MFDgame wrote:My question is whether it is possible to buy a rebuilt MC21 engine and use that in the MC18. What modifications would need to be made to make that work? Would there be any cutting involved or is it just a matter of matching up the correct wires? Basically what would be a best case scenario is if you could straight bolt everything in and use the carburators and everything from the previous engine. I really have no idea what the best option would be but any advice would be much appreciated

An mc21 engine will just bolt in and work, everything will line up.

There is a tiny difference between the mc18 and mc21 flywheel timing though they will both fit and work on either motor and the mc18 flywheel will fit an mc21 crank anyway.
As mentioned the mc21 has a gear postion sensor (GPS) but as the mc18 doesn't use one at all it's irrelevent, wire the neutral light up or swap the whole sensor and you're good to go.
Carbs, exhausts etc all fit and line up perfectly - the mc18 and mc21 crankcases are externally the same, barrels only differ in shape and a tiny bit of porting, not configuration.
Basically the mc18, mc21 and mc28 engines are mix and match, the only things you can't mix are mc21/28 barrels and mc18 heads (and vise versa) and early mc18 cranks with mc21/mc28 cases don't fit either.
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