Difference between Start in Gear (SIG) and Non SIG

[WPB Pitbikes]

Non Start in gear engine with no inner rotor Kit
(Non SIG No IRK)

Its the head of the engine that contains the piston which goes up and down driving the crankshaft via the connecting rod.

On this engine the flywheel balances the crank with the clutch basket on the otherside of the crankshaft.

Good balance good life expectancy of engine......But lacking in that bit extra performance of an IRK.

This engine will not SIG because pulling in the clutch disengages the drive to the crankshaft and the piston does not turn over, also the gearbox remains engaged and turns via the kick so the bike still moves forward .

[WPB Pitbikes]

Start in gear no inner rotor kit
(SIG no IRK)

Flywheel is balanced on crank by the oil spinner and as its factory assembled to have a flywheel and oil spinner the spinner is designed to match the inertia forces of the heavy flywheel.
If you put a IRK on this kind of engine it has been known for the oil spinner to break up.
So its recommended to change the oilspinner at the same time as fitting an IRK

On these SIG engines the clutch will disengage the kicksart drive to the gearbox BUT will still turn the crankshaft and piston over to fire and start.
Therfore you get to kick without the bike moving forward by pulling in the clutch.

[WPB Pitbikes]

NON SIG with IRK

Fitting a Non SIG engine with an IRK can result in damage to the clutch basket as the crank spins up much faster due to the lighter IRK the otherside of the crank. Therfore its not recommended.

[WPB Pitbikes]

SIG With IRK

This is the best overall solution ....a YX engine with SIG and an IRK factory fitted.
The oil spinner will also have been uprated as well.

[WPB Pitbikes]

If the bike is start in gear (SIG) then the clutch is not on the crank, you will have an oil spinner there instead that needs to be changed if you want to fit an IRK.

Start in gears have the clutch actuator on the top of the sidecase, whilst non SIG clutches are on the front facing side.
The oil spinner on these is not in same place, and because you have fitted a IRK on the otherside of the crank the clutch basket will go bang due to the huge loads of acceleration that the quicker spinning up crank with an IRK, will allow.

The heavy flywheel is on the crank for a good reason, it ain't there for the fun of it. It is there to help balance the spinning crank when the clutch basket is fitted the other side.
If you take off the flywheel and leave a heavy clutch basket on the other side it leaves an imbalance that may or may not lead to bearing damage in the inner side cases.

So thats also why you need a stronger clutch basket with an IRK.

Its also why its recommended that IRK's are only fitted to Start in gear engines.

Yx engines with IRK have had all these modifications factory fitted.


If you look at the drive chains of the NON SIG with the SIG you can see why the SIG will turn the head over without turning the gearbox over at the same time as on a NON SIG engine.