02 WRX Wristpin Knock

**Kody Hartlaub** · 01-13-2013 07:22 AM

I noticed the knock a couple months ago when I started the car one morning. The knock has gotten to the point now where it will knock on a cold start for 3-4 seconds and when I'm driving in the 2000-3500 RPM range also when the motor reaches temp it will knock at idle. I took it to my local Subaru dealer and the mechanic said that its the wristpin and it is common on this model. So I am wondering if I am going to replace the motor what kind of motor should I be looking for and if I should maybe jut swap to a STI?

**MainFrame** · 01-13-2013 11:18 AM

Depends on your budget and what you want in a car. If the budget is there and you want a reliable car, by all means buy a new STi.

**Kody Hartlaub** · 01-13-2013 08:40 PM

What's the price difference from buying a wrx motor to a sti motor?

**MainFrame** · 01-13-2013 09:02 PM

I'm not sure, but you can't just put a STi motor with DBW and AVCS heads in your car without also getting both complete wiring harnesses and the ECU.. it would probably be more cost effective and MUCH less trouble to just sell your car and buy a STi.

Or you can build a hybrid motor like mine.. assuming your existing heads are in good condition.

**Kody Hartlaub** · 01-14-2013 05:04 AM

That was another option I was thinking about, thanks for the reply.

**zax** · 01-14-2013 06:25 AM

Are you sure it's the pin and not piston slap? This occurs when a relatively cold piston has not expanded to fill the cylinder and "wiggles" within the cylinder with the side loading force of the rod. Minor piston slap is harmless.

**Kody Hartlaub** · 01-14-2013 03:35 PM

I don't think its a piston slap. I logged some data on my way home today and saw the "Feedback Knock" went to a -4.5. I am not 100% sure what that means though haha.

**zax** · 01-14-2013 07:14 PM

Originally Posted by Kody Hartlaub

I don't think its a piston slap. I logged some data on my way home today and saw the "Feedback Knock" went to a -4.5. I am not 100% sure what that means though haha.

Feedback knock correction is part of the Subaru knock correction strategy. By knock, I mean detonation events. Exactly what mods are on the car and by whom was it tuned?

***mycologist*** · 01-14-2013 07:29 PM

Yeah, the "knock" that is logged by the ECU is detonation and if you hear it it will sound like the rattle things in a paint can when you are accelerating hard. You need to get that FBKC taken care of either by fixing whatever air leak you might have or getting a tune that fits your mods.

If you have real knock "knock" that is very different.

I agree with Zax again on that - Subaru motors can be very noisy and be just fine. I'd get a second opinion as I also don't think I've ever heard of anyone having "wristpin knock" and it certainly isn't "common".

**MainFrame** · 01-14-2013 08:29 PM

Originally Posted by mycologist

I also don't think I've ever heard of anyone having "wristpin knock" and it certainly isn't "common".

I know, right? Spun rod bearing or cracked ringland are the common deaths. Cracked ringland would be evident by loss of power and lots of smoke. Rod knock is pretty niosy and doesn't cause smoke, but it also doesn't keep running. If you hear rod knock coming on it would be unlikely the car would run for more than 20 miles before crapping out entirely.

**zax** · 01-14-2013 08:36 PM

Yes piston wrist pin failure is exceptionally uncommon... Keep in mind that due to the relatively large rod:stroke ratio for a roadcar (1.74:1 for the 2L and 1.65:1 for the 2.5L) the full angular sweep of rod with respect to the piston is quite small compared to most cars. Furthermore, the sideloading force (component of force into the cylinder wall) is small so the actual force on the wrist pin is smaller than most cars. This means less wear on the pin itself.

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**MainFrame** · 01-14-2013 08:41 PM

I have seen one example where the retainer clip came loose (may have been improperly installed) and the wristpin actually walked out to the side of the pistin and gouged the cylinder wall all to hell. But as far as the wristpin actually failing.. nine times out of ten the wristpin will stay intact and basically rip out the bottom of the piston. Not something that happens on a relatively stock car.