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Old 05-18-2010, 12:43 PM   #1
mtcs
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Default Some knocking...rod knock or piston slap?

Well, not too sure whats going on...
Last week I got the car tuned and it was running great at 15lbs. I went to my friends shop and he tricked me into going onto his dyno (real good price). While they were driving the car up the ramps, I noticed a small knocking sound that I had never noticed before. It ran on the dyno like a champ and that night I drove it home without any problems. Now, for the last few days i have been noticing some slight knocking when the car is being driven between 1500-2000 rpm with light load. After about 2000rpm it goes away completely. I first thought it was piston slap, but It really sounds like a rod.

My question is: If it indeed has a rod knock, would I hear it throughout all rpms? I only hear it at 1/5 throttle and between 1500-2000 rpm.

It just freaks me out cause there are such low miles on this fresh Raw/Axis shortblock.

Thanks guys.
-Ben
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Old 05-18-2010, 12:43 PM   #2
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Could this be a bad belt tensioner?
I've been reading and found that rod knock is throughout all rpms. This is only between say 1500-2000rpm.
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Old 05-18-2010, 03:15 PM   #3
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Rod knock is usually when you rev the engine in neutral, it will knock when you blip the throttle at 3-4000rpm, and hold it steady or let off the throttle. Unless you have really run it out of oil, then it`ll be so damaged that it knocks all over the revs.

I`ve had rod knock in a few different engines, and they are all the same, despite boxer engine, inline 4 etc.

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Old 05-18-2010, 03:33 PM   #4
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It never does it in neutral. Only when going through 1st-2nd gear with very light throttle. The car has tons of oil. It has never run out of oil yet in it's short (1 month) life. Always running Rotella T 15-40.

So it might be the timing belt tensioner.
It is over 7 years old afterall

and even worse...(broken)

Going to replace the tensioner and see where that gets me.

Quote:
Originally Posted by Legacy_Turbo View Post
Rod knock is usually when you rev the engine in neutral, it will knock when you blip the throttle at 3-4000rpm, and hold it steady or let off the throttle. Unless you have really run it out of oil, then it`ll be so damaged that it knocks all over the revs.

I`ve had rod knock in a few different engines, and they are all the same, despite boxer engine, inline 4 etc.

YouTube- SR20 GTiR rod knock
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Old 05-21-2010, 05:47 AM   #5
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Hi Ben,

I had the same "scare" with my built engine....

Did you notice any drop in oil pressure? If you are developing rod knock, you would notice a drop in pressure.

Below are a couple videos... the first is of piston slap.. the second is of the tear down and inspection of parts... Everything looked perfect except it had a sloppy piston to wall clearance... could have kept going for many more miles...


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Old 05-21-2010, 03:49 PM   #6
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rod knock happens even in neutral..

so if its not happening in neutral than its something else.
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Old 05-22-2010, 12:05 PM   #7
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It sounds like you didn't replace any pistons to get rid of the piston-slap .... just put a heavier coating on them ???
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Old 05-22-2010, 01:04 PM   #8
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Quote:
Originally Posted by nitro1956 View Post
It sounds like you didn't replace any pistons to get rid of the piston-slap .... just put a heavier coating on them ???
I think you are referring to my post.. and the answer for me is: we are not replacing the pistons just using .002 thickness coating on the pistons..

I think the OP of this thread is experiencing slight piston slap from a first time build

JB
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Old 05-22-2010, 01:19 PM   #9
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Default Some knocking...rod knock or piston slap?

Well, this is my 4th Subaru engine. I have built 3 myself but this one, I had Ron over at Axis/Raw performance build it for me. The block is a brand new oem block from Subaru and ron just put in CP pistons.
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Old 05-22-2010, 01:23 PM   #10
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Default Some knocking...rod knock or piston slap?

Oil pressure is perfect. It isn't fluctuating at all. It is the exact same it was before I started hearing the slight knock. Well I am in the process of replacing the timing belt tensioner but I have hit another snag. Apparently some idiot put helicoils in the tensioner bracket hole that the tensioner bolt connects to. So I unscrewed the bolt and he helicoil came out. Were
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Old 05-22-2010, 02:08 PM   #11
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Damn Turboqueef, it sounds exactly like your first video. So I shouldn't worry about it at all? Even If I'm boosting the crap out of the car?

-Ben


Quote:
Originally Posted by TurboQueef View Post
Hi Ben,

I had the same "scare" with my built engine....

Did you notice any drop in oil pressure? If you are developing rod knock, you would notice a drop in pressure.

Below are a couple videos... the first is of piston slap.. the second is of the tear down and inspection of parts... Everything looked perfect except it had a sloppy piston to wall clearance... could have kept going for many more miles...

YouTube- Piston slap short

YouTube- Rons.m2t
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Old 05-22-2010, 02:20 PM   #12
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Quote:
Originally Posted by mtcs View Post
Damn Turboqueef, it sounds exactly like your first video. So I shouldn't worry about it at all? Even If I'm boosting the crap out of the car?

-Ben
I agree with Turboqueef, subaru motors are just really noise , even worse built. People always get a head of themselves. My car sounds like yours and I believe is pistons slap, I am only taking it apart until a see rod through the block

My vid is crappy I used my phone, you can hear the slap on the end of the vid

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Old 05-22-2010, 02:20 PM   #13
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Quote:
Originally Posted by mtcs View Post
Damn Turboqueef, it sounds exactly like your first video. So I shouldn't worry about it at all? Even If I'm boosting the crap out of the car?

