This puzzles me about forged pistons
Discussion
GreenV8S said:
Ahbefive said:
1/4 of a mm may be too much
It's massively too much for modern engines, but I remember being told that when Tilly Shilling bought herself a new car (a Mini iirc) the first thing she did was strip down the engine and turn 10 thou off the pistons. Using the lathe which, obviously, was bolted down in the middle of her living room. 
I’ll let those that don’t know look it up
Ahbefive said:
I run construction sites and it's common that an M12 bolt does not fit in an M12 hole.
No that's not only not common it's impossible. A 12 mm bolt which has a nominal thread O/D of about 11.8 mm will always fit in a 12 mm hole. If it doesn't then either the bolt is not a 12 mm bolt or the hole is not a 12 mm hole.GreenV8S said:
It's massively too much for modern engines, but I remember being told that when Tilly Shilling bought herself a new car (a Mini iirc) the first thing she did was strip down the engine and turn 10 thou off the pistons. Using the lathe which, obviously, was bolted down in the middle of her living room.
She certainly wouldn't have taken them down in diameter!Mignon said:
Ahbefive said:
I run construction sites and it's common that an M12 bolt does not fit in an M12 hole.
No that's not only not common it's impossible. A 12 mm bolt which has a nominal thread O/D of about 11.8 mm will always fit in a 12 mm hole. If it doesn't then either the bolt is not a 12 mm bolt or the hole is not a 12 mm hole.
Ahbefive said:
I run construction sites and it's common that an M12 bolt does not fit in an M12 hole. It's a very different type of engineering but engineering all day is what we do.
Of course an M12 bolt will fit into a 12.0mm hole...otherwise it is not a correctly made M12 bolt.But an M12 hole would typically denote a threaded hole...so of course that M12 bolt would then screw into that hole if it is as claimed and correct thread.
Otherwise it isnt what is stated.
Ahbefive said:
littleredrooster said:
Ahbefive said:
An 86mm piston won't freely slide inside an 86mm cylinder.
An 86mm piston is only nominally 86mm. The manufacturer makes it slightly smaller to give the correct running clearance, taking into account stuff like thermal expansion, oil film clearance etc.. Thus an 86mm piston actually will freely slide inside a an 86mm cylinder. There are many other aspects of machining to size, but I'm keeping it simple here.Ahbefive said:
Although 1/4 of a mm may be too much tolerance, perhaps the ring makes up the difference.
That 1/4 of a mm is not a tolerance, it's a clearance. I've been intending to write a guide on the differences between allowance, tolerance and clearance for a good while now as these terms are regularly misused on the technical parts of PH.The rings have nothing whatsoever to do with "making up the difference" , they simply (simply...!!) seal against the cylinder wall to make sure that the expanding charge above them doesn't escape to where it isn't wanted.
I run construction sites and it's common that an M12 bolt does not fit in an M12 hole. It's a very different type of engineering but engineering all day is what we do.
Edited by Ahbefive on Tuesday 11th September 18:50
You can't compare the tolerances on a building site to those in an engine, it's just silly. A groundworker will be within an inch or two, roofer to 12mm, job dependent a bricky will work to 5mm, joiner to .5 - 1mm again dependent on what it is and a plasterer will just throw it all over the place and walk off.
It's misleading to say an 86mm piston will slide in an 86mm bore because it won't, it needs clearance, it's just that the .25 you mentioned is vast in engine bore terms. To put that into perspective .1mm is also a bit big for an aftermarket forged and the pistons will make a knocking noise when cold (piston slap), just .02 - .03 less and the noise will go so we're talking now about a bore clearance of .07mm and that's also large compared to some...
I don't know about now, but it used to be said that F1 engines have zero clearance when cold, the engine was effectively a locked up solid mass and needed hot water circulating through it to free it up before starting. Yes, there are hot clearances and cold clearances, two different things.
There are aftermarket piston/bore clearances (which are huge) and factory clearances which are mind blowingly minute, lets take an 18yr old Japanese production car engine with a bore of 87 and factory forged pistons:
Piston/bore clearance: .004 - .027.
When it becomes .04 it's worn out and requires a rebore with new pistons.
Other things worthy of note concerning pistons:
The sides aren't parallel, the top (crown) diameter is smaller than the skirt.
They aren't circular. Average aftermarket pistons are circular at the top, but oval at the bottom, top of the range aftermarket (very rare) and normal OEM are oval at the bottom and around the top.
As you can see it's far from as simplistic as you thought and that's just the basics.
227bhp said:
I don't know about now, but it used to be said that F1 engines have zero clearance when cold, the engine was effectively a locked up solid mass and needed hot water circulating through it to free it up before starting. Yes, there are hot clearances and cold clearances, two different things.
It's said a lot but it's also complete b
ks unfortunately, for a start how on earth would you build one.... they need temperature before starting for a variety of reasons including oil viscosity (or rather load on the pressure and scav pump drives), valve clearances, head sealing, poor closed throttle and low rpm fuel control and piston to bore clearance but none are "locked solid" when cold. The piston/bore clearances are tight, which they need to be with a very small skirt area and a 0.68mm thick compression ring (just the one!) and so are the piston/valve clearances to the point that it's not unusual to see witnessing in any carbon build up on the crown in the later engines but they'll still hand crank from cold (assuming they have air on).poppopbangbang said:
It's said a lot but it's also complete bks unfortunately, for a start how on earth would you build one....
Oh come on, we ain't stoopid. You put the block in an oven and then fit the pistons. Then you turn it over real fast to set the valve clearances before it cools down. Duh!ks unfortunately, for a start how on earth would you build one....Mignon said:
poppopbangbang said:
It's said a lot but it's also complete bks unfortunately, for a start how on earth would you build one....
Oh come on, we ain't stoopid. You put the block in an oven and then fit the pistons. Then you turn it over real fast to set the valve clearances before it cools down. Duh!ks unfortunately, for a start how on earth would you build one....An 86mm bore is a hole in a block which is 86mm in diameter +-the manufacturing tolerance.
An 86mm piston is an 86mm piston in name only. It is designed to fit in an 86mm bore, less the designed running clearance. It does not measure 86mm in diameter.
HTH
An 86mm piston is an 86mm piston in name only. It is designed to fit in an 86mm bore, less the designed running clearance. It does not measure 86mm in diameter.
HTH
Edited by stevesingo on Wednesday 12th September 08:29
Mignon said:
stevieturbo said:
Of course an M12 bolt will fit into a 12.0mm hole...otherwise it is not a correctly made M12 bolt.
He's probably thinking at a level far too high for us "muppets" who know nothing about engineering to even comprehend.Gassing Station | Engines & Drivetrain | Top of Page | What's New | My Stuff