I have a new '94 SC that I bought and put a tranny in for my girlfriend. The car runs great but has a noticable rod knock when its cold. Goes away when its warmed up.
If I were to just pull the pan and toss in a set of new standard bearings, what are the chances it would cure the problem?
I know the proper way would be to have the crank checked by a shop, and at the least polished or maybe even reground.. But trying to save a buck.
Thoughts?
Jeramie
A bearing will never allow a rod to knock when it is cold unless it simply isn't getting any oil. This is highly unlikely since it generally takes about 50rpm to pump full oil pressure on a cold SC motor with a 94 (high volume) oil pump. The only time I've ever seen low oil pressure on a cold engine was when I got a stuck relief valve, but that got worse when the engine was warm. If you think you have a rod bearing issue there are 2 easy steps to find out.
First, install a real oil pressure gauge, even if just for temporary observation. You should see the gauge nail the pressure relief valve immediately on start up. This can be anywhere from 60-80psi, but it should hit max within about 2 seconds of start up and hold steady while idling (cold). When hot it will drop to about 25-40psi depending on wear and oil viscosity.
Second, after a full warm up cycle, change the oil and filter. During initial start up if you hear the rods knock like crazy and then go quiet as soon as the filter fills up, then you have bad rod bearings. Even on a dry start there should not be enough play to cause knocking on a dry start.
Rod knock doesn't sound anything like lifter noises. A rod knock is not really a rod knock anyway (the rods don't have anything to hit). A rod knock is actually the sound of the piston hitting the head. Imagine the sound that will make - a definite "knock". Lifters on the other hand make a clatter noise. One click as slack is taken up in the valvetrain and one "tickle" sound as it bounces when the valve closes. The sound you hear is the rocker arm tapping against the valve tip. A totally different sound compared to a piston hitting the head. Remember, knock vs. clatter, or "tickle". One syllable sound vs. a two syllable sound.
While all SC's have worn rod bearings after 70K or so miles, the knock may not be due to bad bearings. If it passes the oil change test then likely there are other issues. If a rod bearing is bad enough to cause knocking, you have real problems. Typical rod bearings have .001-.003" vertical clearance. Piston to head clearance is typically about .040". For a rod bearing to be bad enough to cause knocking then things would have to be pretty darn ugly in there.
A more likely cause of piston noises is a sticking pin, excessive cylinder bore wear, or carbon. Carbon build up on the top of the pistons will cause the .040" clearance to be dramatically reduced. In fact I've seen .040" of carbon on the tops of many pistons. This carbon is generally due to oil consumption but if the car is puttered around, it can be due to normal use. Many engines leave the piston .020" or further down the hole (OE specs) but the SC engine is built to tighter specs so generally, and especially on the 94's, deck clearance on the SC engine is .00". This means that on a typical 5.0 you might see .060" (.040" gasket + .020" down the hole) clearance, whereas on an SC you will typically never have more than .040" gasket thickness for clearance. Therefore SC engines are more prone to getting a carbon knock. Carbon knocks are interesting because when cold carbon is as hard as aluminum but when it gets hot it softens. This is one reason that a carbon knock will soften or go away once the engine warms up.
There are other reasons why a piston might knock. Head gasket failures are a common cause of piston knock. Many times the fire ring on the OE gasket will actually push out into the cylinder. When this happens the piston will literally hammer the fire ring flat in that area. Because both materials are soft, typically no harm is done to the mechanical parts, but this can be a source of noise. It doesn't typically go away when warm though.
Another cause of piston noise is rocking due to clearance. The SC uses a hyperutectic piston which doesn't require a lot of clearance, but then on the other hand the heat of a supercharged engine dictates clearance for expansion so the piston side clearance spec on the SC is much greater than it is on the NA versions. Typically an NA motor might have .0008" to .0015" piston to wall clearance. An SC engine typically has .0015-.003" clearance which is to say, borderline for a hyperutectic piston. SC engines tend to exaggerate that clearance due to owner ignorance.
If an SC engine is allowed to "rattle" due to bad gas (little old lady owners are the worst) then the resulting detonation will cause the rings to "slam" out into the cylinder walls at the very top of travel. This causes severe bell mouthing of the cylinder walls at the top which in turn makes room for the piston to "rock" even more than it would normally. I've seen the cylinder bores bellmouth out an additional .015" at the top because of this. Now the piston can really rock in the bore and not only make noise rattling against the side of the bore, but the tilted angle of the piston can allow part of it to hit the head even though the middle where the pin is, is still where it belongs. (!)
And finally, in concert with all the things mentioned above, if you have a slightly sticking pin, then the above conditions will cause the piston to again hit the head. A stuck pin generally loosens up with heat because the piston which is aluminum grows when warm and releases it's grip on the pin. This condition is not as common on the 94's because of the full floating pins, but if the rings haven't been sealing well due to piston rock, for example, then there may be a lot of carbon and varnish build up on the sides of the pistons and within the pin bore areas. The pins are oiled partially from the oil scraped off the cylinder walls and well - if that is highly contaminated from blow-by, it can get sticky. But as I said, this is not common on a 94 with full floating pins. In fact I think that is one reason why they went to a full floating pin is in an attempt to reduce cold knock that is more common on the earlier model motors.
And finally, a last possibility for a knocking piston is physical damage. I've taken apart perfectly good running motors and found bent connecting rods. This will put a bind on the piston/pin so that it doesn't rotate properly and can cause the assembly to knock. If this is the case then the term "ticking time bomb" really does apply! I've seen pistons fall .020" below deck due to an "S" shaped connecting rod in the past all the while the engine running "perfectly".
So having said all that, any "guess" is just a guess and you could end up doing a lot of "exploring" to find the root cause of the knock if you chase only one aspect at a time. If you really want to solve it, the best thing would be to pull the motor out and remove both the oil pan and heads. Then you can check all the bearings, check for stuck pins, check for bent rods, measure the bellmouth of the cylinders, clean the carbon, and replace the head gaskets. This will give you the longest lasting results and will tell you the real story. If the cylinders have a lot of bellmouthing, a rebuild is your best alternative.
Oh, and don't overlook anything stupid like broken motor mounts or a loose plug wire, but I'm sure you already did all that.