Help me with this piston movement/noise
01-17-2019, 10:50 PM
#1
Help me with this piston movement/noise first I'm no mechanic, this the probably basic question. We were rolling the LH6 over tonight and it was making some noise. What's the cause of this and what to do about it?
Last edited by dewchugr; 01-17-2019 at 10:59 PM.
01-18-2019, 12:49 AM
#2
It's called piston rock/slap. Basically as pistons wear the distance between the piston skirt and cylinder wall widens and allows the piston to rock in the bore.
Usually requires new cylinder boring and new pistons but that can only be determined by disassembly and measuring everything.
Many engines will run fine with some wear but will make noise on initial start until the piston has a chance to thermally expand.
Usually requires new cylinder boring and new pistons but that can only be determined by disassembly and measuring everything.
Many engines will run fine with some wear but will make noise on initial start until the piston has a chance to thermally expand.
Last edited by LLLosingit; 01-18-2019 at 12:54 AM.
01-18-2019, 08:00 AM
#3
It's called piston rock/slap. Basically as pistons wear the distance between the piston skirt and cylinder wall widens and allows the piston to rock in the bore.
Usually requires new cylinder boring and new pistons but that can only be determined by disassembly and measuring everything.
Many engines will run fine with some wear but will make noise on initial start until the piston has a chance to thermally expand.
rock/slap.
Usually requires new cylinder boring and new pistons but that can only be determined by disassembly and measuring everything.
Many engines will run fine with some wear but will make noise on initial start until the piston has a chance to thermally expand.
rock/slap.
My concern now is whether this would be to much or OK to use. I'm guessing it has around 140-150k on it. When I bought it it was "running but made noise" When we got into it it had a lifter that had come apart in the middle. Also, it's going to be a small turbo setup.
Last edited by dewchugr; 01-18-2019 at 11:05 AM.
01-23-2019, 01:00 PM
#6
LS1Tech Sponsor
Join Date: Jan 2008
Location: Ohio, Georgia, Nevada, Texas
Posts: 1,951
Received 1,121 Likes
on
618 Posts
That is actually normal and not a cause of concern to a certain point. Was the engine known to be noisy...especially when cold? Can you get us a vertical measurement on the intake side? A forged piston's Top land has about .054 clearance cold and this closes to near zero under operating conditions. Cast pistons still have a lot of top land clearance...usually closer to .025 based on memory. I haven't tested a factory new pistons vertical rock from peak to peak, but good forged is about .023".
The video above is incorrect about several things. Piston slap occurs at TDC when the crank pin passes tdc. At that point, rod angle changes and the piston rotates and the cold piston's top land hits the cylinder. Hence the reason it goes away when warm (assuming skirt taper is also correct and is swelling to parallel with the bore).
Pin offset helps a bit, but reducing the overall skirt taper is the most effective way to get rid of piston rock. Pin offset is slightly preloading the piston into the cylinder wall. This makes it "roll" from up the skirt taper the bottom of the skirt tang to the top of the top land less abruptly instead of banging from Top to bottom instantly. The pin offset effects of this aren't as great as some would have you believe though on an LS. Some LS engines have pin offset and others don't.
The video above is incorrect about several things. Piston slap occurs at TDC when the crank pin passes tdc. At that point, rod angle changes and the piston rotates and the cold piston's top land hits the cylinder. Hence the reason it goes away when warm (assuming skirt taper is also correct and is swelling to parallel with the bore).
Pin offset helps a bit, but reducing the overall skirt taper is the most effective way to get rid of piston rock. Pin offset is slightly preloading the piston into the cylinder wall. This makes it "roll" from up the skirt taper the bottom of the skirt tang to the top of the top land less abruptly instead of banging from Top to bottom instantly. The pin offset effects of this aren't as great as some would have you believe though on an LS. Some LS engines have pin offset and others don't.
Last edited by Summitracing; 01-24-2019 at 09:05 AM.
01-23-2019, 06:20 PM
#7
ModSquad
iTrader: (6)
That is actually normal and not a cause of concern to a certain point. Was the engine known to be noisy...especially when cold? Can you get us a vertical measurement on the intake side? A forged piston's Top land has about .054 clearance cold and this closes to near zero under operating conditions. Cast pistons still have a lot of top land clearance...usually closer to .025 based on memory. I haven't tested a factory new pistons vertical rock from peak to peak, but good forged is about .023".
The video above is incorrect about several things. Piston slap occurs at TDC when the crank pin passes tdc. At that point, rod angle changes and the piston rotates and the cold piston's top land hits the cylinder. Hence the reason it goes away when warm (assuming skirt taper is also correct and is swelling to parallel with the bore).
Pin offset helps a bit, but reducing the overall skirt taper is the most effective way to get rid of piston rock. Pin offset is slightly preloading the piston into the cylinder wall. This makes it "roll" from up the skirt taper the bottom of the skirt tang to the top of the top land less abruptly instead of banging from Top to bottom instantly.
