Engine smoking badly after rebuild... HELP PLEASE!...
Discussion
The last time I designed a 94mm piston I used 1.5mm if I recall correctly so pro-rata it would be about 1.2mm for a 73mm piston but I suggest you ask the piston manufacturer. He'll have the specs for other similar sized pistons.
Edit. Actually thinking about it more I may have used 1mm not 1.5mm but it was so long ago I'm not sure. 1.5mm does sound a bit extreme.
Edit. Actually thinking about it more I may have used 1mm not 1.5mm but it was so long ago I'm not sure. 1.5mm does sound a bit extreme.
Edited by Pumaracing on Tuesday 13th April 08:58
Pumaracing said:
I do recall a test Dave Vizard did many years ago (...) I'll ask him if he remembers next time we speak.
Realy NICE... erm... great... or even more!...Soo You know Him personaly. Humm. A Legend for me. His books tought me nearly everithing I khow of engines.
He must be in a nice age though.
Pumaracing said:
The last time I designed a 94mm piston I used 1.5mm if I recall correctly so pro-rata it would be about 1.2mm for a 73mm piston but I suggest you ask the piston manufacturer. He'll have the specs for other similar sized pistons.
Edit. Actually thinking about it more I may have used 1mm not 1.5mm but it was so long ago I'm not sure. 1.5mm does sound a bit extreme.
Just checked a Fiat 1.2 Fire piston (70.80mm bore) and offset is 1.00mm +/-0.10Edit. Actually thinking about it more I may have used 1mm not 1.5mm but it was so long ago I'm not sure. 1.5mm does sound a bit extreme.
Edited by Pumaracing on Tuesday 13th April 08:58
30 years ago when I was building my first engine, a 1275 A series for my Marina, a car magazine gave me his home phone number (can't see that happening nowadays) and he very kindly spent half an hour or so advising me about head porting and other stuff. Then by complete chance about 10 years later I moved out of accountancy, my main profession, into race engines which up till then had only been a hobby. I got back in touch and we've chatted regularly ever since. Trouble is every phone call could last for hours because we start off on one topic then suddenly that branches out into something else and on it goes.
About a year ago you could have asked him questions directly yourself on the Gofastnews.com website but sadly he's left it now. However there's plenty of articles and old postings and you can probably work out who I am. Here's a thread you might enjoy.
http://www.gofastnews.com/showthread.php/263-high-...
About a year ago you could have asked him questions directly yourself on the Gofastnews.com website but sadly he's left it now. However there's plenty of articles and old postings and you can probably work out who I am. Here's a thread you might enjoy.
http://www.gofastnews.com/showthread.php/263-high-...
Soo...
I have sent my sleeved engine back to the machinist to have a second go. This time he bored it to give 0.03-0.04mms piston-bore gap, and a better honing finish (I asked him to let min. 0.05mms for the boring).
Engine assembled, started in situ for 10 minutes around 2500. Then hit the road.
Voila: the engine smoked. Although the some was only half of that was last time. But still present. At least the plugs kept dry. I was not happy, but had a full tank, soo why not? Let's mini a bit. After 500kms, things changed a bit. The smoke is barely visible on idle and the end of the exhaust started to clean up. But if You rev it, it still puffs blue smoke.
Better, better, but still not good enough...
I have sent my sleeved engine back to the machinist to have a second go. This time he bored it to give 0.03-0.04mms piston-bore gap, and a better honing finish (I asked him to let min. 0.05mms for the boring).
Engine assembled, started in situ for 10 minutes around 2500. Then hit the road.
Voila: the engine smoked. Although the some was only half of that was last time. But still present. At least the plugs kept dry. I was not happy, but had a full tank, soo why not? Let's mini a bit. After 500kms, things changed a bit. The smoke is barely visible on idle and the end of the exhaust started to clean up. But if You rev it, it still puffs blue smoke.
Better, better, but still not good enough...
I am still not sure about the cause of this situation.
My recent mini engines NEVER smoked after rebuild.
The machinist replaced the liners.
I cannot replce the builder - thats me
. Although assembly misstake is not impossible, as I am not a I am not a man without misstakes... just dont know what can I do wrong?...
What if we have piston shape problem?...
