Piston clearance
Apr 25, 2011 | 01:00 PM
#1
Thread Starter
Staging Lane
Joined: Apr 2011
Posts: 78
Likes: 0
From: Montgomery County, TX
Piston clearance
Need advice on piston to wall clearance.
Just got a call from the machine shop and he mic'd the pistons and cylinder walls. Appears the walls are spot on but the pistons are .004 too small!
He said it may cause a piston rattle.
These are the original pistons with 165K miles but you could still see the crosshatch in the walls and the machining marks on the pistons.
They never rattled before and still trying to get by for a grand or less for this rebuild.
Just got a call from the machine shop and he mic'd the pistons and cylinder walls. Appears the walls are spot on but the pistons are .004 too small!
He said it may cause a piston rattle.
These are the original pistons with 165K miles but you could still see the crosshatch in the walls and the machining marks on the pistons.
They never rattled before and still trying to get by for a grand or less for this rebuild.
Apr 25, 2011 | 01:12 PM
#2
Thread Starter
Staging Lane
Joined: Apr 2011
Posts: 78
Likes: 0
From: Montgomery County, TX
Just broke out the Chiltons and looking at piston to bore clearance.
Gosh I hate decimals.
It says 0.0020 " between piston and wall.
So the machine shop said pistons were .004 smaller than the bore which is 3.780 inch exactly what the book says.
So the 4/1000 vs. 20/1000 seems OK if I'm reading the decimal correctly.
Not sure if he just mic'd the pistons or actually installed it in the bore and measured the clearance.
Gosh I hate decimals.
It says 0.0020 " between piston and wall.
So the machine shop said pistons were .004 smaller than the bore which is 3.780 inch exactly what the book says.
So the 4/1000 vs. 20/1000 seems OK if I'm reading the decimal correctly.
Not sure if he just mic'd the pistons or actually installed it in the bore and measured the clearance.
Apr 25, 2011 | 01:29 PM
#4
Have your machine shop clarify if it is .004 under spec, or under the block measurement. If your pistons were worn, it would be ovaled. The skirts will be worn more than the measurement right under the pin.
Apr 25, 2011 | 01:32 PM
#5
Thread Starter
Staging Lane
Joined: Apr 2011
Posts: 78
Likes: 0
From: Montgomery County, TX
He said he measured the skirt. Not sure exactly where though.
Apr 25, 2011 | 02:37 PM
#6
Thread Starter
Staging Lane
Joined: Apr 2011
Posts: 78
Likes: 0
From: Montgomery County, TX
Good grief!
Got my daughter to help with the decimals.
Looks like I'm .0012 too much in a negative way. If that makes since.
Appears not enough to worry about. 12/10000.
40/10000 mic'd. 28/10000 service limit.
Left me with 12/10000 over the service limit.
What do ya think?
Thanks.
Got my daughter to help with the decimals.
Looks like I'm .0012 too much in a negative way. If that makes since.
Appears not enough to worry about. 12/10000.
40/10000 mic'd. 28/10000 service limit.
Left me with 12/10000 over the service limit.
What do ya think?
Thanks.
Last edited by CodyBoy; Apr 25, 2011 at 02:44 PM.
Apr 25, 2011 | 05:16 PM
#7
.0012 doesn't add up...I came up with a .002 difference 
Even within spec, expect the pistons to rattle (or "slap" as it's referred to as "piston slap"), when the engine is cold, all of the GENIII engines do it. The LQ9's started getting teflon coated skirts in 03 in an attempt to correct this problem.
IIRC from one of your other threads, this engine suffered from oil starvation, which could account for excessive wear on the piston skirts. You mentioned the crosshatch, which it seems to me is also always present on these engines, unless the block is toast, but often when there are oiling issues, there will be vertical scores or marks on the cylinder walls. How about cylinder taper and roundness, how did they check out? I have to assume that you're at the very least getting a mild hone, and would think that a 165k mill that suffered from oil starvation may need the cylinders bored over .020-.030, but I could be wrong. If the cylinders are "checking out spot on", then it sounds like you need a new set of factory replacement size slugs. My gut tells me that if everything is properly inspected and measured that you'll be ordering some .030 over slugs though...