The pin offset effects of this aren't as great as some would have you believe though on an LS. Some LS engines have pin offset and others don't. LS bores are offset 24.1 degrees away from crank centerline toward the driver's side of the car. The effect of this is to have the piston and rod straight up and down at TDC at Peak combustion pressure with the crank already well past TDC. This gives combustion more leverage against the crank pin angle to rotate it (rather than trying to blow it vertically down out the main caps). It also reduces friction by not trying to drive the piston into the cylinder wall from the same thing.
The video above is incorrect about several things. Piston slap occurs at TDC when the crank pin passes tdc. At that point, rod angle changes and the piston rotates and the cold piston's top land hits the cylinder. Hence the reason it goes away when warm (assuming skirt taper is also correct and is swelling to parallel with the bore).
Pin offset helps a bit, but reducing the overall skirt taper is the most effective way to get rid of piston rock. Pin offset is slightly preloading the piston into the cylinder wall. This makes it "roll" from up the skirt taper the bottom of the skirt tang to the top of the top land less abruptly instead of banging from Top to bottom instantly.
The pin offset effects of this aren't as great as some would have you believe though on an LS. Some LS engines have pin offset and others don't. LS bores are offset 24.1 degrees away from crank centerline toward the driver's side of the car. The effect of this is to have the piston and rod straight up and down at TDC at Peak combustion pressure with the crank already well past TDC. This gives combustion more leverage against the crank pin angle to rotate it (rather than trying to blow it vertically down out the main caps). It also reduces friction by not trying to drive the piston into the cylinder wall from the same thing.
Trending Topics
01-23-2019, 08:34 PM
#8
I think youve confused the GM LS engine with the Honda LS engine that has the offset bores? It’s a great idea for power but requires a counterbalance shaft due to vibrations. If a v-8 engine were to try this technology, the cylinder heads would have to be assemetric meaning a dedicated left bank head and a dedicated right bank head, along with head gaskets, and the intake would be an offset unit, I would think...
01-23-2019, 08:54 PM
#9
ModSquad
iTrader: (6)
01-24-2019, 09:08 AM
#10
LS1Tech Sponsor
Join Date: Jan 2008
Location: Ohio, Georgia, Nevada, Texas
Posts: 1,951
Received 1,121 Likes
on
618 Posts
I think youve confused the GM LS engine with the Honda LS engine that has the offset bores? It’s a great idea for power but requires a counterbalance shaft due to vibrations. If a v-8 engine were to try this technology, the cylinder heads would have to be assemetric meaning a dedicated left bank head and a dedicated right bank head, along with head gaskets, and the intake would be an offset unit, I would think...
01-24-2019, 05:01 PM
#11
Teching In
Join Date: Apr 2017
Location: Western NC
Posts: 42
Likes: 0
Received 0 Likes
on
0 Posts
What were talking about here is a stock engine not a forged unit with lots of miles. If your going to add boost and you don't want it to end up in the crankcase a rebuild is recommended.
At least tear it down and check clearances.
At least tear it down and check clearances.
02-05-2019, 10:35 AM
#12
That is actually normal and not a cause of concern to a certain point. Was the engine known to be noisy...especially when cold? Can you get us a vertical measurement on the intake side? A forged piston's Top land has about .054 clearance cold and this closes to near zero under operating conditions. Cast pistons still have a lot of top land clearance...usually closer to .025 based on memory. I haven't tested a factory new pistons vertical rock from peak to peak, but good forged is about .023".
The video above is incorrect about several things. Piston slap occurs at TDC when the crank pin passes tdc. At that point, rod angle changes and the piston rotates and the cold piston's top land hits the cylinder. Hence the reason it goes away when warm (assuming skirt taper is also correct and is swelling to parallel with the bore).
Pin offset helps a bit, but reducing the overall skirt taper is the most effective way to get rid of piston rock. Pin offset is slightly preloading the piston into the cylinder wall. This makes it "roll" from up the skirt taper the bottom of the skirt tang to the top of the top land less abruptly instead of banging from Top to bottom instantly. The pin offset effects of this aren't as great as some would have you believe though on an LS. Some LS engines have pin offset and others don't.
The video above is incorrect about several things. Piston slap occurs at TDC when the crank pin passes tdc. At that point, rod angle changes and the piston rotates and the cold piston's top land hits the cylinder. Hence the reason it goes away when warm (assuming skirt taper is also correct and is swelling to parallel with the bore).
Pin offset helps a bit, but reducing the overall skirt taper is the most effective way to get rid of piston rock. Pin offset is slightly preloading the piston into the cylinder wall. This makes it "roll" from up the skirt taper the bottom of the skirt tang to the top of the top land less abruptly instead of banging from Top to bottom instantly. The pin offset effects of this aren't as great as some would have you believe though on an LS. Some LS engines have pin offset and others don't.
Thank you