Stroker crank case clearence??

[FL-R]

Im putting a 4mm stroker crank in my bike, I was wondering how much of the case neeeds to be ground down to clear the new crank. Thanks

[86 Quad R]

typically you shouldnt/wouldnt get into an issue until you get into the +5mm range and at that depends on the rod being used.

[C-LEIGH RACING]

Just to be safe,
right at the very top where the cases curve around & blend in with the cylinder skirt, the top part of the big end on the rod can get mighty close to that area & all would need to happen, is a little play in that big end bearing or some play in those main bearings for that rod & case to touch each other.
Take that + 4 crank & sit it in the clutch side case half, put a piston on the rod, sit the cylinder on the case & look at that area I'm talking about as you rotate the crank around, you'll see how close the rod & case are to each other.

The + 6 ESR strokers, the rod is clearanced so no need to trench the 250R cases, except for the little area at the top of the cases.
Neil