piston slap
06 February 2001, 06:47 PM
#31
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Nito, if i was you then i`d get the car in for the work doing.I`ve just had a short engine fitted (950 miles on it now 
) , not only has the knocking stopped from cold, but the most obvious thing is that the whole car is so much smoother.My m8 got in it, ( last week ), and even he said it felt alot smoother, and that was without any prompting from me at all.
This been the case, why is it alot smoother?...possibly because the engine is running as it was designed to, and not with a worn piston that is moving around in the bore?
O.K,the knocking stops when the engines warm, but has it really stopped? .....or is it so slight that you just carnt hear it??
Look at the physics, for the worn piston to operate correctly when the engine is warm, it would have to expand to the same size as the other pistons(because its smaller than them in the first place..hence the knocking). Now unless said piston was made out of a different material to the other pistons ( which its not ) , then it carnt possibly do it.
Your car has a problem , it could be running alot smoother, and last alot longer, if you decide you don`t want it to, thats your problem.
Upto you m8.
) , not only has the knocking stopped from cold, but the most obvious thing is that the whole car is so much smoother.My m8 got in it, ( last week ), and even he said it felt alot smoother, and that was without any prompting from me at all.This been the case, why is it alot smoother?...possibly because the engine is running as it was designed to, and not with a worn piston that is moving around in the bore?
O.K,the knocking stops when the engines warm, but has it really stopped? .....or is it so slight that you just carnt hear it??
Look at the physics, for the worn piston to operate correctly when the engine is warm, it would have to expand to the same size as the other pistons(because its smaller than them in the first place..hence the knocking). Now unless said piston was made out of a different material to the other pistons ( which its not ) , then it carnt possibly do it.
Your car has a problem , it could be running alot smoother, and last alot longer, if you decide you don`t want it to, thats your problem.
Upto you m8.
08 February 2001, 07:45 PM
#32
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Can someone please tell me if the sound of piston slap is a 'knocking' sound or a 'tick tick'sound. My 98 STi4 makes the former most of the time,louder on cold start up.The sound is not audible from inside the car.If this sounds like the demon piston slap,is it a problem that mechanics can definitavely diagnose.I have a warranty on the vehicle until June that covers all major mmechanicals.Will the warranty company accept this concept of piston slap;ie can a mechanic prove it exists without taking the engine apart.Any thoughts on this would be very appreciated
08 February 2001, 08:19 PM
#33
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On my car, it was like a hollow knocking come slapping sound, which could be heard inside the car, especially under high load/low revs i.e. reversing out of the driveway.Defently not a tapping or ticking sound.
Hope that helps
Hope that helps
08 February 2001, 09:38 PM
#35
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I work on these scooby things and thought i might be able to clear up a few points ref piston slap.....Normally only number 4 piston is effected (If you are sat in the car it is left hand rear cylinder) Although I have had problems with number 3. up until recently Subaru would supply a new short engine .They now have modified pistons available ,so complete short motor now not needed..
08 February 2001, 10:52 PM
#36
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Dealer contacted me today. They wanted to check the engine number (plus another number which seems to relate to the type of piston fitted to the individual engine).
Apparently the fix will involve replacing No. 4 piston with a "modified" one. The dealer did not know whether it was oversize piston or not.
I asked about checking the clutch (and changing if it is worn) whilst the engine is out of the car. Said this could be done. He also suggested that the cam belt be changed since this would normally be due at the next service - having this done while engine is out will save alot on labour charges. Sounds sensible.
Not sure about replacement piston. How can this help if the bore if worn (which it must be even if its only slight).
D.
Apparently the fix will involve replacing No. 4 piston with a "modified" one. The dealer did not know whether it was oversize piston or not.
I asked about checking the clutch (and changing if it is worn) whilst the engine is out of the car. Said this could be done. He also suggested that the cam belt be changed since this would normally be due at the next service - having this done while engine is out will save alot on labour charges. Sounds sensible.
Not sure about replacement piston. How can this help if the bore if worn (which it must be even if its only slight).
D.
09 February 2001, 12:00 AM
#37
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DJB..Oil pressure is actually at its highest when the engine is cold. Mine reads about 7bar cold on idle and more on throttle, as opposed to 4-5bar when on throttle when warm and 1.5bar on idle warm!
But you're right the engine isn't properly protected until the oil has reached working temperature which is why I never thrash mine until it hits 70 degrees!
I've heard so many different versions of the piston slap problem, that it's the skirt design on the piston or the piston is slightly undersized etc.
Apparently the forged cossie pistons do make this piston slap noise too so I'm not sure how detrimental it is to the engine, I'm sure if what you said above were true it would cause massive engine damage in no time, the tolerances must be so minute that considering the way most of these are driven I can't see it causing engine damage. I haven't heard of any scooby engine failures resulting from slap, it's usually from the bearings or no.3.