Even within spec, expect the pistons to rattle (or "slap" as it's referred to as "piston slap"), when the engine is cold, all of the GENIII engines do it. The LQ9's started getting teflon coated skirts in 03 in an attempt to correct this problem.IIRC from one of your other threads, this engine suffered from oil starvation, which could account for excessive wear on the piston skirts. You mentioned the crosshatch, which it seems to me is also always present on these engines, unless the block is toast, but often when there are oiling issues, there will be vertical scores or marks on the cylinder walls. How about cylinder taper and roundness, how did they check out? I have to assume that you're at the very least getting a mild hone, and would think that a 165k mill that suffered from oil starvation may need the cylinders bored over .020-.030, but I could be wrong. If the cylinders are "checking out spot on", then it sounds like you need a new set of factory replacement size slugs. My gut tells me that if everything is properly inspected and measured that you'll be ordering some .030 over slugs though...
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Apr 25, 2011 | 06:02 PM
#8
Thread Starter
Staging Lane
Joined: Apr 2011
Posts: 78
Likes: 0
From: Montgomery County, TX
.0012 doesn't add up...I came up with a .002 difference Even within spec, expect the pistons to rattle (or "slap" as it's referred to as "piston slap"), when the engine is cold, all of the GENIII engines do it. The LQ9's started getting teflon coated skirts in 03 in an attempt to correct this problem.
IIRC from one of your other threads, this engine suffered from oil starvation, which could account for excessive wear on the piston skirts. You mentioned the crosshatch, which it seems to me is also always present on these engines, unless the block is toast, but often when there are oiling issues, there will be vertical scores or marks on the cylinder walls. How about cylinder taper and roundness, how did they check out? I have to assume that you're at the very least getting a mild hone, and would think that a 165k mill that suffered from oil starvation may need the cylinders bored over .020-.030, but I could be wrong. If the cylinders are "checking out spot on", then it sounds like you need a new set of factory replacement size slugs. My gut tells me that if everything is properly inspected and measured that you'll be ordering some .030 over slugs though...
Even within spec, expect the pistons to rattle (or "slap" as it's referred to as "piston slap"), when the engine is cold, all of the GENIII engines do it. The LQ9's started getting teflon coated skirts in 03 in an attempt to correct this problem.IIRC from one of your other threads, this engine suffered from oil starvation, which could account for excessive wear on the piston skirts. You mentioned the crosshatch, which it seems to me is also always present on these engines, unless the block is toast, but often when there are oiling issues, there will be vertical scores or marks on the cylinder walls. How about cylinder taper and roundness, how did they check out? I have to assume that you're at the very least getting a mild hone, and would think that a 165k mill that suffered from oil starvation may need the cylinders bored over .020-.030, but I could be wrong. If the cylinders are "checking out spot on", then it sounds like you need a new set of factory replacement size slugs. My gut tells me that if everything is properly inspected and measured that you'll be ordering some .030 over slugs though...
OK. I still hate decimals and arithmetic in general.
Bore measured to spec at 3.780"
Pistons measured .004 under that to bring them to 3.776
Service limit is .0028 under which would bring it to 3.7772
So the difference from my calculation(with daughters help) would be .0012 difference. (piston too small)
.0015 is the smallest feeler gauge I have and if .0012 is smaller than that (it is according to calcs and daughter) then it don't seem much of a difference to worry about.
But if I need new pistons then I'll go for it. Just not seeing it.
Apr 25, 2011 | 06:06 PM
#9
Thread Starter
Staging Lane
Joined: Apr 2011
Posts: 78
Likes: 0
From: Montgomery County, TX
The pistons never rattled or slapped.
The only difference is honing the cylinders to help with seating the new rings.
The machine shop said maybe a thousandths from the honing.
If they rattle a little not to worried about it.
Now if it will sound like a diesel then yea, that may be something to consider.
They worked fine before dissassembly so why not now.
Just trying to understand.
Thanks
The only difference is honing the cylinders to help with seating the new rings.
The machine shop said maybe a thousandths from the honing.
If they rattle a little not to worried about it.
Now if it will sound like a diesel then yea, that may be something to consider.
They worked fine before dissassembly so why not now.
Just trying to understand.
Thanks
Apr 26, 2011 | 10:05 AM
#10
What piston is it, that definitely makes a difference in where you should set piston to wall clearance!
FYI: When you buy a piston that is labeled 4.030" bore, that will typically be right at the actually bore size of the hole, and the manufacturer will have built the clearance into the piston by making it 4.026" or so. Also, be sure you measure the piston 90º from the pin. Pistons are sort of egg-shaped, and measuring anywhere else is not right and will distort your readings.
FYI: When you buy a piston that is labeled 4.030" bore, that will typically be right at the actually bore size of the hole, and the manufacturer will have built the clearance into the piston by making it 4.026" or so. Also, be sure you measure the piston 90º from the pin. Pistons are sort of egg-shaped, and measuring anywhere else is not right and will distort your readings.