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Leak-down results after blow up

Long and the short- I popped my motor this weekend after whacking the rev-limiter to hard. Went home did, a compression test, didnt like what I saw, pulled the motor, did cold leak down, found bent valves, tore off heads, and now a real *****ty problem. I blew off two rockers and bent the exhaust valves on 1-3, cylinders 4-6 were fine (All had 3% Leak-down). When I did the leak down I felt like there was more air coming through the breather on 1-3- so in turn I took the good heads from 4-6 and tried them on cylinders 1-3. Heres the fun part- 1-3 with the good heads still had bad and inconsistant leakage numbers. Took the heads back off along with the cylinder from #3. I looked and looked and looked, tried puring oil into the cylinder and re-trying the leakdown and still got a bad number and the rings were perfect. Now I think I have a cracked piston/s, Before I go and blow my head off I would like to know if anyone has any ideas as to what the problem could be and/or maybe a solution.

Oh and by the way- all of the chain ramps on 1-3 were broke- I pretty sure that I had a bad tensioner from the looks of it. This should have been the cause of the problem.
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86' 930/GT-40R Sold
94' Rustang GT daily (long gone)
2008 C6/Z51 Corvette
Old 05-08-2008, 04:22 PM
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Kind of a moot point since it all has to come apart, but it could be cracked rings or cracked piston. Air coming through the case breather is a pretty sickening sound....
Old 05-08-2008, 05:00 PM
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Im thinking cracked piston because I looked at the rings and they were fine. But I cannot for the life of me see where it is cracked. It just dont make sense.
Yea, I wanted to puke when I heard it...
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Old 05-08-2008, 05:16 PM
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I had 3 cracked pistons that didn't show up until we sent them out for a crack test.
Old 05-08-2008, 05:30 PM
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You're not going to be able to tell ring condition by a visual inspection. so I'd check the pistons and replace the rings. They're much cheaper than another tear-down.
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Old 05-08-2008, 05:57 PM
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cstreit- I forgot to mention that they are only 6k miles old- sorry about that

Gumba- Did you do a leakage test on them? If so how much were the cracks leaking if you

couldnt see them. Im not trying to say I will reuse them if theyre cracked I just want some kind of base line to go by.
I have a lineon some pistons and cylinders- only thing that sucks is that I have to have the pistons machined for valve pockets and clearance for my setup- wish I had the extra ching to spring for a set of C2 turbo pistons and cylinders.
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2008 C6/Z51 Corvette
Old 05-08-2008, 07:56 PM
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Sorry to hear about that.

When you mentioned the cause, maybe bouncing it off the rev limiter too hard, I was wondering the type/numbers of your valve springs, then you mentioned some contact, the broken rockers.

But you think it was the tensioner first?

Are some guys running solid tensioners (on the track)? Anybody installing collars?

Regards,
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Old 05-08-2008, 09:02 PM
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Dude... Kris that sucks so bad man. I've got a good set of 930 valves I think. Let me know if you want to check them out. They would be dirrrrrt cheap for you man...
Old 05-09-2008, 06:37 AM
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Thank you rsrmike- I may take you up on that.
As for the valve spring numbers- I had verdan do all of the head work which is why I am not concerned- they perform their work to the highest standard and I am not thinking it was them. I believe that my "contact" problems are a result of my behavier coupled with bad tensioners. If the tensioners were good I wouldnt have had a problem. Last summer I bounced that car off of the rev limiter about 300 times in every gear and nothing ever resulted. If you think about it; my cracked chain ramps, my bent valves, my rocked rockers on only cyls 1-3- they all point to a cam mistiming of some kind. The "origonal" cam timing was good because the car ran great for a while, so either the tensioners caused the mis-time or the ramps being broke caused it. But now my problem is the # 2 and #3 pistons/cylinders. Even if I put the heads from 4-6 (not bent up) on #2 and #3- I still get a bad leak-down number- with no visible damage. Rings, pistons, and cylinders look mint to the naked eye. The only thing left to do is to have them crack checked and then replace.
When I do put her back together I am never using hydro tensioners again- Im just going to get mechanicals and call it a day. Wish I did that from the begining...foolish.

