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jberger
Samba Member
Joined: November 17, 2003
Posts: 2476
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 Posted: Tue Sep 10, 2013 4:01 pm Post subject: |
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| boof1306 wrote: |
| In my experience the reason we see a lot of out of round qsc liners is that at 96mm the lower skirt thins out and becomes malleable. All the discrepancies I have seen are in that lower section. So no surprise the cofap liners are straight at the original 94mm. I wonder how many qsc liners are bent when they leave the factory and how many are damaged in transit. |
That has not been my experience. They were pretty effed right at the top, do not pass go. Out of round is first, taper comes second. Boring too quickly? Dull tooling? Out of tolerance? Every once in while they are ok. Even quality shops have difficulty boring the stock cylinders, they are so hard that as they are boring them the cylinder grows so the dimensions change. I used kibblewhite to bore my set and I don't think they would do it again. After checking 6 places on each cylinder I could not measure more than .0001" between any of them. As a reference, brand new Mahle cylinders are .0000", for real. |
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morymob
Samba Member
Joined: November 09, 2007
Posts: 4683
Location: east-tn
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| Posted: Wed Sep 11, 2013 4:08 am Post subject: |
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| Have u considered contacting EGGE, they make up 1 to sets of them. |
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tencentlife
Samba Member
Joined: May 02, 2006
Posts: 10163
Location: Abiquiu, NM, USA
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| Posted: Sat Sep 28, 2013 7:43 am Post subject: |
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What bore size were these cyls?
What combustion sealing ring was used?
If a stock sealing ring was used on any oversize bore without cutting relief around the top edge of the piston then unless the crown is kept low in the bore there is danger of collision which will break off the top land just as seen in the pic. At normal operating temps the rod-piston combo will grow in length by ~.010" as it is. That cyl did have chronic deto going on, apparently confirmed with the discovery of a bad injector, but the evidence is right there on the piston crown regardless. Running hotter, that one would have expanded even further increasing the likelihood of collision with the sealing ring.
If enlarged-ID seals were used in this case, then judging by the severity of the deto I think it alone could easily account for the breakage.
A little more on this topic here:
http://www.shoptalkforums.com/viewtopic.php?f=37&t=140150
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tencentlife
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Joined: May 02, 2006
Posts: 10163
Location: Abiquiu, NM, USA
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Dave Harrison
Samba Member
Joined: March 02, 2012
Posts: 14
Location: Athens, GA
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| Posted: Mon Sep 30, 2013 6:09 am Post subject: |
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What bore size were these cyls?
What combustion sealing ring was used?
Tencent,
The bore on these cylinders was stock 94mm, and yes, a combustion sealing ring was used. So in this case, there was no sealing ring/piston crown contact. These AA piston crowns were actually shaved down (I assume they were modified this way) a little too much, so that at the top of their stroke, they came up shy of being at zero deck (shy by over 1mm)...
It's very hard to say what has caused all the erosion and pitting; it could be detonation, piston debris or steam erosion. I have had non-VW engines that have had slight water ingress (when hot) due to cracks between the valve seats and they leave a very similar pattern of destruction. Is the top of the barrel pitted above the top ring witness?
silverbulletuk,
There was no pitting on the cylinder anywhere. A small 1-2sqcm patch of melted aluminium from the piston was deposited on the inside of the cylinder inline with the spot of greatest piston destruction. Otherwise, the cylinder appeared pretty undamaged.
rsxsr,
I seriously considered going the route of custom pistons, but since I would have to track down new cylinders and rings, I figured if I could get a complete set of QSCs or Cofaps, I'd go that way.
I managed to get a great deal (under $250.00 shipped) on a new complete set of cofaps from a seller in the classifieds here (with the 35.8mm compression height), so I'll be getting these machined to zero deck and hopefully, I'll have time to put this thing back together in the next month or so.
Thanks again everyone for all the input, and "suggested reading".
Dave |
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