A friend of mine has just fitted JW Forged Racing pistons to his rebuilt STi7. However there is a strange knocking that is in time with the beat of the engine.

The noise has been there from the first time that the engine was started after the rebuild and 1000 km later it is still there!!!

It sounds like "tack...tack...tack...tack...tack" in time with the pulse of the enging about 12 times every 10 seconds or around 720 times per minute, which is around the idle speed.

It does not get louder with an increase in rpm only more frequent.

The sound is not like that of the piston slap that was on this site a few days ago, so I am not sure what it is.

The engine was rebuilt due to detonation which damaged the #3 piston. The block did not need to be re-bored, only polished, and the crank and con-rods are all fine.

A complete set of new seal and bearings were used in the rebuild.

Has anyone rebuilt one of these engines with forged pistons and experienced this noise.

Please help as my friend has not yet paid for the rebuild as he is not happy with the noise.

Any ideas anyone?

 

Valve clearances? Sticking hydraulic tappet?
Alcazar

 

These engines have solid valve lifters. Any changes made to the cylinder heads or were they left alone?

My best guess would be piston slap, or very noise injectors, but the latter is very unlikely. Odd Q: alternator is not making the noise?

 

Sorry, but another thought crossed my mind: Maybe the camshaft stays have not been tightened correctly (they must be removed in order to losen and tighten the head bolts) and that can result in free play on the cam(s) and the resulting noise...

 

the clue is "forged racing pistons" I would lay money on piston slap, although it should go away as they reach working temp.
I'm not sure if Subaru pistons have offset pins (a known method of reducing slap) if so let's hope they haven't fitted one piston the wrong way round!
JohnD

 

Piston to bore clearance will/should be much greater than std, to that effect the noise that you hear is probably just slap, its not a problem apart from the noise unless it is excessive, he should check with the builder what clearances were set, each bore and piston should have been matched together, the noise will gradually get louder as time /mileage goes on since the pistons and bores bed in increasing the clearance. Pistons should be installed in an orientation, cylinder bores would also probably be both A and B grade across the four. Normal practice is to bore the cylinder limners to suit the pistons.

bob

 

Thank you everyone for your help and to Bob Rawle for getting it right.

We called JW Racing in California and they say that the sound is piston slap. This they say is normal as the forged pistons are made with a greater piston to wall clearance than the stock ones to allow for expansion under extreme heat and presure.

The car absolutely flies as we also fitted a 4 into 1 header with a 3" Zeigen fireball to a catless downpipe. All purchased from Vivid Racing in California.

Add to this a Greddy profec E controller and we have a hell of a car.

With only the forged pistons as beefed up internals what would be a safe boost to run as maximum? Presently it is set to 17.5 psi as the low boost. Still to set the high and overtake boost.

The car is fitted with a AFR and Knocklink sensor from Link Engineering to ensure safety.

Thanks