Cracked piston...$^*#@!!!
Thread Starter
Had a 1976 L-82, 4-sp
Joined: Mar 2011
Posts: 5,338
Likes: 1,213
From: Some days your the dog and some days your the hydrant.
Royal Canadian Navy
Cracked piston...$^*#@!!!
While progressing with my top end rebuild I started to clean the small amount of carbon off the top of my KB hyper pistons. On closer inspection of #4, I noticed a hairline crack just to the left of centre in the exhaust valve relief about an 1.5" long. Then I see a shorter hairline crack across the intake valve relief. My finger nail just barely catches when I run it across the crack. So, it's toast! Is there anything preventing the removal of the piston while the engine remains in the car? The car is on jackstands and I don't want to remove the block which I know is a better way to do it.
Pro
Joined: Feb 2012
Posts: 577
Likes: 6
While progressing with my top end rebuild I started to clean the small amount of carbon off the top of my KB hyper pistons. On closer inspection of #4, I noticed a hairline crack just to the left of centre in the exhaust valve relief about an 1.5" long. Then I see a shorter hairline crack across the intake valve relief. My finger nail just barely catches when I run it across the crack. So, it's toast! Is there anything preventing the removal of the piston while the engine remains in the car? The car is on jackstands and I don't want to remove the block which I know is a better way to do it.
Can of worms. Technically all you have to do is rotate the crank so you have access to the piston rod cap to remove the fasteners, remove the cap, and push the piston up and out through the top of the bore. So technically, not much is keeping you from changing the piston. You have to be very careful though that the bearing nor journal are altered or damaged and you can re-use them when the piston is replaced and the rod is reinstalled in the block. This isn't a perfect world, and it's not that easy unless you can find that exact same part number piston. Unless you get the exact same piston you'll need to change all of the pistons using this same method and that puts you back to the best option of removing the block and doing it right.
Le Mans Master
Joined: Jan 2006
Posts: 6,504
Likes: 139
From: Butler Pa
Chances are what you're seeing isn't a crack. A piston cracked like you describe would've already come apart. It's not difficult to take it out, so go ahead and have a shop check it out before you decide to replace it.
Melting Slicks
Joined: Aug 2008
Posts: 2,200
Likes: 3
From: Lehigh county Pennsylvania
Depending on how many miles are on your engine, there could be a ridge at the top of the cylinder that needs to be removed before you push out the piston. Most of the big box auto parts stores will lend you this 'ridge reamer.'
Team Owner
Joined: Oct 2004
Posts: 81,241
Likes: 3,062
From: altered state
St. Jude Donor '05
Id think if it would have come apart being a hyper.
Got any pics of it? Sure its not a scratch?
Somtimes castings will have some funny looking stuff on them that can really
throw you off.
Get some pics up see what Carl has to say
Got any pics of it? Sure its not a scratch?
Somtimes castings will have some funny looking stuff on them that can really
throw you off.Get some pics up see what Carl has to say
Last edited by cv67; Apr 7, 2012 at 10:18 AM.
Le Mans Master
Joined: Jun 2004
Posts: 6,231
Likes: 65
From: Seattle WA
St. Jude Donor '14
That would be my take as well.A picture might be worth a thousand words here before you take the next step.
Thread Starter
Had a 1976 L-82, 4-sp
Joined: Mar 2011
Posts: 5,338
Likes: 1,213
From: Some days your the dog and some days your the hydrant.
Royal Canadian Navy
Great input! Thanks lads. I too was kinda miffed that if it was a crack then why hadn't the piston grenaded. I assembled this engine myself many years ago (maybe 15K miles on it now) but don't recall any "scratches" on any of the pistons but who knows. I'll take it out and have it examined. Maybe get it x-rayed at my work! A replacement piston can still be had from KB with the same specs for about $65 from Jeg's if I need to replace it.
Melting Slicks
Joined: Feb 2006
Posts: 3,428
Likes: 5
From: Japan
Get a couple pieces of fuel hose or similar material and push it up onto the rod bolts after you've removed the rod cap...if the threaded part of either rod bolt even lightly taps the crank journal it will leave a mark, so you need some protection on there when removing the piston/rod assembly. From the way you're describing it though that sounds more like a typical casting imperfection; I'd make absolutely sure before I pulled that piston out of the hole.
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Le Mans Master
Joined: Oct 2006
Posts: 7,123
Likes: 433
From: Gladstone MO
C3 of Year Finalist (appearance mods) 2019
Before you rip the pan off and do all the work to get that piston out, think about getting a Spotcheck kit:
http://www.grainger.com/Grainger/SPO...26-128580020-2
That will prove there's a crack or a casting flaw.
http://www.grainger.com/Grainger/SPO...26-128580020-2
That will prove there's a crack or a casting flaw.
Melting Slicks
Joined: Feb 2006
Posts: 3,428
Likes: 5
From: Japan
Before you rip the pan off and do all the work to get that piston out, think about getting a Spotcheck kit:
http://www.grainger.com/Grainger/SPO...26-128580020-2
That will prove there's a crack or a casting flaw.
http://www.grainger.com/Grainger/SPO...26-128580020-2
That will prove there's a crack or a casting flaw.
What he said...if you can blend that thing smooth with some cartridge roll-type sanding barrels or a mounted stone and then do a liquid penetrant test on it that would tell you what you've really got...just make sure you get ALL of the debris out of there before you button it all back up.
Drifting
Joined: Aug 2005
Posts: 1,682
Likes: 3
From: League City TX
Thread Starter
Had a 1976 L-82, 4-sp
Joined: Mar 2011
Posts: 5,338
Likes: 1,213
From: Some days your the dog and some days your the hydrant.
Royal Canadian Navy
I'm replacing the base pan gasket anyway so I will remove the piston. It's no big deal at this point. I'll have the guys at work do their NDT magic. Then there will be no doubt as to the integrity of the piston and I save $87 for a dye penetrant kit! Another option is to get a 383 rotating assembly
Melting Slicks
Joined: Feb 2006
Posts: 3,428
Likes: 5
From: Japan
Well now, if you've got NDT equipment at work, why not get a little bit of penetrant and some developer and save yourself all that work and the risk of nicking the crank or breaking a ring? Just do it on the car...just a thought.
Thread Starter
Had a 1976 L-82, 4-sp
Joined: Mar 2011
Posts: 5,338
Likes: 1,213
From: Some days your the dog and some days your the hydrant.
Royal Canadian Navy
Here are some pics. The cracks are just inside the black marker lines.
http://forums.corvetteforum.com/album.php?albumid=25340
http://forums.corvetteforum.com/album.php?albumid=25340
Race Director
Joined: May 2005
Posts: 10,674
Likes: 122
From: Millington Illinois
St. Jude Donor '08-'09
Pull the piston and have it checked. It looks like a crack to me also. If it is cracked don't buy another KB POS and replace it. You got what you paid for. Buy some good low expansion forged pistons and redo it....Rebalance, bearings, rings. at a minimum. That sucks!