Has anyone had engine problems resulting directly from slap on this board?
I believe Stef had a new short block fitted due to slap and the new block engine failed shortly after (Stef??) in which case I'd be very cautious about having my short block replaced since it may never be quite like it was when it left the factory!
In short, I think piston slap would probably be one of the last things to cause my engine to go, while I think that (not having a pop at dealers in anyway whatsoever here) a dealer tampering with the engine (when they are not proffessional engine builders) would do more damage than good imho!
Nito
But you're right the engine isn't properly protected until the oil has reached working temperature which is why I never thrash mine until it hits 70 degrees!
I've heard so many different versions of the piston slap problem, that it's the skirt design on the piston or the piston is slightly undersized etc.
Apparently the forged cossie pistons do make this piston slap noise too so I'm not sure how detrimental it is to the engine, I'm sure if what you said above were true it would cause massive engine damage in no time, the tolerances must be so minute that considering the way most of these are driven I can't see it causing engine damage. I haven't heard of any scooby engine failures resulting from slap, it's usually from the bearings or no.3.
Has anyone had engine problems resulting directly from slap on this board?
I believe Stef had a new short block fitted due to slap and the new block engine failed shortly after (Stef??) in which case I'd be very cautious about having my short block replaced since it may never be quite like it was when it left the factory!
In short, I think piston slap would probably be one of the last things to cause my engine to go, while I think that (not having a pop at dealers in anyway whatsoever here) a dealer tampering with the engine (when they are not proffessional engine builders) would do more damage than good imho!
Nito
09 February 2001, 09:24 AM
#38
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Dick,
I see what you mean about the physics, but you've overlooked an important point. What if the piston slap problem is caused because the piston in question (3 or 4) heats up more than the others, which I have heard as an explaination a few times. In that case wouldn't this piston also expand more than the others when the engine is fully warmed up. This means it may actually become the same size despite the extra wear...
I see what you mean about the physics, but you've overlooked an important point. What if the piston slap problem is caused because the piston in question (3 or 4) heats up more than the others, which I have heard as an explaination a few times. In that case wouldn't this piston also expand more than the others when the engine is fully warmed up. This means it may actually become the same size despite the extra wear...
09 February 2001, 10:48 AM
#39
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Scoobysnacks
This may be true but I'm most concerned about the damage which is being done in the first few minutes after starting from cold. This is the very time when the oil pressure is low and the oil is more viscous and this is when engine wear is greatest. The knocking sound must equate with wear.
D.
This may be true but I'm most concerned about the damage which is being done in the first few minutes after starting from cold. This is the very time when the oil pressure is low and the oil is more viscous and this is when engine wear is greatest. The knocking sound must equate with wear.
D.
09 February 2001, 11:34 AM
#40
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Agreed, but don't some higher powered engines slap from new? eg. those with special Cosworth pistons. In these engines, is there damage being done every time the engine is running cold?
On our Subarus though, will a point be reached where no further wear will be done to the bore? I'm not a mechanic, so was just wondering.
Another question that springs to mind is that if the new fix is a new piston, will the same thing not happen when the new one overheats as well (which I guess it would as it's in the same place in the engine)?
On our Subarus though, will a point be reached where no further wear will be done to the bore? I'm not a mechanic, so was just wondering.
Another question that springs to mind is that if the new fix is a new piston, will the same thing not happen when the new one overheats as well (which I guess it would as it's in the same place in the engine)?
09 February 2001, 01:01 PM
#41
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The Cosworth (forged) pistons definitely lead to some piston slap.
My 98 STi had to have the first set of Cos pistons replaced (after a rebuild) because it was so noisy (and it still sounds like crap in the morning....).
The clearances are generally larger on high performance pistons,as you expect them, to run hotter.
I agree that it's probably not doing any damage - although on an engine where there isn't normally slap, it could be evidence of bore/piston wear, rather than the cause of it.
You can try to do clever stuff with the design of the slirt to minimise the change in shape when the piston heats up, but ultimately aluminium expands a lot, and you can't argue with physics....
I used to do Finite Element Analysis on various stuff (including F1 and roadcar pistons) at Cosworth.
Pistons are still a black art.
Nick
My 98 STi had to have the first set of Cos pistons replaced (after a rebuild) because it was so noisy (and it still sounds like crap in the morning....).
The clearances are generally larger on high performance pistons,as you expect them, to run hotter.
I agree that it's probably not doing any damage - although on an engine where there isn't normally slap, it could be evidence of bore/piston wear, rather than the cause of it.
You can try to do clever stuff with the design of the slirt to minimise the change in shape when the piston heats up, but ultimately aluminium expands a lot, and you can't argue with physics....
I used to do Finite Element Analysis on various stuff (including F1 and roadcar pistons) at Cosworth.
Pistons are still a black art.
Nick
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