piston moves sideways in cylinder
05-27-2007, 08:01 AM
#16
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A very simple explanation... piston to bore clearance is necessary to allow for heat expansion of the piston.. Imagine the engine has lenght & width like a car...
As the engine rotates it places more force on the piston across the width of the engine, this is known as major & minor thrust side. This is where most piston & cylinder wear takes place with the top of the bore wearing to an oval and tapered... With the pistons removed measure the diameter of the piston & bore and compare to spec's. The pistons will usually show more signs of wear than a bore as they are cheaper than an block and easily replaced, the aluminium bores are treated (nikisil from memory) and quite wear resistant...
good luck,
irisblue
As the engine rotates it places more force on the piston across the width of the engine, this is known as major & minor thrust side. This is where most piston & cylinder wear takes place with the top of the bore wearing to an oval and tapered... With the pistons removed measure the diameter of the piston & bore and compare to spec's. The pistons will usually show more signs of wear than a bore as they are cheaper than an block and easily replaced, the aluminium bores are treated (nikisil from memory) and quite wear resistant...
good luck,
irisblue
05-27-2007, 08:50 AM
#17
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I see - so, left and right of the piston and cylinder will be more worn out than the front and back? I can move my pistons sideways too. If the tolerance is there for heat expansion, then that means the pistons expands more than the block... I guess that's feasible because the pistons are aluminum (right?) and the block is a aluminum/silicon alloy.
Then compression numbers must get even better with temperature. 195 is already very high. Actually, isn't that too high? Could that be caused by carbon buildup? There was an unbelievable amount of carbon on these pistons. I had to scrape it off with a dull screwdriver and finish it off with 320 grit sand paper.
At any rate, I would love for this to be ok and normal, so I can ignore it and slap it back together. I'll rebuild it sometime down the line, but I was expecting the engine to be ok for now. Everything was performing and behaving well; the only indication of tired internals was the crap in the intake ports and a little in the intake manifold, from blowby. But that could have been from the valves too, right?
Then compression numbers must get even better with temperature. 195 is already very high. Actually, isn't that too high? Could that be caused by carbon buildup? There was an unbelievable amount of carbon on these pistons. I had to scrape it off with a dull screwdriver and finish it off with 320 grit sand paper.
At any rate, I would love for this to be ok and normal, so I can ignore it and slap it back together. I'll rebuild it sometime down the line, but I was expecting the engine to be ok for now. Everything was performing and behaving well; the only indication of tired internals was the crap in the intake ports and a little in the intake manifold, from blowby. But that could have been from the valves too, right?
05-27-2007, 09:19 AM
#18
Originally Posted by FRporscheman
I see - so, left and right of the piston and cylinder will be more worn out than the front and back? I can move my pistons sideways too. If the tolerance is there for heat expansion, then that means the pistons expands more than the block... I guess that's feasible because the pistons are aluminum (right?) and the block is a aluminum/silicon alloy.
Then compression numbers must get even better with temperature. 195 is already very high. Actually, isn't that too high? Could that be caused by carbon buildup? There was an unbelievable amount of carbon on these pistons. I had to scrape it off with a dull screwdriver and finish it off with 320 grit sand paper.
At any rate, I would love for this to be ok and normal, so I can ignore it and slap it back together. I'll rebuild it sometime down the line, but I was expecting the engine to be ok for now. Everything was performing and behaving well; the only indication of tired internals was the crap in the intake ports and a little in the intake manifold, from blowby. But that could have been from the valves too, right?
Then compression numbers must get even better with temperature. 195 is already very high. Actually, isn't that too high? Could that be caused by carbon buildup? There was an unbelievable amount of carbon on these pistons. I had to scrape it off with a dull screwdriver and finish it off with 320 grit sand paper.
At any rate, I would love for this to be ok and normal, so I can ignore it and slap it back together. I'll rebuild it sometime down the line, but I was expecting the engine to be ok for now. Everything was performing and behaving well; the only indication of tired internals was the crap in the intake ports and a little in the intake manifold, from blowby. But that could have been from the valves too, right?
Carbon insde the chamber only raised your static compression.
Your car will run much better now because of the LOWERED compression.
This is due to the very sophisticated 2-sensor knoc system on your 968, which was definitely saving you by cutting much timing before, especially if you ran **** gas.
05-27-2007, 11:02 AM
#19
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Originally Posted by FRporscheman
I had another question. If you want me to drop the oil pan, I need to remove the 1-piece crossmember. How do I remove the crossmember if the head is off? Remember the rear engine lift eye in on the head.
