All these motors can have their performance improved by a considerable amount, but it is a step by step process, some of which can be expensive and complicated to carry out. But because all these motors are fairly similar, the modifications are similar also.

The major differences are:

Series 1 - 175cc motors have a unique kick-start in the middle of the chaincase. Only a basic porting job is desirable on these motors.

Series 1 - 125/150cc motors have petroil-lubricated flywheel bearings, bronze bush small-ends and shorter

cylinder lengths.

Series 2 - 125/150/175cc motors have mainly bronze small-end bearings but use the normal flywheel bearing

lubricated by grease on assembly with 2 seals - one either side in the magneto flange. The later Series 2's can

have needle roller small-bearings. 125 and 150 motors use conrods that are 107mm long. TV175 use conrods

that are 116mm long. The pistons used are the same overall length but the gudgeon pin is positioned to allow

the extra length of the conrod. Therefore, as 175 pistons are available with both gudgeon pin positions, it is

important to state whether it is a TV175 piston or a 150-175cc conversion piston you require.

Series 3 - 125/150/175cc motors use needle roller small-ends and are the same as Series 2 in general layout.

SX/GP - 125/150/200cc motors (Series 3) use needle small-end bearings but have a thinner conrod at the big

end to allow better lubrication to the big-end bearing. The small-end eye has shims either side to centralise the

conrod in thepiston so that the conrod does not rub the crankshaft webs and cause extra wear and heat. These

motors use 2% oil (125/150cc) rather than 4% oil (200cc) in standard format.

The GP 125/150/200 crankshaft uses the same floating conrod assembly but with a thicker flywheel taper. The GP150/200 crankshaft also uses a wider flywheel bearing, different shaped crankshaft webs for balance and a stronger big-end pin.