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Jab pistons. gen4 in a gen 3.


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Has anyone fitted the new gen 4 piston in a gen 3 with any success . I'm told the gen 3 piston is no longer available. probably because of skirt's cracking.

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Has anyone fitted the new gen 4 piston in a gen 3 with any success . I'm told the gen 3 piston is no longer available. probably because of skirt's cracking.

gday, did that come from the Jabiru Factory?

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not directly to me but im told it did , not about the cracking though , my comment, because I know it happens and have seen several engines with cracked pistons in all cylinders.

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I never like it anyhow. Aero air cooled pistons should be solid skirt. (generally) Nev

nev, are you referring to the slot under the oil ring. dmech

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In the parallel universe inhabited by lycoming and Continental those split skirt pistons may have had an AD, airworthiness directive issued. If I owned a jab I would be upgrading to the HD piston, my personal opinion worth what you paid for it.

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In the parallel universe inhabited by lycoming and Continental those split skirt pistons may have had an AD, airworthiness directive issued. If I owned a jab I would be upgrading to the HD piston, my personal opinion worth what you paid for it.

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when I did my automotive mechanical engineering and for the last 45 yrs that I have been actively involved in it , a split skirt referred to a vertical split with a hole drilled each end , similar to the old Bedford truck was an example .The opening under the oil ring was referred as a slot . Which as most mechanics will say it is to allow oil to drain , however that is not its main purpose .

I know why these pistons are cracking, and how to rectify it. The heavy duty pistons because of its design will crack in a gen 3 and more likely cause cylinder problems in a gen 4 eventually. I know of a gen 3 fitted with hd pistons that seized with in3 hours, was just curious whether any one has had good results with hd in a gen 3.

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Dmech there are T slot, "W" slot. Invar strutted etc all car or touring type pistons designed for quietness primarily and not heavy duty work. Aircooled Aero cylinders run much hotter than Castblock liquid cooled designs so the expansion relationship that is built into the Auto type is not as effective in the aero types They are also intrinsically weaker and run higher piston crown temps than "solid" types which run more skirt clearance when cold of necessity but decrease to a suitable clearance when hot is it's properly allowed for. Nev

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Dmech there are T slot, "W" slot. Invar strutted etc all car or touring type pistons designed for quietness primarily and not heavy duty work. Aircooled Aero cylinders run much hotter than Castblock liquid cooled designs so the expansion relationship that is built into the Auto type is not as effective in the aero types They are also intrinsically weaker and run higher piston crown temps than "solid" types which run more skirt clearance when cold of necessity but decrease to a suitable clearance when hot is it's properly allowed for. Nev

"Solid" I take it, you mean no slot plenty of petrol trucks run slotted /split skirt pistons . They used to work hard, from memory nearly always overloaded back in those days. Jabiru uses an auto piston and can be made to work ok in the jabiru engine .

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when I did my automotive mechanical engineering and for the last 45 yrs that I have been actively involved in it , a split skirt referred to a vertical split with a hole drilled each end , similar to the old Bedford truck was an example .The opening under the oil ring was referred as a slot . Which as most mechanics will say it is to allow oil to drain , however that is not its main purpose .

I know why these pistons are cracking, and how to rectify it. The heavy duty pistons because of its design will crack in a gen 3 and more likely cause cylinder problems in a gen 4 eventually. I know of a gen 3 fitted with hd pistons that seized with in3 hours, was just curious whether any one has had good results with hd in a gen 3.

Sorry for the incorrect terminology, got that from the jab engine manual. I know nothing about piston design but lycoming use a piston that has no slot with great success in a steel barrel, why can't this type work in a jab steel barrel?

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The Bedford truck is a very low output motor and is watercooled. In those days a lot of trucks were powered by the larger car engines and all were lazy larger displacement petrol motors.. These slotted pistons are designed to distort and expand in such away as to use the cam (oval) shape to be forced to become more round and expand less. Steel struts do the same. This continuous distorting and the slots themselves make the pistons quite weak and prone to skirt failure. even in the cars they are fitted to. They are never used in higher performance motors by anyone who knows what they are doing.. Nev

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Sorry for the incorrect terminology, got that from the jab engine manual. I know nothing about piston design but lycoming use a piston that has no slot with great success in a steel barrel, why can't this type work in a jab steel barrel?