While you have the pistons out you might as well put in some low compression pistons and throw a turbo on there
05-27-2007, 04:11 PM
#20
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Thanks special tool - it all makes sense now. I think I'll leave the pistons alone for now.
Chris, if this were my own 968........... oh man, it would have been so hard to say no. But this is my mom's car. I just now asked her if she knows what a turbo is... I got owned. She told me exactly what a turbo is, then she explained to me that she doesn't need turbo because SHE IS the car's turbo.
Chris, if this were my own 968........... oh man, it would have been so hard to say no. But this is my mom's car. I just now asked her if she knows what a turbo is... I got owned. She told me exactly what a turbo is, then she explained to me that she doesn't need turbo because SHE IS the car's turbo.
01-24-2008, 06:53 AM
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Old topic, but half a mm is way too much. The design clearance is 0.2mm (diametral). If it's too big the piston will tilt and bang against the sides too much.
Somebody must have swapped the pistons once and got the wrong size, or they're insanely worn. Within the nominal 100mm there's 3 diameter groups for the cylinders/pistons, each varying by 0.2mm. So you have a 99.8, 100.0, and 100.2mm cylinder, and the pistons are 0.2mm smaller in diameter for each group. Denoted by a '0', '1', or '2' stamped on the piston (between the valve cutouts).
In your case you probably have a tolerance group '0' piston, and a '2' bore.
Somebody must have swapped the pistons once and got the wrong size, or they're insanely worn. Within the nominal 100mm there's 3 diameter groups for the cylinders/pistons, each varying by 0.2mm. So you have a 99.8, 100.0, and 100.2mm cylinder, and the pistons are 0.2mm smaller in diameter for each group. Denoted by a '0', '1', or '2' stamped on the piston (between the valve cutouts).
In your case you probably have a tolerance group '0' piston, and a '2' bore.
01-24-2008, 09:30 PM
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Check with a Inside Diameter gauge to see if the cylinder is within out of round tolerence. My car is cuttently over the out of round tolerance and my piston rocks alot. It is running but starts up very rough. Worst case you piston skirt breaks off and falls into the engine then your piston ***** and jams into the block. Praying to the P-gods that this doesnt happen to me. If your still able to get in there its worth the look.
01-24-2008, 09:54 PM
#24
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That car has been back together and driving in rush hour for months.
The motor is running very strong, makes no noises, does not overheat, and loses a normal amount of oil for a stock bottom end and oil pan gasket.
I don't think that engine will be open for a while. But when it is open again, it will be getting a complete rebuild from the bottom up. Maybe I'll leave the mains alone, but if I take out the crank I'll want to get it balanced and that's more $$$!!!
The motor is running very strong, makes no noises, does not overheat, and loses a normal amount of oil for a stock bottom end and oil pan gasket.
I don't think that engine will be open for a while. But when it is open again, it will be getting a complete rebuild from the bottom up. Maybe I'll leave the mains alone, but if I take out the crank I'll want to get it balanced and that's more $$$!!!
01-24-2008, 10:08 PM
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when you do a complete rebuild do they bore out the engine and put in sleeves if its out of tolerance? That seems like it could be expesive because of aluminum? What would a complete rebuild mean for you engine?
01-25-2008, 12:56 AM
#26
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What I meant was:
rod bearings
rings
head gasket (again, ugh)
oil pan gasket
BS reseal
front seals, belts, maybe water pump (it's still fairly new)
The head was completely redone with new guides, stem seals, 3-angle grind, fresh seats, at 222k miles (car has 224k miles). The RMS is new too.
I won't take out the crankshaft for just the main bearings. Those last "forever" so I don't want to mess with them. If I have to take the crank out for other work, like balancing/lightening, then so be it.
I have heard many success stories of just putting new rings into old bores and it works just fine with our alusil blocks. As long as the cylinders are not scored (which they weren't).
rod bearings
rings
head gasket (again, ugh)
oil pan gasket
BS reseal
front seals, belts, maybe water pump (it's still fairly new)
The head was completely redone with new guides, stem seals, 3-angle grind, fresh seats, at 222k miles (car has 224k miles). The RMS is new too.
I won't take out the crankshaft for just the main bearings. Those last "forever" so I don't want to mess with them. If I have to take the crank out for other work, like balancing/lightening, then so be it.
I have heard many success stories of just putting new rings into old bores and it works just fine with our alusil blocks. As long as the cylinders are not scored (which they weren't).
