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No, CA18's don't have forged pistons..very few factory engines do. With the exception of some of the early turbo cars (they didn't know any better), and the factory racing specials eg. Ford Sierra.

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Educate me here coops, a buggered big end is bad news by all accounts, but why would it drop the compression

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I hope (for your sake) that it is just a cracked ring, or a head-gasket as lagwagon suggested, but I'd be willing to bet $$$ that you've spun the 3rd & or 4th big end bearing..... This is a common problem for CA18's.
I'm pretty sure the stock CA18DET has cast pistons, but unless you are looking for a 10 second car you should be o.k to stick with the cast ones.

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You say that your engine is a CA18, then you refer to boost pressures, so i'm guessing that it's actually a CA18DE that you have fitted a turbo to, or it's a CA18DET.

If it's a CA18DET, then i believe it will have forged pistons anyway.
Can anyone verify?

if you like i have a complete ca18 engine with brand new gasket kit, sump to cam covers, reco'd head and heavy duty valve springs for around $1000 which is a lot cheaper than a rebuild if you need the option.

i would put my money on head gasket, passenger side rear of the motor. firstly tear down your motor and see what has happened, if the pistons rods and crank are all fine, i'd just re-use them.

if all of these things are fine...
chrome/moly rings are just a different type of metal, and a lot harder. they take a lot longer to bed in than the standard cast rings (up to 3000km's) and you do run a small risk that they wont bed in. having said that they are a good thing and cost less than a set of genuine rings from nissan roughly $100 - $120.

no need to balance your crankshaft if you are using all of the same bottom end components as it is done from the nissan factory.

i believe forged pistons are only needed if you are going for some really big power and want to tune your car really close to its detonating threshold. if your engine is pinging it will last 10 seconds longer if the pistons are aftermarket forged items. the factory CA turbo pistons are a good thing.

don't o-ring the head.

there are a few fairly priced clutches on the market. adelaide extreme, daikin exedy, just have a look around and speak to them.

i rebuilt my CA18t with fresh bearings and molly rings. the rest is standard. at the drags i was running 16psi (T28bb) and revving to 7500-8000. my pistons are fine so slight boost increase will be more than ok.

Just get it rebuilt to standard. USE A GENUINE NISSAN HEAD GASKET. $90, can't be beaten! buy a complete gasket kit for the rest and throw the head gasket on ebay or the bin.

or you could buy that other fella's CA18 package for a 1200, its very fairly priced for the motor alone!

good luck

Unfortunately my ca18 shat itself - compression in cylinders was 160, 160, 155, 120. drove worse than the a12!

I am forced now to take the engine out (probably cracked piston ring) and was wondering if there was anything i should tell the engine rebuilders to do when it's out.

What are the advantages and costs of these things for example:
chrome/moly rings, balance crankshaft, forged pistons, o-ringed head. Obviously i will fit a new clutch, any ideas here? Will forged pistons extend the life of the engine?
are the standard pistons ok for slightly increased boost to 10psi?
Any other ideas will help!!!
thanks, justin