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because of the jab engine design!, pin does not exit the cylinder on a Lycoming . Piston has to be slab sided on a jab. to fit down into crank case , the jab is very compact compared to Lycoming and jab has a much shorter conrod , conrod stroke ratio of approx. 1.5:1, the engine [vn v6 that the pistons were designed for] has a cr stroke ratio of 1.98:1 nearly 2:1. Generally the shorter the cr/stroke ratio the more offset on the pin. The holden v6 piston has an offset of .020", another vehicle I looked at [ Toyota] with similar c/r/stroke ratio to the jab has an offset of.080".how ever in the jab case it has nothing to do with the cause of piston breakage. I don't know what the ratio is on the Lycoming but it looks quiet long , I have one in the workshop at the moment , I will measure it tomorrow. The jab engine is what it is ,and we have had to work with its design to overcome the piston issues , at the moment we have two engines that have just achieved 1000 hrs without breaking pistons ,and these engines have seen some hard work and run only on 95 mogas. these engines have been stripped down for top end o/haul as mandated by jab. otherwise were running fine. The pistons are almost as good as the day we installed them 1000 hrs ago. These are being crack tested and I'm sure they can go another term. Over the last 2 yrs we have tried another ring package ,[ that we had made], in 2 other engines one a 6 cyl, which has now done 200 hrs over 2 and abitt yrs and in all that time has only put about 25cc of oil in the catch can. I need to mention before I go, the slot in the piston has nothing to do with the pistons breaking up. We have not tried jabs hd piston, which looks like a reincarnation of the old acl piston in shape at least, doubt metallurgy is the same. ACL made good pistons, pity they had to close up.

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I've seen aircooled engs go from heavy oil usage to negligible by fitting a good modern ring package,and my series one 3300

i would say uses too much oil,so would be interested in a set of quality rings,care to share any information?

cheers

colin

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dmech your rod length/stroke ratio is very high and must contribute to side loads . The person I got all my information from was running the technical side of ACL years ago and was a personal friend. He is/was the only person who ever explained how "W" slots work. Most people have NO idea. I've also made plenty of my own pistons and never had any failures.How can you be assured the slots which MUST weaken the piston, aren't a problem? They certainly impose extra loads on it). Nev

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dmech your rod length/stroke ratio is very high and must contribute to side loads . The person I got all my information from was running the technical side of ACL years ago and was a personal friend. He is/was the only person who ever explained how "W" slots work. Most people have NO idea. I've also made plenty of my own pistons and never had any failures.How can you be assured the slots which MUST weaken the piston, aren't a problem? They certainly impose extra loads on it). Nev

dmech your rod length/stroke ratio is very high and must contribute to side loads . The person I got all my information from was running the technical side of ACL years ago and was a personal friend. He is/was the only person who ever explained how "W" slots work. Most people have NO idea. I've also made plenty of my own pistons and never had any failures.How can you be assured the slots which MUST weaken the piston, aren't a problem? They certainly impose extra loads on it). Nev

Hi Nev Not my rod length/ stroke ratio , that's the ratio used in the jab engine and with piston offset of .020" the wrong way around . The piston was designed for a longer rod, as used in the holden v6 nearly 2:1. I would never build an engine with a cr/str ratio of 1.5:1 in a light crank case. I have been involved in engine reconditioning [now retired] for the last 46 yrs and had a lot of contact with ACL tech dept. during that time especially in regard to race engines that I was involved with, I used to use their off the shelf pistons and brgs etc. ,while others were using special forged pistons and other expensive components from speed shops. I could build an engine for around$700-$1500 , while others were spending4-5 times that . We won a lot of races including several state titles & classics , with a lot of different makes of engines over a period of more than 10 yrs , all with normal every day parts supplied by ACL, not once did we have a piston or brg failure. had lot of respect for their parts. With regard to the slots , I absolutely know for sure ,that it is not a factor or cause of the piston failure in the jab engine, also the rod length is not a part of it either , the pistons are brittle because of the high silicon content and the frozen in solution heat treatment, so the slot is beneficial,, as it allows the somewhat thin skirt to spring/flex , it also controls the heat flow from the crown into the skirt, thus having an effect on the expansion characteristics .

