Rebuild

Hi All
Well I have finished rebuilding a St70 Dax 6V ,It had a 12v piston fitted by someone after the old one had hit the valve and wrecked the head a new head has been fitted (E22) with the letters SUNK engraved on one of the fins.The bore was ok no marks and was a Std size, I fitted the 6v piston and rings along with new gaskets and seals but when I started it up its burning oil ( very light blue ) a black oil residue is coming out if the exhaust breather that is above the engine filler cap , Has anyone got any ideas what the problem could be .

Correct head for the piston?
Check the
- piston dome (50 / 70cc)
- combustion chamber (50 / 70cc)
- Exhaust valve gasket
- piston rings (condition, position, overlap)
- all rubber rings in place
- head nut torque

I wondered about the Head myself, then I asked myself, how do you tell the difference between a 6v and 12v head?

Measure up the combustion chamber. (diameter)
OT50 = NT70 = 39mm
OT70 = 47mm

Valve diameters:
OT50= NT70 = 23mm IN 20mm OUT
OT70 = 25mm IN, 22mm OUT

As a general rule blue smoke is excess fuel and grey/black smoke is oil

it's the other way round
blue = oil
white = water / condensation
black = excess fuel (or lack of air)

Oops.........., I shouldn't post after a saturday night's drinking session

Waynus wrote:Measure up the combustion chamber. (diameter)
OT50 = NT70 = 39mm
OT70 = 47mm

Valve diameters:
OT50= NT70 = 23mm IN 20mm OUT
OT70 = 25mm IN, 22mm OUT

Thanks Waynus, I assumed OT means "Old Type" So this is the measurements for 6v heads?
But then Im confused when you say "OT50 = NT70" ?

Meassurements for the Old Type (6v) 50cc heads are the same as the New Type (12v) 70cc heads.
So make sure you don't mix these up. (look at the camshaft bearings)

NT50 (AB27 for instance) are the 'eco' cylinder heads with even smaller valves.
But.. they can perform great with high-compression kits.
(bigger valves doesn't necessaraly provide greater performance, especially not in the low/mid revs range)

HELP
Hi guys I am still having trouble , Now i am getting a knocking noise from the cam adjustment side . I got hold of a new cam chain and rollers I fitted the new rollers but when I tried it fit the new chain after a struggle it went on but was very very tight , So I took it off and I have fitted the old one back on and have the loud knocking noise that wasn`t there before I changed the cam rollers . I have followed the instruction in the manual about adjusting the chain but it makes no difference , My question is will or should a new chain be very tight I have counted the links on both chains and both have 82