The worst of all of this is that I dont have any money to upgrade anything. My biggest pet peev is when I have to work my a$$ off to just get myself back to square 1.
Oh well, the season is just beggining...
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2008 C6/Z51 Corvette
Old 05-09-2008, 07:33 AM
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Kris , This is a pic of a fitting I made to deal with the clearence with the -6 lines. And a pic of the modified DFI dizzy. On Big blocks after time the pistons become oval from high compession and fatigue. I figured as bad as the valves hit the pistons I wouldn't be surprised if the pistons aren't round anymore causing the blow by. Just an idea.
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Old 05-09-2008, 04:31 PM
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When I took my motor apart the first time, the rings looked ok from above, but fell into hundreds of pieces when I slipped the piston out. Thats when I changed to molly rings (steel cylinders tho).

I bumped my oil pressure up to keep chain tensioners pumped up with the GTII EVO cams. Is your car only track?

Gerry - clean that motor up, thats embarrassing.
Old 05-09-2008, 08:21 PM
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I think the leak down was in the 20-40% range on 3 cylinders. What started out as a minor service turned into a complete rebuild. Hard to say how much the cracks were leaking. When they removed the pistons the rings fell out in many pieces. The pistons had 2-3 cracks each.
There seem to be different opinions on chain tensioners. When we had our 3.4 n.a. race motor build the builder installed solid tensioners, when we rebuild the motor, the next builder used the hydraulic tensioners without the stops. The motor is being rebuild again (talk about bad luck), not sure what tensioners they're using.
I would install a shift lift so your not bouncing off the rev limiter.
Old 05-10-2008, 07:44 AM
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Really sorry to read the news Kris, will give you an excuse to make it look like GSF's, OMG that's nice!!! Good luck Kris, look forward to hearing what you fin out.
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Old 05-11-2008, 04:27 AM
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Gerry,

What's the background on the dizzy? Have any more info you can share?

Jim

Quote:
Originally Posted by GJF View Post



...And a pic of the modified DFI dizzy...
Old 05-11-2008, 08:53 AM
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You know GJF- I was thinking the same thing aobut how round they were- but why now? It is stille feasible
Thank all of your for your replies- I think Im just going to buy a set of 3 used pistons and stick them in there for the year and save my money for a set of nice C2 Turbo cylinders and Mahle pistons- maybe a set of JEs, but I dont like the Idea of running big clearance for them. I still have not found any cracks and my rings look brand new with no cracks.
I will keep you all posted- I am working hard and taking on some jobs right now to pay for the parts- I just did a 5 speed conversion on a 94' 928 GTS this weekend that should yeild some money to rebuild my motor.
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Last edited by 930gt-40r; 05-11-2008 at 11:48 AM..
Old 05-11-2008, 11:43 AM
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hi what year is the car i have some pistons and cylinders from a 86 930
Old 05-11-2008, 03:51 PM
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Mines an 86' also- how many miles on the pistons?
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2008 C6/Z51 Corvette
Old 05-11-2008, 03:58 PM
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not sure the motor replaced
Old 05-11-2008, 07:11 PM
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are they beat or are they good?
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2008 C6/Z51 Corvette
Old 05-11-2008, 07:17 PM
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Hey Jim,
The dizzy I use is a dual synch unit from Accel. It uses hall effect triggers and has one for the ignition and one for the cam. This unit provides the two signals required for sequential injection. Using Accel DFi it is a plug and play unit. But..... it comes from Accel with a 6 plug cap. I had Mike over at JB Racing modify it to adapt the twin plug cap and rotor so it would still be plug and plug with my 7 series DFI fuel injection. I showed this pic to Kris as he runs DFI also and has the dizzy to. And since he is rebuilding it would make sense to atleast have the heads opened up for the second plug. This dizzy would work with any fuel injection system if wanted. $200+ for the dizzy from Accel and I think Mike charges around $700+ for the modification.
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87 911 Carrera platform
Twin Garrett's, Big Valve, Xtreme Cylinder Heads, JB Racing cylinders and machine work, custom JE pistons, GT2 EVO cams, GT3 oil pump, modified 3.2L intake, 80mm TB, custom Intercooler, equal length headers, DFI, KEP Stage 2 custom twin disc, G50 custom gear set, LSD, Tilton trans pump, Fluidyne cooler
Old 05-11-2008, 08:44 PM
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