Not very big at all it almost seals with just the clamp, less than a millimetre. The exhaust manufacturer recommends the OEM gaskets which is what I have purchased.

http://www.ebay.co.uk/itm/1726259089...%3AMEBIDX%3AIT

http://www.pipewerx.com/yamaha-xt-66...l-exhaust.html

"We recommend fitting new Yamaha gaskets with these, part number 4EL-14714-00"

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Now have a 2017 S1000XR Sport SE

2006 XT660X - Cat D write off due to someone pulling out on me - Pipewerx carbon fibre exhausts - Snorkel removed and airbox drilled - DNA stage 1 + 2 air filters - Kevs fuel controller mod - Scottoiler V system - Oxford heated grips - HEL braided brake hoses - Wilbers 540 rear shock - Brembo brake pads - AFAM chain and sprockets - Michelin Pilot Street Radial's - Denali DX Xtreme dual intensity spotlights