-Ben
Yea.. it probably is just piston slap... I was very worried about it until I ran the 11.44 with it... and all of my oil samples kept coming back perfect...

My slap was much less noticeable with the stock motor mounts... but when I installed the aftermarket mounts it sounded like in the video...

You can talk to Ron about it but I bet it is no big deal!!!

JB
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Old 05-22-2010, 02:50 PM   #14
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It's just weird. On my other built motors I built, I used:

Top: .019
Second: .022
Oil: .015

Had no pistons slap, but I think I had one of my rings butt with itself and cracked a cylinder a little.

What does Ron set his rings to? anyone know?
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Old 05-22-2010, 03:27 PM   #15
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Quote:
Originally Posted by mtcs View Post
It's just weird. On my other built motors I built, I used:

Top: .019
Second: .022
Oil: .015

Had no pistons slap, but I think I had one of my rings butt with itself and cracked a cylinder a little.

What does Ron set his rings to? anyone know?
Not sure what he uses for ring gap but I don't thing that has any impact on the slap.. My understanding is slap is caused by piston to wall clearance... mine was set to .003 on the first build...

jb
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Old 05-22-2010, 03:46 PM   #16
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So it depends on the cylinder bore and which pistons were used...
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Old 05-22-2010, 04:34 PM   #17
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Would piston slap effect a compression test at all?
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Old 05-22-2010, 04:56 PM   #18
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Quote:
Originally Posted by mtcs View Post
Would piston slap effect a compression test at all?
no unless it was HUGE piston slap... mine was fine when I pulled the engine
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Old 05-22-2010, 06:59 PM   #19
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To TurboQueef:

..........If I understand correctly...you had one lose piston causing the slapping noise. But you're coating all 4 pistons with 0.002" coating. Doesn't this mean some of your pistons will end up too tight ?
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Old 05-22-2010, 07:09 PM   #20
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Quote:
Originally Posted by nitro1956 View Post
To TurboQueef:

..........If I understand correctly...you had one lose piston causing the slapping noise. But you're coating all 4 pistons with 0.002" coating. Doesn't this mean some of your pistons will end up too tight ?
Yes... that would be the case but ... all of the cylinders will be honed to provide proper clearance considering each individual piston size!
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Old 05-22-2010, 10:00 PM   #21
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TurboQueef...

On a stock MY 2003 2.5 in an Outback...what PTW clearance would you suggest ?
My 2003 Outback runs good......but I want to change out a lose piston which is giving slap noise.
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Old 05-22-2010, 10:22 PM   #22
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Quote:
Originally Posted by nitro1956 View Post
TurboQueef...

On a stock MY 2003 2.5 in an Outback...what PTW clearance would you suggest ?
My 2003 Outback runs good......but I want to change out a lose piston which is giving slap noise.
Not sure... I let the pro's determine this... maybe Ron will chime in... I think it depends on the pistons you use and a couple other variables... !!
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Old 05-22-2010, 10:32 PM   #23
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Quote:
Originally Posted by nitro1956 View Post
TurboQueef...

On a stock MY 2003 2.5 in an Outback...what PTW clearance would you suggest ?
My 2003 Outback runs good......but I want to change out a lose piston which is giving slap noise.
Just noticed your user name... what does it stand for?

FYI... From 1986 ~ 2006 I raced a Suzuki drag bike... did not run nitro but I raced in the IDBA then ProStar sanctions... they always had top fuel drag bikes at the events I raced in... what a blast that must have been. One of my race friends was Tony Lang... he ran a top fuel Suzuki... what a rush...



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Old 05-25-2010, 01:51 AM   #24
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I just built a complete long block myself (except for setting valve lash) and I used wiseco standard bore pistons with just a hone. For the first 400 miles I couldn't hear them at all. Now though on cold mornings I can hear them very slightly for about a minute then the sound goes away, and sometimes I still can't here them on cold starts at all.
While I was building my engine I had a guy come over with a built STi that had 100mm CP's and it sounded like a Mack Truck! He took me for a ride in it and it was the fastest car I have ever been in and he claimed it was 430 WHP but I couldn't get over how loud his pistons were.
I was expecting my wisecos to be as loud but like I said they are very quite.

It might just be a CP piston thing, but usually it is the piston to wall clearance.
However CP's do use huge ring gaps so maybe the rings aren't holding the piston very tight in the cylinder?
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Old 05-25-2010, 09:26 PM   #25
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Quote:
Originally Posted by STIStalker View Post
I just built a complete long block myself (except for setting valve lash) and I used wiseco standard bore pistons with just a hone. For the first 400 miles I couldn't hear them at all. Now though on cold mornings I can hear them very slightly for about a minute then the sound goes away, and sometimes I still can't here them on cold starts at all.
While I was building my engine I had a guy come over with a built STi that had 100mm CP's and it sounded like a Mack Truck! He took me for a ride in it and it was the fastest car I have ever been in and he claimed it was 430 WHP but I couldn't get over how loud his pistons were.
I was expecting my wisecos to be as loud but like I said they are very quite.

It might just be a CP piston thing, but usually it is the piston to wall clearance.
However CP's do use huge ring gaps so maybe the rings aren't holding the piston very tight in the cylinder?

What PTW clearance did you use for your engine ?
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