A bit like L'Oreal 'Replumper'


Right. So because the block happens to expand at the the same rate as these pistons, you don't have to allow for any extra clearance within the bore (other than the absolute accepted minimum)?

Does this mean that 'normal' pistons expand more, so extra clearance has to be allowed for, so as to stop them from seizing in the bores?

 

I would if the same was being said by everyone.

 

[quote=joz8968;8261187]A bit like L'Oreal 'Replumper'


Right. So because the block happens to expand at the the same rate as these pistons, you don't have to allow for any extra clearance within the bore (other than the absolute accepted minimum)?

Does this mean that 'normal' pistons expand more, so extra clearance has to allowed for, so as to stop them from seizing in the bores?[/quote]

That would be my take on it,

David

 

What do I know ?

 

FTR, what is the absolute min. piston ring clearance for those 'Hyper-blah blah' pistons?

And what is it for 'normal' ones?

 

I can confirm that the V5 and V6 do have forged pistons.

These are easy to identify from cast pistons too. The cast pistons have casting marks underneath the pistons and will display a criss cross pattern on them.

Forged Pistons have a very smooth underneath.
Here is a set of V5 forged STI pistons (which are for sale btw) as you will see the underside of the pistons is smooth.

 

Nice sales pitch Daz, wish I'd thought of that

David

AND

regarding ring gaps and clearances I have no idea, as we don't use the B/S pistons.

BUT generally, 3 thou per inch of bore.

David APi

 

Right.

So, for example, taking the std bore's diameter of 92mm (c. 3.6"), then you have to allow about 0.011" (0.28mm) of clearance (i.e. 3.6 x 0.003)?

 

Yeah about that for a top ring.

David

 

How many rings does the Impreza piston have: 2 or 3?

Are they called:

1) 'Top ring'
2) 'Compression ring'
3) 'Oil control ring'

What are they technically called and what is each one's 'job'?

 

V8 Euro STI engines have forged pistons.

 

You got it right there.

 

The other two are self-explanatory but what is the 'Top ring's' job

Also, did I get the order correct?

 

The order is right and the top ring is a compression ring also.

 

Cheers

 

Top ring - Compression control ring + takes most of the heat from the combustion process. Most top rings are coated eg moly/chrome/gas nitrided to reduce wear and improve the durability in the harsh environment they work in.
Second Ring - Compression control and oil control, they provide a secondary seal and help dissipate heat. Most have a Tapered lower edge to scrape the oil from the cylinder wall to prevent it getting into the combustion chamber.
Oil Control Ring - Does what it says on the Tin.



This is a good bit of high speed camera work


YouTube - From Inside a Working Petrol Engine


Mick

 

Wow. That's fascinating and rather beautiful (gayer ).

Really illustrates the flame front's controlled burn when the spark plug fires.

I'd love to see footage of an engine detting or pre-igniting!

 

I knew it has been confirmed elsewhere, thank you.