I will keep on running the car. How long can it take for rings o fully bed in? I mean if I reach 1500 (2000?, 3000?)kms and still have smoke, it wont get better?
Although the engine is much better it is still not good enough. Only perfect is good enough.
My recent mini engines NEVER smoked after rebuild.
The machinist replaced the liners.
I cannot replce the builder - thats me
. Although assembly misstake is not impossible, as I am not a I am not a man without misstakes... just dont know what can I do wrong?...What if we have piston shape problem?...
I will keep on running the car. How long can it take for rings o fully bed in? I mean if I reach 1500 (2000?, 3000?)kms and still have smoke, it wont get better?
Although the engine is much better it is still not good enough. Only perfect is good enough
.You really only have three possibilities.
1) Rings/bores
2) Valve guides/stem seals
3) Overly rich fuel/air mixture
If you think you have eliminated 1 and 2 I suggest you look at 3. I have to assume this time round you examined the bores and honing before you built the engine and were satisfied about the size and honing pattern. Maybe it's time to find an experienced engine builder locally who can actually look at what you're doing.
Personally I've never known a rebuilt engine to smoke at all, even at first start up, and I can't imagine it will get better with more mileage if it isn't right to start with.
Could you be drastically over filling the engines with oil for some reason like the wrong dipstick being fitted? Measure the amount of oil you're putting in compared to the book figure.
1) Rings/bores
2) Valve guides/stem seals
3) Overly rich fuel/air mixture
If you think you have eliminated 1 and 2 I suggest you look at 3. I have to assume this time round you examined the bores and honing before you built the engine and were satisfied about the size and honing pattern. Maybe it's time to find an experienced engine builder locally who can actually look at what you're doing.
Personally I've never known a rebuilt engine to smoke at all, even at first start up, and I can't imagine it will get better with more mileage if it isn't right to start with.
Could you be drastically over filling the engines with oil for some reason like the wrong dipstick being fitted? Measure the amount of oil you're putting in compared to the book figure.
"3) Overly rich fuel/air mixture"
Tomorow I will put my WBO on and set the carb spot on. Although I have fitted drasticaly different needles giving no difference at all. I dont think this is the cause, the electrodes of the pugs are clean and the engine is responsive, pulling fine without hesitation.
"If you think you have eliminated 1 and 2 I suggest you look at 3. I have to assume this time round you examined the bores and honing before you built the engine and were satisfied about the size and honing pattern. Maybe it's time to find an experienced engine builder locally who can actually look at what you're doing."
I have had two good engine builders look at my assembly, but they had found no real problem. Although they are not mini experts, and have not even build one mini engine. Bore look straight, honing fine, no marks of the boring "under" the honing. Piston-bore gap 0.03-0.04mms - just as it should be.
"Personally I've never known a rebuilt engine to smoke at all, even at first start up, and I can't imagine it will get better with more mileage if it isn't right to start with."
The smoking of this engine has got better with time. At first it had visible smoke on idle, and the end of the exhaust was oily black. Now the smoke is invisible at idle (though it smells bad), and the en of the exhaust is dry, but black.
"Could you be drastically over filling the engines with oil for some reason like the wrong dipstick being fitted? Measure the amount of oil you're putting in compared to the book figure."
From the very start I have controlled the amount of oil. The mini needs to be filled 4.6-4.8litres including the fillter - just as I did.
Now the oil consumption is about 0.6-0.8litres/1000km - I guess.
The engine did 800kms after the last assembly.
Tomorow I will put my WBO on and set the carb spot on. Although I have fitted drasticaly different needles giving no difference at all. I dont think this is the cause, the electrodes of the pugs are clean and the engine is responsive, pulling fine without hesitation.
"If you think you have eliminated 1 and 2 I suggest you look at 3. I have to assume this time round you examined the bores and honing before you built the engine and were satisfied about the size and honing pattern. Maybe it's time to find an experienced engine builder locally who can actually look at what you're doing."
I have had two good engine builders look at my assembly, but they had found no real problem. Although they are not mini experts, and have not even build one mini engine. Bore look straight, honing fine, no marks of the boring "under" the honing. Piston-bore gap 0.03-0.04mms - just as it should be.
