GasGas/Sherco/Scorpa 250 Head Insert
S3 Cylinder Heads on a Sherco
The 300 GasGas and Sherco use the same piston and therefore the S3 cylinder head insert is the same shape for both bikes. I assume to save on stocking lots of different models, S3 now only send the GasGas insert out for both bikes.
The inserts are identical except for the O-ring groove so you have to use the GasGas inner head O-ring, GasGas part number MT300416065
The 250 inserts are also now coming as the GasGas version, the GG uses a 0.24mm smaller diameter piston so the insert isn't ideal but it will work ok in the Sherco. Again you will require the GG inner O-ring.
Whenever you swap the head insert please fit new all new O-rings and new copper sealing washers, the O-rings squash over time leaving them oval in shape also the inner O-ring is often burnt from exposure to the cylinder gasses. The copper washers are very soft to allow them to squash and form to the imperfections of the screw and the cylinder head, this creates a very good seal. Once the washers have been squashed they work harden so they won’t form to the imperfections again, if they are reused and hence they may leak.
Most Sherco’s use six small O-rings to seal the coolant out of the bolt holes, the design of the S3 head leaves the joint between the inner and outer cylinder head parts right over the O-ring so it can no longer create the required seal. Because of this these small bolt O-ring do not need replacing with new ones, simply leave the old ones in place and use the new copper washers (Sherco part No. 2390) to create the required seal.
Fitting on newer bikes only (2011 to 2018)
Unfortunately Sherco turned the cylinder head around 180° when they released the new engine in 2011. So if you have the newer engine S3 say to remove the locating pin from the inner insert to allow you to position the spark plug in the position that works best for your bike. Originally the spark plug was leaned back towards the rear of the bike but on some of the newer bikes there isn’t enough clearance so it is best to angle it to the right hand side of the bike. To remove the Pin I think it is best to clamp the pin in a vice and then twist and pull the insert off the pin being careful not to scratch the insert on the vice.