Jabiru Pistion pin offsett

[Camel]

Here is link to engineering report bulletin . http://www.jabiru.net.au/images/AVDALSR088-1_Piston_Offset.pdf

 

 

[ozbear]

Even if the pistons are Holden I think you will find that they have been reworked. I know they reworked the gudgeon pin retaining ring groove in early models.

From memory they were 060 oversize v6 Holden pistons to prevent reboreing of the cylinders and subaru bearings on the crank with honda valves in the earlier engines not sure about the later engines .

 

 

[facthunter]

The original Holden pistons have NO gudgeon pin retaining grooves as the gudgeon pin is originally a firm press fit on the conrod eye in the car engine The fit in the rod locates the pin.

 

In the Jabiru application, a shorter gudgeon pin is used with grooves to locate circlips machined into the pistons. There are precautions to be observed when installing these circlips are there are now two (2) types which must not be mixed. They are round section and square section, and only fit corresponding pistons. This is all covered in relevant Jab publications..

 

The offset is small but the reduction of sideload on the thrust side is the main purpose and less piston slap when cold. The ignition timing would be affected if the piston was installed in the wrong direction . The direction of engine rotation is the critical factor when fitting. Nev

 

 

[turboplanner]

So would fitting them the wrong way round leave us with slightly retarded ignition timing?

 

 

[facthunter]

In my head, the difference would be 1 mm (2 x .5) and TDC would be reached earlier. It has the same effect as moving the cylinder on skirt loads. The offset (off centre gudgeon) causes the piston to tilt and supposedly reduces the slap sometimes heard. When the cylinder was moved (in the direction of rotation) it was called the De Saxxe principle.

 

Check my logic on the itgnition timing issue.

 

The Jabiru advance figures in the doc seem conservative. The engine should run cooler with more advance (and more HP) but that is easily confirmed on the dyno. Not rocket science. I think the advance and compression ratio were both reduced a while back. Nev

 

 

[Downunder]

In my head, the difference would be 1 mm (2 x .5) and TDC would be reached earlier. Nev

So the pistons reaching TDC earlier would actually mean ignition IS retarded?

 

 

[turboplanner]

If the piston reached TDC earlier, you would need to set the ignition timing earlier to match putting the piston in that way, otherwise no serious issues?

 

The more you retard the timing the more heat you are going to get past the exhaust valve, but the question is whether the timing was retarded enough to create significant heat.

 

The ignition timing also needs to be adjusted for a compression change.

 

Any thoughts of this burning exhaust valves/overheating guides in the way some people are talking about.

 

Leaner main jets also move the heat up.

 

 

[facthunter]

Chromed exhaust pipes blue quickly when the engine is retarded. Some vertical twin (360degrees firing) show blue on one pipe, fairly quickly. This is nearly always due to the cam in the magneto being ground incorrectly with different timings resulting. Nev

 

 

[turboplanner]

I've previously mentioned that we need to know combustion temperatures to really know what is going on in there.

 

CHT roughly measures in the 100 - 200 deg C range

 

Combustion Chamber temperature roughly measures in 1000 to 2000 deg C range.

 

EGT roughly measures temp cooling as it comes out roughly in between the two.

 

I've never bothered measuring CHT; if you want do and don't want to spend the money, stick a sausage between the fins, and if you smell it cooking, that's some sort of sign.

 

In developing one race engine I used CCT to see where I was going because I had to advance ignition timing to cater for slower burning methanol fuel, and I had to retard the timing because I had a higher compression rate. I got it right after a season, but it was very expensive.

 

So if the compression ratio was reduced the ignition timing would normally be advanced to compensate, another possible cause of increased heat.

 

 

[facthunter]

Sometimes with a very high compression ratio the combustion chamber shape becomes like an orange peel and the flame doesn't propagate well. Coming back a bit in CR, may make the shape more conducive to burning more rapidly. Any how sometimes you need a lot of advance. When this gets to extremes you obviously have some kind of problem. Some engines end up with 50 degrees of advance and not a particularly high revving engine either. Nev

 

 

[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.

 

 

[dmech]

So would fitting them the wrong way round leave us with slightly retarded ignition timing?

YES Factory fitted wrong way, offset IS relevant to rotation ,has nothing to do with prop thrust, or flywheel , thrust , because this engine rotates opposite to the holden engine that these pistons were originally designed for and since adapted by jabiru for their engine, the arrow should go towards the flywheel . This will definitely improve smoothness of engine and performance . The conrods are way to short also 1.53:1 c.r.s.r, I have made longer rods and barrels for my engine ,but have not had it running as yet. offset was originally introduced in the auto industry to correct angularity problems due to shorter c/rods as the demand for more compact engines were required by the auto industry. Piston slap is certainly caused by fitting these pistons with offset to minor thrust side , and this would have to be happening in the jab engine ,absolutely! Offset has often been thought to control slap, but its main roll to correct angularity of c/rod at max cylinder pressure ie. 15- 17 deg atc. There is a number of modern engines that don't have any offset on pins and these do not suffer from piston slap , 2 examples are Subaru ea81 /ea 82. I have both of these engines ea 81 176k and a ea 82 282k and these engines don't have any piston slap noise. some engines ,Honda for example have actually offset the cylinder centre line to the c/shaft centre line by as much as 14mm t achieve simular results .

I made up a model 20 months ago which clearly demonstrates the effects on the engine with pistons fitted wrong way around, since then a number of engines have been changed with good results . and no detrimental effects at all!.

