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About dmech

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  1. what has happened to the old classifieds , will they return?
  2. Hi nev what do you think of those specs I gave you?. Would like to hear your comment
  3. 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
  4. 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 th
  5. Thank's for info one-track, just an article I read inside a dc3 on display at the Mareeba air port QLD. Was just outside a repair facility there . That was 15 yrs ago.
  6. I read some where , I think it was at Mareeba airport, that the Russians copied and made more dc3 s than the original manufacturer never paid a cent for license or copyright .
  7. 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 sp
  8. 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 th
  9. "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 .
  10. 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
  11. nev, are you referring to the slot under the oil ring. dmech
  12. 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.
  13. 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.
  14. Was just a by product of changing to a different type piston, piston change wasn't intended for increased power. Piston were changed to prevent piston break up which seems to have worked, this project has cost us a huge amount of money and time , we are now working on a ring pack to complement this piston and we have had some amazing results so far with one engine 3300 has done over 130 hrs and absolutely no oil in the bottle, bottle is bone dry, we havnt striped this engine to see how cylinders are going ,[ owner wont allow us ] but I hope to bore scope cylinders next week
  15. Sorry did not reply earlier , we went on nbn wireless , nothing but trouble since. All the pistons , except for one set , were VR pistons. the other 4 were Silverlite brand, we tried these because they had a lot more material in the skirt area where the VR pistons were cracking, they only made 130 hrs before cracking , they cracked from the oil ring slot downwards and inwards, where as the VRs cracked from the bottom of the skirt upwards and inwards. We had 2 sets of VRs only made around 260hrs each with about 8 hrs difference between them. We are experimenting with another piston , and ha
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