Giannelli vs stock
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examble here
https://www.moto-hame.fi/tehopakoputki-yamaha-dt125rdt125x-athena-p-8028.html
the seller said it came with 4bl 170cc big bore kit
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That's odd that, as I can't say I'd recommend either of those pipes.
I'm sure they are fine. As I say, the DT just isn't a quick bike, so definitely don't spend too much $$$ trying to make it into something it's not.
By all means release some free ponies and just enjoy it for what it is.
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Stock gutted is pretty decent.
The benefit is more weight.
If you are handy with a welder, then I'd look at having your current pipe derestricted.
Check this out.
https://dt125r.co.uk/topic/20/yamaha-dt125re-x-stock-exhaust-pipe