"Personally I've never known a rebuilt engine to smoke at all, even at first start up, and I can't imagine it will get better with more mileage if it isn't right to start with."
The smoking of this engine has got better with time. At first it had visible smoke on idle, and the end of the exhaust was oily black. Now the smoke is invisible at idle (though it smells bad), and the en of the exhaust is dry, but black.
"Could you be drastically over filling the engines with oil for some reason like the wrong dipstick being fitted? Measure the amount of oil you're putting in compared to the book figure."
From the very start I have controlled the amount of oil. The mini needs to be filled 4.6-4.8litres including the fillter - just as I did.
Now the oil consumption is about 0.6-0.8litres/1000km - I guess.
The engine did 800kms after the last assembly.
That's a crazy amount of oil consumption. I still think there must be a simple obvious reason for all this that an experienced engine builder could spot. If you were a bit closer to me than Hungary I would say bring it round and we'll have a look. If you're mad enough to drive 1000 miles each way you still could I suppose
I still have to suspect the bores are the most likely culprit but it never pays to assume anything else is correct.
Let's make sure the heads are ok first. Stem to guide clearance, good oil seals and valve caps not hitting the oil seals. Also just setting the carb mixture at idle means very little. The mixture needs to be right at full throttle and I have to say your valves look very black to me. Normally with the right fuel mixture inlet valves are blackish and exhaust valves are tan colour like this.
http://3.bp.blogspot.com/_hREX3F5tUZg/Sap0J0vjnGI/...
I still have to suspect the bores are the most likely culprit but it never pays to assume anything else is correct.
Let's make sure the heads are ok first. Stem to guide clearance, good oil seals and valve caps not hitting the oil seals. Also just setting the carb mixture at idle means very little. The mixture needs to be right at full throttle and I have to say your valves look very black to me. Normally with the right fuel mixture inlet valves are blackish and exhaust valves are tan colour like this.
http://3.bp.blogspot.com/_hREX3F5tUZg/Sap0J0vjnGI/...
"That's a crazy amount of oil consumption. I still think there must be a simple obvious reason for all this that an experienced engine builder could spot. If you were a bit closer to me than Hungary I would say bring it round and we'll have a look. If you're mad enough to drive 1000 miles each way you still could I suppose "
Thnaks for Your offer. Gentleman-kind. Unfortunatly distance is, well...
"I still have to suspect the bores are the most likely culprit but it never pays to assume anything else is correct."
Cant it be some kind of piston-shape related problem? I have compared the new AE pistons to the old Hepolite items. the crown diameter differed quite radicaly. The hepolite wa 0.3mms less on top of the crown than on the bottom of the skirt, the new AE was 0.65mms less... I know it may be related to piston material, unleaded gasoline, but it sounds a bit strange...
"Let's make sure the heads are ok first. Stem to guide clearance, good oil seals and valve caps not hitting the oil seals. Also just setting the carb mixture at idle means very little. The mixture needs to be right at full throttle and I have to say your valves look very black to me. Normally with the right fuel mixture inlet valves are blackish and exhaust valves are tan colour like this."
Head had been tested by two companies. Every type of test was carried out. I must say that they are ok. I have even tried an old head on the engine, with no difference.
"Thnaks for Your offer. Gentleman-kind. Unfortunatly distance is, well...
"I still have to suspect the bores are the most likely culprit but it never pays to assume anything else is correct."
Cant it be some kind of piston-shape related problem? I have compared the new AE pistons to the old Hepolite items. the crown diameter differed quite radicaly. The hepolite wa 0.3mms less on top of the crown than on the bottom of the skirt, the new AE was 0.65mms less... I know it may be related to piston material, unleaded gasoline, but it sounds a bit strange...
"Let's make sure the heads are ok first. Stem to guide clearance, good oil seals and valve caps not hitting the oil seals. Also just setting the carb mixture at idle means very little. The mixture needs to be right at full throttle and I have to say your valves look very black to me. Normally with the right fuel mixture inlet valves are blackish and exhaust valves are tan colour like this."
Head had been tested by two companies. Every type of test was carried out. I must say that they are ok. I have even tried an old head on the engine, with no difference.
The machinist suggested that the oil consumption can be connected to the exhaust system. I doubt it, but he is much more experienced than me.