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dmech I'm not suggesting YOU made the ratios. but you DO have high side thrust loads as a result. and you will also get more vibration. It's to make the engine thinner. How far do the pistons come out of the bore spigots at BDC?Is there enough support for the piston? The ring land area doesn't really count.

Flexing and high silicon don't go together. Some of that stuff is like tombstones. Drop one on the floor and a bit will break out. Brittle as. Nev

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dmech I'm not suggesting YOU made the ratios. but you DO have high side thrust loads as a result. and you will also get more vibration. It's to make the engine thinner. How far do the pistons come out of the bore spigots at BDC?Is there enough support for the piston? The ring land area doesn't really count.

Flexing and high silicon don't go together. Some of that stuff is like tombstones. Drop one on the floor and a bit will break out. Brittle as. Nev

nev I will look at measurements on piston over the week end. High sill and flex does work if their working environment is engineered correctly , as it does in the holden v6 ,However not in a jab. You might say the same about high carbon steel and flex, like valve springs for instance , set them up wrong and they break to. The pistons are designed to flex but they have limitations , like most things metal. The piston does come a considerable way out of the cyl. spigots as you suggest, however there is very little angle load as it is bdc . I and others here ,are using a different piston to the jab, whi

I've seen aircooled engs go from heavy oil usage to negligible by fitting a good modern ring package,and my series one 3300

i would say uses too much oil,so would be interested in a set of quality rings,care to share any information?

cheers

colin

we are using different pistons , not sure how they would go on the jab pistons , we had these made in the u.s and quiet expensive , but worth every cent I reckon.

ch is lower sill. content .about 10% the jab piston is hyper sill content and hardened to t6 t5 would be better which is what acl used to use. However a small mod to the present pistons will sort the cracking problem.

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dmech I'm not suggesting YOU made the ratios. but you DO have high side thrust loads as a result. and you will also get more vibration. It's to make the engine thinner. How far do the pistons come out of the bore spigots at BDC?Is there enough support for the piston? The ring land area doesn't really count.

Flexing and high silicon don't go together. Some of that stuff is like tombstones. Drop one on the floor and a bit will break out. Brittle as. Nev

Hi nev, got some measurements from an old barrel wear marks yesterday from a gen3 jab piston exits the barrel spigots by 20.12 mm [792"]. Piston we are using exits by [ .204" ] measured from bottom of oil groove . Skirt engagement of jab piston 25.9mm [ 1.019"], piston we are using 31.5mm [1.241"] [ oil ring groove higher up] all measurements are at bdc . the piston we are using is a light weight forged piston with a more resilient material , strong as, but comes with its own set of problems as well, but at least no breakups after a 1000 hrs and inspection indicates they could be used again. dmech.

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Thanks for all that dmech. Some well known US piston manufacturer just went out of business. Regards Nev

Hi nev what do you think of those specs I gave you?. Would like to hear your comment

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I think you are headed in the right direction. Forged pistons often have slightly more to lot's more clearance needed but they are generally reliable. It's not that much of an issue if managed properly. I don't like the 3 piece oil ring. Seen too many scratched bores on the Jab cylinders., which I put down to the stainless ring being incompatible with the bore metal. I don't know what else you are going to use. I have a batch of hi sil castings here that I've half machined up but will not use as they are too hard and brittle. You can get an idea by tapping them with a small metal rod and like any hard thing, they ring. You just about have to grind them also. Don't machine well any other way. Nev

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