This puzzles me about forged pistons

[delcbr]

If I have an engine with cylinder bores of 86mm and I decide I want to fit forged pistons surely I should have the block bored to 86.25mm and have 86.25mm forged pistons.
Why would someone want to fit 86.00mm forged pistons to a block that’s already been in use?

[Inline__engine]

if the bores are good then you dont need to pay someone to make the bores+0.25

[Ahbefive]

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.

[stevieturbo]

Why would anyone want to fit 86.25mm pistons when they could have 86.5mm ?

[delcbr]

Keeps the weight down on the pistons for a start. When engine is stripped down for Rebuild can increment up in bore size. Saves having to buy another block.

[Boosted LS1]

delcbr said:

If I have an engine with cylinder bores of 86mm and I decide I want to fit forged pistons surely I should have the block bored to 86.25mm and have 86.25mm forged pistons.
Why would someone want to fit 86.00mm forged pistons to a block that’s already been in use?

You could hone the block and have an under 86 mm piston if you want to save weight and preserve the block.

[stevieturbo]

Now I want 86.75mm...

[delcbr]

My pistons are 86.02mm block is honed to 86.10mm these are factory sizes.
This is why it puzzles me that piston manufacturers sell 86.00mm pistons.

[Boosted LS1]

stevieturbo said:

Now I want 86.75mm...

Some people really get bogged down with this stuff. Lets go 87 mm.

[Boosted LS1]

OP, have you already got your pistons, if so what o/d?

[delcbr]

Just speaking of what the factory specs are for the engine. I haven’t bought pistons yet because they are £1000 it’s for a 6 cylinder.
But Iv oticed this before for other engines they offer forged pistons for original bore size.

[Inline__engine]

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.

they will give you pistons to suit a 86 mm bore, not a piston that is 86 mm diameter. this means the pistons would be 85.9x so it has the right clearance to a 86 mm bore

[Inline__engine]

delcbr said:

My pistons are 86.02mm block is honed to 86.10mm these are factory sizes.
This is why it puzzles me that piston manufacturers sell 86.00mm pistons.

different pistons expand a different amounts and different applications require different clearance. its not clear if you are talking OE or aftermarket

[gareth_r]

The second post answered the question.

86mm bore 6 cylinder - JZ or RB?

[delcbr]

S50b30 Bmw m3 3.0

[Boosted LS1]

If they are Mahle pistons they often have the clearance built into them. You may find your current rings could fit in which case you won't need to hone the block. Or you may use new rings and hone the block so as to start 'afresh'. Are the technical specs available?

[Mignon]

What a bizarre thread.

[delcbr]

Mignon said:

What a bizarre thread.

Please elaborate what you mean

[delcbr]

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

[gareth_r]

delcbr said:

S50b30 Bmw m3 3.0

86 x 86 is popular.