Jack Donsen

Google Jabiru Sth Africa and ask for Len Alford.

Google offset wrist pin and see that the pin should go towards the major thrust side. Then measure an old piston and see that the wrist pin is to the anti thrust side because the direction of rotation has been reversed for RH prop compared to SAE spec auto engine.

I have corrected wrist pin offset in several Jabiru experimentals and they work heaps better.

CHT drops 30F, Oil drops 10F and you gain 50 RPM - guaranteed. Just rotate the pistons so the arrow faces rearwards.

Gooday all i am in the prosess of top end over hall on my jab 3300 humm.I have a question ?

 

 

 

 

 

 

 

Jabiru use a holden piston, the holden motor rotates in the clockwise direction the jabiru rotates in a anty clockwise, thus should mean on a Jabiru the pistion notch on the piston crown should be fitted to the rear.

 

,

 

Offset is always to the major thrust side of the piston, as the Jabiru motor rotates the oppsite direction the pistion notch should face the rear. 'Not the front' as stated in the manual.

 

Quote' from Jabiru manual

 

Note: Arrows on inside of pistons point in direction of rotation. Oil Rings and

 

 

 

 

 

 

pistons and bore well. Notch on the piston crown faces the Propeller Flange.

 

When i arsked Jabiru engine shop they told me in short that the thrust is from the propeller on a plane and a car it is from the flywheel and according to them the manal is correct.

 

I for the thrust is from the power stroke ???

 

"humm" :hittinghead: I dissagree.

 

All the reserch i can find states that,

 

"Most engines today utilize pistons with an offset pin bore. That is, the pin

 

bore is "moved" a specific distance from the centerline of the piston. In gas

 

engines the offset is always to the major thrust side of the piston. The

 

piston thrust side is the part of the piston perpendicular to the pin bore

 

that carries the majority of side loading during the power stroke.

 

The primary reason for pin offset is to prevent the piston from slamming into

 

the cylinder bore after the connecting rod journal passes top dead center.

 

This problem is referred to as piston slap. The desired scenario for piston

 

movement is to gently rock from side to side within the bore. This rocking

 

motion eliminates slap. "

 

by Hunter Betts

 

Can any one else shed some light on this

 

Thanks Mick

Your assessment is correct. The reversal of the crank rotation effectively moved the wrist pin offset to the wrong side of the centreline. Jabiru Sth Africa have been reversing pistons from new for over 10 years and are getting great results in achieving 1000 hours TBO. If you record your T's and P's prior to correcting the offset and comparing them after, you will drop CHT by 30 degrees F and the oil temp will drop about 10F and you will pick up an extra 50 RPM at full static.