Valve hitting piston ... ?
08-13-2006, 08:16 PM
#1
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Valve hitting piston ... ?
Pulled the heads off today and discovered 4 of 6 pistons with small divots on the exhaust side of the piston. The picture below is #5, where I found a 18% leakdown, and the most pronouced mark. No obvious damage to the valve. Could it be slightly bent from contact, thus the high leakdown number?
Last edited by rs911t; 08-13-2006 at 09:19 PM.
08-13-2006, 11:15 PM
#5
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I just pulled my apart yesterday and all six that look like that way. I would not be surprised if you bent a valve. I couldn't get any leak down number on cylinder 3 and I'm sure a valve is bent although nothing is obvious.
Last time was obvious...
Last time was obvious...
08-13-2006, 11:22 PM
#6
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Oooh ... yes, that's obvious. Ouch.
The heads are being redone anyway ... so I'm getting new exhaust valves, too.
What about the pistons? Does the divot require they be replaced?
The heads are being redone anyway ... so I'm getting new exhaust valves, too.
What about the pistons? Does the divot require they be replaced?
08-13-2006, 11:40 PM
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I don't think the divot mandates that they be replaced, but I would get them checked out. Mine are pitted from detonation and just about all the top rings were broken as well as having the divot issue, so I'm going to replace mine with the 964 style 3.3 P&C's.
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08-13-2006, 11:58 PM
#9
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Originally Posted by A930Rocket
I don't think the divot mandates that they be replaced, but I would get them checked out. Mine are pitted from detonation and just about all the top rings were broken as well as having the divot issue, so I'm going to replace mine with the 964 style 3.3 P&C's.
08-14-2006, 12:01 AM
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The only thing you want to do with the divots is smooth them out so they don't become a hot spot, and provide a method for detonation.
If you can get a machine shop to smooth them out they are fine (more than likely).
This weekend we took A930Rocket's engine apart and every single cylinder had busted rings.
It was kinda funny, we'd pull a cylinder off and you'd hear all this metal hitting the pan on the floor, you'd look down and it was just pieces of rings everywhere.
Brian
If you can get a machine shop to smooth them out they are fine (more than likely).
This weekend we took A930Rocket's engine apart and every single cylinder had busted rings.
It was kinda funny, we'd pull a cylinder off and you'd hear all this metal hitting the pan on the floor, you'd look down and it was just pieces of rings everywhere.
Brian
08-14-2006, 12:25 AM
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It kinda looked like rust. It was vary small pitted holes and rough to the touch. I left the camera in the garage, so I'll post some tomorrow.
Originally Posted by rs911t
No broken rings on mine. What does pitting look like? Any pictures? I'm assuming mine are just showing carbon buildup.
08-14-2006, 08:08 AM
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FWIW - On all turbo motors I have had my pistons machined with valve relief pockets (both intake and exhaust side) to insure a safer clearance margin (for racing).
This was true for my 3.0 Turbo, 3.3 Turbo, 3.8 GT2 Twin Turbo...
But to see if you need to do this (or how much of a relief pocket you need or want) you need to measure your valve to piston clearance.
I used to measure it well before TDC all the way to after TDC to find the closest point as a starting reference. Every enegine is a bit different due to P&C, Heads, case and the like...
Norm
This was true for my 3.0 Turbo, 3.3 Turbo, 3.8 GT2 Twin Turbo...
But to see if you need to do this (or how much of a relief pocket you need or want) you need to measure your valve to piston clearance.
I used to measure it well before TDC all the way to after TDC to find the closest point as a starting reference. Every enegine is a bit different due to P&C, Heads, case and the like...
Norm
08-14-2006, 08:56 AM
#13
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Norm brings up a good point, the Piston to Valve clearance is tightest around 7 degrees BTDC for the intake and 7 degrees ATDC for the exhaust. Therefore, you want to check the clearance from 20 degrees BTDC to 20 degrees ATDC. As Norm suggests, you CAN cut the a valve relief into the pistons, however, you will reduce the compression and that is not something you want to do when your engine is already sitting between 6.6 and 6.8:1 compression for a stock engine.
I'd also replace your rod bolts. Most rod bolts that break are because they were stretched at some point in time. A missed downshift is most often the reason.
I'd also replace your rod bolts. Most rod bolts that break are because they were stretched at some point in time. A missed downshift is most often the reason.
08-14-2006, 09:30 AM
#14
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Originally Posted by A930Rocket
I just pulled my apart yesterday and all six that look like that way. I would not be surprised if you bent a valve. I couldn't get any leak down number on cylinder 3 and I'm sure a valve is bent although nothing is obvious.
Last time was obvious...
Last time was obvious...
08-14-2006, 02:38 PM
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That's what happens when you downshift from 4th to 3rd at 100 mph and some how really go from 4th to 1st going into turn one at Road Atlanta years ago. What dumba$$ I was...
Originally Posted by sand_man
GAGAGAGONG! I think I just threw-up in mouth a little bit!