taco eater :
Will Caldwell from San Francisco, CA United States
lil bit misleading but like mr. treat said there are a lot of different derbi motors out there. the pp motor i pulled off a '80 variant uses 17x28x6 seals. most likely the earlier piston porters used this seal, then when derbi was producing both piston port and pyramid reed at the same time, they both used 16x28x7 seals. these i believe are the pp motors that have the pyramid reed case induction that is simply blocked off. yeah.
update!
ALL seals from a 1980 piston port
2 - 17x28x6 single lip crankshaft seals (replaced with double lip seals cause that's what was available) 2 - 16x24x4 single lip (or 5mm doesn't matter) pedal shaft seals (again these were replaced by double lip seals because they were what was available locally) 1 - 15x30x7 single lip sprocket shaft seal 1 - 25x40x7 double lip DOUBLE SPRING rear pulley shaft seal (i replaced this with a double lip single spring seal...shouldn't be a problem but we'll find out!)
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multiple sizes git yer measurin stick
December 21, 2011
taco eater :
a treat from treatland
actually there are multiple sizes of seals used on differing years of derbis. i asked a dear friend about this and his response was... I asked XXX about this and he said the seal in stock has been fine for a few of his motors (and one he just installed them on). My theory: there are two different piston port cases, the early ones which are completely flat on the top minus the front upper frame mount, and the later ones which are cast for the later pyramid reed blocks but are filled in. XXX's recent motor was one with the older cases and your seal worked fine. XXXX has also used the seal on older cases with no issues as well. I'm guessing that the newer cases use a different seal (which would also carry through for pyramid reeds). That said, everything derbi is always different. I've seen 3 or 4 different rear brake hub mounting styles on same and different year piston ports. There is no consistency.
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jesses right
December 19, 2011
taco eater :
Ryan Nash from Columbia, MO United States
Pyramid Reed Yes! Piston Port Maybe! 
jesses right