This puzzles me about forged pistons

[stevieturbo]

delcbr said:

Description
model: BMW M3 3.0 24V 3-Ring Version

engine: S50B30

part no.: K9400DA

bore size: 86mm/86,5mm/86,75mm/87mm

stroke: 86.00mm

Wössner comp. ratio: 12.6:1

OE comp. ratio: 10.5:1

dome volume: +5.0cm³

comp. height: 31.50mm

piston weight: 306g

rod length: 142mm

pin dia.: 21mm

That list buggers your 86.25mm idea.

If the bore is good, just leave it alone. If it isnt, then bore it to whatever size is needed to clean it up. It isnt rocket science.

[Inline__engine]

delcbr said:

Please elaborate what you mean

https://en.oxforddictionaries.com/definition/bizarre

[PaulKemp]

Mignon said:

What a bizarre thread.

Indeed

[PaulKemp]

Popcorn anyone?

[Boosted LS1]

^ Bit late really. We're going home.

[Mignon]

Ahbefive said:

An 86mm piston won't freely slide inside an 86mm cylinder. Although 1/4 of a mm may be too much tolerance, perhaps the ring makes up the difference.

As you clearly know nothing about engines or engineering why do you feel the need to comment?

[Boosted LS1]

^ Because he thought he might be helpful? Even if he is a bit adrift.

[Ahbefive]

Mignon said:

As you clearly know nothing about engines or engineering why do you feel the need to comment?

So where do you think I am going wrong? I have had a read up and can't see that I have said anything incorrect or contraversial.

Also why do you think that just because I may be wrong about piston/cylinder bore sizings that I know "nothing about engines or engineering"?

Edited by Ahbefive on Tuesday 11th September 06:41

[227bhp]

Mignon said:

Ahbefive said:

An 86mm piston won't freely slide inside an 86mm cylinder. Although 1/4 of a mm may be too much tolerance, perhaps the ring makes up the difference.

As you clearly know nothing about engines or engineering why do you feel the need to comment?

I don't know if you realised it, but a lot of these threads are incorrectly started in 'General Gassing' so the general public get to put their uninformed comments in before it gets moved to here.
Personally i'd be happy with them left there. If an uniformed person seeks uniformed answers he's happy with then everyone is happy, it's just forum filler.
It all comes down to the same thing, we need two engine tech sections for two levels of knowledge. Sadly it won't happen as: A. PH can't be arsed.
B. You'd need a moderator who can differentiate between the two to sort them out.

Until then you'll just have to put up amateurish idle threads and similar responses.

[227bhp]

Ahbefive said:

Mignon said:

As you clearly know nothing about engines or engineering why do you feel the need to comment?

So where do you think I am going wrong? I have had a read up and can't see that I have said anything incorrect or contraversial.

Also why do you think that just because I may be wrong about piston/cylinder bore sizings that I know "nothing about engines or engineering"?

Edited by Ahbefive on Tuesday 11th September 06:41

I have a hard time believing what i'm reading sometimes, the very fact you've posted up drivel indicates you have no knowledge. How can that be difficult to understand?

[Mignon]

Ahbefive said:

So where do you think I am going wrong? I have had a read up and can't see that I have said anything incorrect or contraversial.

If you didn't know how stupid it was when you first typed it why would you suddenly realise anything by reading it again?

Ahbefive said:

Also why do you think that just because I may be wrong about piston/cylinder bore sizings that I know "nothing about engines or engineering"?

Well you clearly have no idea what piston to bore clearances should be or how piston rings work so I can't imagine you're some sort of genius on other parts of the engine.

[Ahbefive]

And yet neither of you muppets are capable of explaining what I have allegedly gotten so wrong?

Are you suggesting that an 86mm piston is correct for an 86mm cylinder? So far you are just coming across as a pair of condescending idiots rather than sharing any sort of knowledge.

[Mignon]

You didn't ask for anyone else's knowledge. You just ventured an uninformed opinion and displayed your own lack of it.

[Ahbefive]

Mignon said:

You didn't ask for anyone else's knowledge. You just ventured an uninformed opinion and displayed your own lack of it.

If you say so. Now I am asking for your expert knowledge on what exactly I have said that is wrong and why.........

[littleredrooster]

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.

[Mignon]

littleredrooster said:

That 1/4 of a mm is not a tolerance, it's a clearance.

Actually it's neither. It's an oversize. The bore to skirt clearance on a piston will normally be between about 1 and 4 thou (0.025 - 0.10 mm). The tolerance on a finish honed bore size is usually -0.00/+ 0.001". The tolerance on a piston skirt diameter will be within 0.0005".

Other than that though, what you said.

[littleredrooster]

Mignon said:

Actually it's neither. It's an oversize.

Other than that though, what you said.

Sorry - I just meant the 1/4 of a mm to which he was referring, i.e. the difference between piston size and bore size being a running clearance rather than a tolerance.

[Mignon]

littleredrooster said:

Sorry - I just meant the 1/4 of a mm to which he was referring, i.e. the difference between piston size and bore size being a running clearance rather than a tolerance.

Yes that would certainly be an amusing engine to listen to.

[Ahbefive]

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.

Thanks for that, a helpful and informative post unlike anyone elses. Again, appreciated.

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

[GreenV8S]

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.