My exhaust is an LCB 3-1 and a 1.75" RC40. I have replaced the rear box as the original was a bit too loud fo this car. But for the feel it is not that restricktive.
Expected engine output is 75bhps.
My exhaust is an LCB 3-1 and a 1.75" RC40. I have replaced the rear box as the original was a bit too loud fo this car. But for the feel it is not that restricktive.
Expected engine output is 75bhps.
camelotr said:
Cant it be some kind of piston-shape related problem? I have compared the new AE pistons to the old Hepolite items. the crown diameter differed quite radicaly. The hepolite wa 0.3mms less on top of the crown than on the bottom of the skirt, the new AE was 0.65mms less... I know it may be related to piston material, unleaded gasoline, but it sounds a bit strange...
Normally the diameter of a cast petrol engine piston in the ring land region is about 0.5% to 0.6% smaller than the nominal bore diameter to allow for thermal expansion of the crown which is obviously the hottest part. With normal casting alloys such as LM13 with coefficients of thermal expansion of about 19 millionths per degree C that provides sufficient room for expansion for crown temperatures up to about 300C or even 350C and frankly if they get much hotter than that the piston is going to break anyway. Even high silicon piston alloys start to lose strength very rapidly above 300C although aluminium doesn't actually melt until over 700C.0.5% should be quite safe for road use and one might go up to 0.6% for very severe use and high compression ratios. For a 71mm diameter piston that would indicate a ring land diameter about 0.35mm to 0.43mm below bore size. 0.3mm below skirt diameter equates to about 0.35mm below bore size so that's at the bottom end of the range and maybe a tad tight but sufficient for normal operating temperatures and certainly 0.65mm below skirt diameter i.e. 0.7mm below bore size is very much higher than I would expect or than ought to be necessary.
Those forged VW pistons I mentioned on page 2 also had a much smaller ring land diameter than normal and I suggested to the client that in conjunction with the absence of gudgeon pin offset plus short slipper style skirts this was also contributing to the problem of piston rock and the pistons slapping themselves to death. It was actually the ring land area that was seizing and the piston manufacturer, god bless their little cotton socks, took this to mean it needed to be machined even smaller still and told the customer to do this when in fact the solution was the very opposite.
Anyway, this will obviously create more piston rock than otherwise might be the case but whether that would be sufficient to prevent rings sealing to the extent you are experiencing is very doubtful. Strictly speaking the rings float about fairly independently of the piston itself and should seal regardless of whether the piston is rocking, especially by the time you get down to the oil ring which is situated much closer to the full diameter of the skirt.
However it's unusual and raises something of a warning flag to me. I would like to know what AE have to say about this.
What are the Mini Spares pistons like in this respect?
Pumaracing said:
camelotr said:
The machinist suggested that the oil consumption can be connected to the exhaust system.
I'm buggered if I can see why. You might as well blame the make of tyres being used.camelotr said:
The "EVO" 73 piston is 0.7mms less on top than on bottom.
I'd appreciate an actual diameter that I can compare to bore size. Just giving me a difference between top and bottom is comparing one unknown to another unknown. Piston dimensions should always be compared relative to a known nominal size i.e. bore size so you know where you are.However it seems far too much. Maybe the original H&G pistons worked ok without pin offset because of full skirts and large piston land diameter to prevent rock. Maybe now with small ring land diameter and still no pin offset things are conspiring to cause a problem.
Pumaracing said:
camelotr said:
The "EVO" 73 piston is 0.7mms less on top than on bottom.
I'd appreciate an actual diameter that I can compare to bore size. Just giving me a difference between top and bottom is comparing one unknown to another unknown. Piston dimensions should always be compared relative to a known nominal size i.e. bore size so you know where you are.However it seems far too much. Maybe the original H&G pistons worked ok without pin offset because of full skirts and large piston land diameter to prevent rock. Maybe now with small ring land diameter and still no pin offset things are conspiring to cause a problem.
The greatest diameter of the piston is 72.95mms (app 10mms from the bottom of the skirt). And the diameter just above the compression ring is 72.25mms. Piston-bore clearance is now 0.08mms. Soo closest gap: 0.08mms, greatest gap 0.78mms.
No pin offset.
Edited by camelotr on Monday 3rd May 11:02
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