dmech Posted November 30, 2014 Share Posted November 30, 2014 I'll rephrase the question. Looking at the bottom of the piston, is the gudgeon pin offset towards the lump or away from the lump? It can only be one or the other, surely. Ornis Offset is neither towards or away from lump lump is on pin axis looking at bottom of piston with lump towards you the offset will be to the right of pin axis, and is obvious by just looking at open end /bottom of piston Link to comment Share on other sites More sharing options...
facthunter Posted November 30, 2014 Share Posted November 30, 2014 IF you have two different weights you could get away with orientating then in opposite positions. Ie for the FOUR cyl horizontally opposed, have both rear or both front cylinders the same weight pistons. Nev Link to comment Share on other sites More sharing options...
Downunder Posted November 30, 2014 Share Posted November 30, 2014 Maybe someone with a piston could take a picture? Link to comment Share on other sites More sharing options...
dmech Posted November 30, 2014 Share Posted November 30, 2014 Offset is actually 0.5 mm. ........... Bob How do you arrive at that figure bob , holden manual quotes 1.00mm , are they wrong!! Link to comment Share on other sites More sharing options...
facthunter Posted November 30, 2014 Share Posted November 30, 2014 It doesn't really MATTER (for our purposes here) You are only trying to fit it the right way around.. Nev Link to comment Share on other sites More sharing options...
biggles Posted November 30, 2014 Share Posted November 30, 2014 How do you arrive at that figure bob , holden manual quotes 1.00mm , are they wrong!! dmech , you quoted the offset as .05 mm . I was just bringing to your attention that the figure quoted by Jabiru is , in fact 0.5mm . Bob 1 Link to comment Share on other sites More sharing options...
jetboy Posted November 30, 2014 Share Posted November 30, 2014 IF you have two different weights you could get away with orientating then in opposite positions. Ie for the FOUR cyl horizontally opposed, have both rear or both front cylinders the same weight pistons. Nev Nev if youre meaning my question about the weight difference with the new new flycut pistons variation, no you cant get dynamic balance with the offset pistons in the engine because the flycuts will be on the offset side of some pistons and on the other side for the rest, because thats where the valves are. It wasnt about sustituting one piston for a new new type. I was asking if there could be a future issue due to these lighter pistons regarding piston rocking/slap effects due to the cutouts being in different sides on the different pistons. Of course if this was an issue then all pistons could be flycut on both sides. Jabiru seem to be desperate to lower the compression ratio anyway. So long as there are no hot spots created on the crowns due to this. ralph Link to comment Share on other sites More sharing options...
facthunter Posted November 30, 2014 Share Posted November 30, 2014 I get what you mean. Some flycut (relieved) pistons are done to suit any installation, by doubling up on the recesses. They do produce sharp edges. Re the balance in the sense that concerns you. I don't think it's enough to make much difference as the skirt length is reasonable and there is already an effect by having the pin off centre, which the remachining will add to on one side and subtract on the other. Nev Link to comment Share on other sites More sharing options...
Guest Ornis Posted December 1, 2014 Share Posted December 1, 2014 Here is the Jabiru engineering report: http://www.jabiru.net.au/images/AVDALSR088-1_Piston_Offset.pdf Looking from the back of the engine, the right side of the engine is on the right and the crankshaft turns clockwise. Looking at a piston, the offset is on the same side as the lump shown in figure 6. Looking at the diagrams 4, 5, 10 and knowing the lump is towards the propeller in all cylinders, the gudgeon pin offset is, on the power stroke on the right side of the engine: below the crank centre line on the left side of the engine: above the crank centre line This orientation means the conrod is more parallel to the cylinder early in the power stroke than otherwise. The force from the piston is parallel to the cylinder whatever the vector is through the conrod. I would say that placing the conrod to be more parallel to the force is the rationale behind the choice, particularly in an engine with short stroke and conrod. The specifications in the box with the MAHLE (formerly ACL) pistons, give the offset as 0.5mm. Link to comment Share on other sites More sharing options...
facthunter Posted December 1, 2014 Share Posted December 1, 2014 I agree with your conclusions. The de Saxxe principle does the same by moving the cylinder to achieve the same result regarding the sideloads on the piston skirt. This is impossible with the type of engine construction here due to the need to have the opposing cylinders at the same height in the crankcase. The moving of the cylinder has gone out of fashion anyhow. Nev Link to comment Share on other sites More sharing options...
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