Rb25 Det Neo , Just Forged It

[DaveB]

I have had my rb25 neo rebuilt and have discovered the joy of piston slap with forged pistons when cold. When they're cold there's like an 8 thou clearance from piston to bore to allow for mega expansion; it blows smoke and sounds like a diesel until it's warm (which generally happens by about the end of the street); I suggest there's no way the compression will be as high under these conditions, and perhaps it could even be a 25-50% difference like you're seeing here.

[aerofocker]

was the test performed properly as in, fuel pressure dumped, throttle wide open all plugs removed?

[Simon-S14]

I have had my rb25 neo rebuilt and have discovered the joy of piston slap with forged pistons when cold. When they're cold there's like an 8 thou clearance from piston to bore to allow for mega expansion; it blows smoke and sounds like a diesel until it's warm (which generally happens by about the end of the street); I suggest there's no way the compression will be as high under these conditions, and perhaps it could even be a 25-50% difference like you're seeing here.

sounds like it was built a bit looser then it probably should have been.

[AngryRB]

nothing worse than piston slap, and it just gets worse... my first car ( cordia turbo) had it bad and even after a rebuild was still slapping away, if your very particular about your car it annoys you everytime you start it, i think all that rocking scuffs the piston and rings, and just accelerates wear and tear.. not to mention sounding like its a worn out engine....

Its very worthwhile to do that important work yourself or inspect it, cause what i thought was a good boring out of my block turned out to be a rough as guts quick job and resulted in piston slap and low compression on a rebuilt motor ( no forged pistons), rings wouldnt bed in and it was a mess...

all those measurements of a thousandth of an inch make big differences, and mechanics can be very rough and approximate instead of precise and exact...

the difference between a motor with perfect clearance and precision compared to others can be huge, sounds rough and dowy compared to sweet and quiet....

[Rolls]

I have had my rb25 neo rebuilt and have discovered the joy of piston slap with forged pistons when cold. When they're cold there's like an 8 thou clearance from piston to bore to allow for mega expansion; it blows smoke and sounds like a diesel until it's warm (which generally happens by about the end of the street); I suggest there's no way the compression will be as high under these conditions, and perhaps it could even be a 25-50% difference like you're seeing here.

8 thou is pretty massive, why so big?

If it is blowing smoke when cold it means the rings haven't properly bedded in, this isn't right. 4 thou sounds better for a street car to me.

[r33_racer]

Depends on the piston brand and application. Its possible 0.008" is the recommended piston to bore clearance.

It would be interesting to see the spec sheet for the pistons. What brand are they?

[Rolls]

Depends on the piston brand and application. Its possible 0.008" is the recommended piston to bore clearance.

It would be interesting to see the spec sheet for the pistons. What brand are they?

It is possible but unlikely.

[urtwhistle]

Wow 8th, I thought my old 30 was bad with it's 6.5tho. are you running like a million psi with nitrous and methanol? I would have thought 4tho would be more on the money.

[r33_racer]

It is possible but unlikely.

Yeh that is true. But you never know! Maybe some unheard of brand of piston with a super high expansion rate lol.