 

The piston used in jab engine has 1.00 mm [.040] pin offset . jab claim .05mm [.020"] offset [see pin offset link ] on jab site. see attachments below. Changing pistons creates a more favourable angle of rod between piston and crank centre line for the smoother transmission of load. With piston wrong way ,it imparts a more jerky motion into crank ,which then is absorbed by the engines rotating mass which results in abnormal loads on flywheel/ prop hub/ prop/and their fasteners, it also imparts an opposite and equal force to the engine block and accessories ,which to some extent is restrained by engine mounts[ vibration felt in air frame]. A recent survey indicating top thru bolts on oil filter side and lower bolts on opposite side failing , tend to indicate that this opposite force may contributing to the bolt failure

 

Piston pin orientation does not change max piston velocity but does change the position that max velocity occurs . Changing pistons around will retard ign tming as it will bring tdc earlier . however as the piston dwells longer as it travels thru tdc this would have a correcting factor, and does not appear to have effected the engines that have been changed , as they seem to have a small increase in power, and run cooler , side benefit of changing pistons is better oil control [ less oil in catch can].Changing pistons according to my model indicates a small effective change in valve timing [ retarding ] , however this engine has long manifold runners and probably results in less flow thru of a/fuel mix into the exhaust during valve overlap resulting in lower egt. Another indication from model is that more of the power stroke is used before ex valve opens ,should result in less stress on ex vale and contribute to lower egt These engines have a history of valve problems , mainly when using avgas , im sure I have a simple fix for this , I mentioned this to Ian Bent some weeks ago & he indicated he would try it out , so wont mention that tonight.

 

Another problem I cant under stand ,is why jab have fitted the oil pressure switch between the pump and filter instead of the main oil gallery this would give error readings especially when cold , also fittings for oil cooler at filter are to small.

 

Also using a sealer over the complete crank case halve is an invitation for disaster, as is the use of Loctite on certain other parts

 

could not load holden manual or jab pin offset link . Could be checked at your holden dealer manual for vn commodore v6

 

A.D.

 

 

[planesmaker]

Dmech, thanks for that information. Tom

 

 

[jetboy]

Dmech,

 

are the old type pistons the same (offset) etc? I'm about to put new rings in (@500 hrs) which is an opportunity to consider this (depending on what else needs to happen)

 

also is there any effect in your model with regards to the flycut pistons - is the weight imbalance (and one bank of pistons has that imbalance on the opposite side) of any significance?

 

Ralph

 

 

[Guest Ornis]

Question. Is the lump under the piston on the side of the offset? YES/NO

 

http://www.jabiru.net.au/images/AVDALSR088-1_Piston_Offset.pdf

 

 

[dmech]

Dmech,are the old type pistons the same (offset) etc? I'm about to put new rings in (@500 hrs) which is an opportunity to consider this (depending on what else needs to happen)

also is there any effect in your model with regards to the flycut pistons - is the weight imbalance (and one bank of pistons has that imbalance on the opposite side) of any significance?

 

Ralph

will talk to night or tomorrow, off to fly behind one of those modified motors today. would like to upload holden engine manual but must be to large for this site if you have an email I can email to you / would prefer every one could see it though ,if you know of a way to get it onto this thread/ forum, please let me know how.

A.D.

 

 

[reggie]

And is this what's creating the harmonics that's braking flywheel bolts?

 

 

[facthunter]

Very unlikely. The order of magnitude is very small compared with the real forces operating. Regarding the offset, it's dead easy to use a pair of callipers with the piston pin in place to determine which side it is offset to. Put an arrow on the top of each piston in texta indicating front so there is no doubt. The pistons are probably only made by one maker, Probably ACL originally. so you should get a reference that way. Nev

 

 

[planesmaker]

Question. Is the lump under the piston on the side of the offset? YES/NOhttp://www.jabiru.net.au/images/AVDALSR088-1_Piston_Offset.pdf

This only covers the wear etc on the piston and basically admits there is little difference and really won't matter which way it is put in. However it does not address possible vibration ,however small , that may lead to the possibility of through bolt breakages.

 

 

[Guest Ornis]

Someone please help. Does the position of the lump under the piston indicate the side of the offset wrt the centre? YES/NO

 

 

[dmech]

Question. Is the lump under the piston on the side of the offset? YES/NOhttp://www.jabiru.net.au/images/AVDALSR088-1_Piston_Offset.pdf

holding the piston upright and arrow or lump away from you,or forward , , the pin will be offset to your right , that side also happens to be the major thrust side when fitted to the holden. Please note : jabiru 's engineering paper's quote offset of only .05mm [ .020"] . Offset that I have measured is 1.00mm [ .040"] this is also confirmed by the holden vn engine manual. Pistons with valve relief 's is still the same holden piston .

A.D.

 

 

[biggles]

this is also confirmed by the holden vn engine manual. Pistons with valve relief 's is still the same holden piston . A.D.

Offset is actually 0.5 mm. ........... Bob

 

 

[Guest Ornis]

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.

 

 

[facthunter]

0.5 mm is the quoted figure. . I'm not going to talk about lumps here if I don't know what pistons have them. Just measure it . That's the safest way. You are not checking the amount . Just the way it is to be orientated... Nev

 

 

[dmech]

Dmech,are the old type pistons the same (offset) etc? I'm about to put new rings in (@500 hrs) which is an opportunity to consider this (depending on what else needs to happen)

also is there any effect in your model with regards to the flycut pistons - is the weight imbalance (and one bank of pistons has that imbalance on the opposite side) of any significance?

 

Ralph

NOT SURE ,if fly cuts are the same size,then you would have to swap from one side to the other to change piston orientation , will look at this but sure that can be done