket headers had some joints of this spring loaded or other flexible type .. i'm guessing, possibly because they thought that if stock headers had it, they need it too.
"2nd" generation of headers mostly hadn't such, probably because due collected more statistics it seemed that in most cases headers worked fine without observed excessive welds cracking or alike even without those flexible joints, so decided to not include them to simplify & cheapen design/production.
So imho no, it shouldn't be an issue if there is such or not, as far as in general you don't see much issues reported regarding particular header you intend to buy. Some worse quality headers (or other aftermarket exhaust bits, eg. catback) may see cracking welds even with such joints, and opposite, good ones may last just fine without.

JDL UEL headers actually used to have braided flex joints.

https://www.ft86club.com/forums/atta...5&d=1476499948

They got rid of them though and it apparently hasn't caused any issues. There is some thought that flex joints are fragile and susceptible to failure.

https://www.perrin.com/images/blog/2...pe-550x366.jpg

I wouldn't worry about it too much as long as your header is made by a reputable manufacturer with experience on the platform.