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Built RB25 piston slap?

KimHaugli
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KimHaugli Rookie

KimHaugli

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Hi. I have a built RB25. Since I got the car a year ago, it's had this annoying knocking sound. Does not sound like it's any different depending on engine temp.

I don't think the sound has gotten much worse. Been two times for mapping in dyno, one drag race and 13000km on the road just last summer. Now after using the car for a few days the oil starts to get a strong smell of fuel. Injectors is tested and cleaned. Nothing wrong with them. Does not look like the engine has any noticable blow by either.

Engine is bored, CP Pistons (87mm), Eagle rods. All bearings are ACL Race series. Compression test was: 9.9, 9.9, 10, 10, 10, 10.5

Anyone had the same sound from their's RB? Listened with a stethoscope and knocking is most noticeable from sylinder 6 on block- intake side. 

 

Edited by KimHaugli
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KiwiRS4T Grand Master

KiwiRS4T

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Your compressions are quite low but I'm guessing it was built that way.  The noise sounds a bit of a worry .  Were you able to  narrow down the source to high up (valve gear) or lower down in the block?  Is your VCT working OK?

KimHaugli Rookie

KimHaugli

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8 hours ago, KiwiRS4T said:

Your compressions are quite low but I'm guessing it was built that way.  The noise sounds a bit of a worry .  Were you able to  narrow down the source to high up (valve gear) or lower down in the block?  Is your VCT working OK?

Sound was faint in the head and a lot more noticeable on the block. VCT should be fine. I guess the sound would come from the front if that was the problem?

GTSBoy Grand Master

GTSBoy

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Yeah, so, the obvious suspects for block noise are piston slap, gudgeons and rod bearings. Bit hard to say which it might be from here though.

The video from inside the car sounds more like a spun bearing than piston slap. Hard to believe that it has been making that noise for 13000km without either it exploding (if it was a bearing) or you taking it apart to find out what the hell is wrong. There's no way that I would drive it if it sounded like that.

The video from inside the engine bay sounds so much more like a stuck/collapsed lifter than anything from lower down. There's no way that I would drive it if it sounded like that.

KimHaugli Rookie

KimHaugli

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All lifters has been opened and cleaned out before I started using the car again this year. Many were saying lifters last year but they looked perfect. 

Last year when I started to get fuel in the oil is when I parked the car. Got the map checked and it was way too rich. Assembled the head, got off to dyno. Noise was still there and 3 days after, strong smell of fuel in oil. 

KimHaugli Rookie

KimHaugli

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On 20/06/2020 at 2:10 AM, GTSBoy said:

That just sounds like lifters too.

TAKE IT APART!

Or at least take it to a decent mechanic.

I find it weird for it to be the lifters as everyone told me it was last year. Took them out this winter and they have been opened and cleaned out. Still the same sound, and no one looked like beeing broken. 

After using the car a little the last weeks the sound is getting worse when the engine is cold, and is more noticeable now when warm. So the engine is coming out ?

Dil-Dog Collaborator

Dil-Dog

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Posted

This actually is a really interesting post... to start, a small amount of  Piston slap Is actual a normal occurrence in forged builds under certain conditions believe it or not.

1# Forged pistons are honed/bored at slightly higher than stock piston to wall clearance over factory cast pistons 

2# Forged pistons and cast pistons dont have the same expansion under heat, forged pistons expand more hence and take more time too and thats why they need a greater tolerance. Sometimes depending on the application... forged pistons when in the bores can even rock back and forth to the point where they go “click clack” when moving them by hand. Crazy to think about but look it up on youtube from HPacademy. 

#3 Forged pistons usually have a central gudgeon pin location, where as cast/stock are made with an offset pin location on all OEM pistons, which helps reduce noise.

Now, from my experience with my forged rb25neo, i do have some very slight slap on cold start but within 5-10 mins it is 100% Gone. This is considered normal with forged motors

In your case with ur engine warmed up still slapping loud like hell and with ur compression readings, it may be that ur clearances are completely off and this is 100% the machinist/engine builders fault for not doing it or checking it correctly. 
 

Sorry to hear about it but keep us updated, very curious to know what happened or if its something else entirely.

KimHaugli Rookie

KimHaugli

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5 hours ago, Dil-Dog said:

But keep us updated, very curious to know what happened or if its something else entirely.

Yeah, I will come with updates when I get it out.

I guess there will be scarring on the side of pistons or cylinders if the engine has piston slap and been driven that far with it? When I had the head off I could not see any damage to the cylinder walls, but you never know what hides lower in the cylinder when piston is in.

KimHaugli Rookie

KimHaugli

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1 hour ago, Dil-Dog said:

Yeah man, good luck and take pics and get back to us 

Just got it confirmed 30 minutes ago. Piston slap ? but the mechanic said that the sound was not that bad and that he would not strip the engine because of that. Since my engine is built with larger pistons, he told me to replace the thermostat to a normal one (85-90c). The one that is in now opens at 63c. He thought the sound might go mostly away if the engine got up to normal temperatures. So I'll try that first ?

  • 5 months later...
KimHaugli Rookie

KimHaugli

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On 28/06/2020 at 6:25 PM, Dil-Dog said:

Yeah man, good luck and take pics and get back to us 

Sorry for the really late answer, had a lot to do the latest months. But I guess I figured out what the problem was. Previous engine builder/company have done everything wrong... With the sound in the engine and it starting to fill fuel in the oil I was pretty sure that rings or pistons were worn. What I didn't expect after 40k km was this.

-Completely worn piston skirts

-Completely worn cylinders, and honing was done wrong. Honing marks were straight as an arrow in 80% of ALL the cylinders.

-Cylinders were so oval that if you turned the piston 90 or 180* it would not fit down in the cylinder.

All bearings looked great with no marks or wear, so that was not were the sound came from. New build starting early January when the parts arrive. Just got my new block finished at the machine shop, bored to 86.5mm and decked. Can't wait to get started and do it right this time!

 

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