Technical Damage to piston skirts
- Thread starter Toshi 975
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Technical Damage to piston skirts
Hi Dave, I have had the same nearly identical marks on my pistons
The attached pic shows my old 80.0mm BIS piston which my engine man commented as 'over heating' and 'excessive piston to bore clearance' I forget what the actual figures were, but could find out if you need a steer.
Ian.
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Thank you for your input.
These are first oversize (0.2mms) Pistons from a 594ccs engine that only ran for 50 miles. Now fitted with new pistons and running well. There was an issue with lubrication as the wrong big end shells had been fitted so some oil squirt jets in the con rods were blanked off. However the performance con rods that are available for the 500 & 126 engines do not have these jets and I am told that they rely on the splash effect in the sump to lubricate the bores so that puzzles me. Not sure about overheating but the engine before strip down did turn over quite freely.
It could well be the piston to bore clearance was tight but not noticeable when engine cold. When engine heated up the piston expansion could have taken up all the clearance.
The pic posted by Ian shows some wear marks toward the bottom of the piston skirt rather than centre.
The pic posted by Ian shows some wear marks toward the bottom of the piston skirt rather than centre.
looigi
New member
I had exactly the same marks on one of my engines.
It was in a 500F I bought from my neighbour which had an engine in it that was overhauled by a "Fiat expert" in Palmerston North, NZ. I forget the exact figure, but the book says there should be something like 0.020" piston to cylinder clearance.... I found 0.005" when I pulled it apart! That and the loose bolts I found all over the internals of the engine led me to believe that this idiot was in fact an expert..... X being an unknown quantity and a spurt being a drip under pressure.
New pistons, rings and a quick hone put it all right again.
Chris
It was in a 500F I bought from my neighbour which had an engine in it that was overhauled by a "Fiat expert" in Palmerston North, NZ. I forget the exact figure, but the book says there should be something like 0.020" piston to cylinder clearance.... I found 0.005" when I pulled it apart! That and the loose bolts I found all over the internals of the engine led me to believe that this idiot was in fact an expert..... X being an unknown quantity and a spurt being a drip under pressure.
New pistons, rings and a quick hone put it all right again.
Chris
looigi
New member
No Jackwhoo, you have the decimal in the wrong spot. The piston to barrel clearance on my engine was 0.005" (5 thousandths) when I pulled it apart.
It should have been, and now is 0.059" (59 thousandths). Actually mine is half a thou under, but I can live with that.
Chris
Thanks chris , I don't have the correct specs for this engine.
I think there may be confusion between mm and inches . 0.059 inch is about 1.5mm.
The important thing is you have direct experience of similar air cooled engine with insufficient piston to bore clearance.
I think there may be confusion between mm and inches . 0.059 inch is about 1.5mm.
The important thing is you have direct experience of similar air cooled engine with insufficient piston to bore clearance.
looigi
New member
I have been thinking about this today, so I checked.
I was wrong in my earlier assertions and I apologize. The correct piston to bore clearance is 0.03 - 0.05mm (.012" to 0.020").
I was wrong in my earlier assertions and I apologize. The correct piston to bore clearance is 0.03 - 0.05mm (.012" to 0.020").
Nicely said; plenty of people simply sidestep the matter when they make inaccurate assertions.
I think many of us, with a non-engineering background, find these micro-measures a nightmare to deal with,
No need to apologize , you have shared good practical experience.
0.03mm is about 0.0012 inch
0.05mm is about 0.002 inch
berkeleyman
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Skirt clearances aside, I know there is a right and wrong way round to fit the pistons, (or so I have been told), if the pistons had been fitted the wrong way round could this have caused the scoring?
No, I don't believe so. The main reason for ensuring pistons are fitted the correct way round is that the gudgeon (piston/wrist) pins are offset slightly to one side of the centre line of the piston. This is to minimise piston rock within the cylinder as it's driven down by the rapidly expanding gases. The only consequence of fitting the piston the wrong way round is possible 'piston slap' -a minor noise during engine operation.
Of course a secondary reason is that some pistons have pockets in their crown for valve clearance. Fitting these pistons the wrong way round will result in valves colliding with the piston crowns (lots of damage!).
AL.
What is peculair about the marks on the above pistons is that they don't extend down to bottom of the piston skirts. The piston's diameter is at it's largest on the piston skirt right down to the bottom.
'Jackwhoo' did ask if the marks were on both sides of the pistons but I don't think Toshi975 answered this? Also, he didn't say if there was any damage to the cylinder bores?
If both sides of the pistons have similar damage then I'd suggest insufficient piston to cylinder clearance. There will tend to be more severe damage on the thrust side of the pistons...
As was said (again, I think by 'Jackwhoo') the piston to cylinder clearance might be fine with the engine cooled down but be used up as the engine heats up. The clearance recommended is to allow for piston expansion as the engine heats up. It includes a margin to allow for an engine running hotter than normal e.g. hot climate, labouring up a hill, full loaded, poor quality fuel? etc. It also has to leave sufficient clearance for an oil film to exist under all these conditions. If the engine gets very hot and piston to bore clearance were to be reduced to absolutely zero, then oil cannot get between the piston skirt and the cyl. bore.
I'm wondering if this engine was running exceptionally hot? Various factors can affect engine temperature on air-cooled engines - incorrect fan, slipping fan belt, thermostat not opening, cylinder fins covered in dirt etc. incorrect ignition timing, especially too far retarded! (some other air cooled engine have oil coolers e.g. VW, Citroen 2CV, the fins on these need to be kept clean). Increasing the engine capacity may surpass the cooling ability of the standard system, fan, fan drive speed etc.
Toshi975 may actually have already found the cause of the problem with his mention of oil sprays holes in the con rods being accidentally blocked-off. Some con rods have no holes.
Some have one hole spraying to one side - this is usually to lubricate camshaft lobes and perhaps a little of the spray does hit the cylinder bore (you don't want too much being sprayed onto the cylinder wall as it can overwhelm the oil control rings on the pistons).
Some have 2 holes, one for the above reason, the other to cool the underside of the piston crown and lube the small end bearing.
Remember also that there is oil being sprayed out each side of the big end bearings and the crank is rotating quickly so the oil get sprayed everywhere anyway.
(Anyone who has ever pressurized the main oil gallery of an engine at rest with the oil sump removed in order to check for a faulty/worn crank bearing will know what I mean!!).
So, How to fix the problem? I think these pistons might be re-useable. There doesn't seem to be any damage at the ring lands or at the bottom of the piston skirts. I'd measure them and check against their original dimensions to check that the skirts haven't collapsed.Then clean up the torn surfaces. Check the cylinder bores for roundness everywhere. Note that larger cylinders especially if standard cylinders are significantly over-bored can distort in strange ways in use. You might need to increase the piston to cylinder clearance a bit more than you already have by honing the cylinders. Depending on what I found, I'd either hone to the max clearance specified by Fiat or go a little larger due to the larger pistons now being used.
(In the old days, piston and ring clearances used to be given in terms of thousandths of an inch per inch of bore size, there were different figures for air vs water-cooled engines. It was normal practice to increase the clearances beyond this on modified/race engines.)
AL.
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It is vitally important to apply the piston to bore clearance specified by the piston manufacturer. The piston manufacturer will have tested the expansion of the piston at running temperatures and specifies the bore clearance from there.
