View Full Version : Need opinions on bottom end knock

02-09-2010, 10:46 AM
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.



Quick Cat 89
02-09-2010, 11:54 AM
Hi friend. I wouldn't do that if it was me not saying in a mean way.. if you was to do that and your crank is needing turned or a good polish all it would do is damage your new bearings in a matter of time. I would go ahead and pull it and have it polished or turned, depending on how rough the crank looks. Thats just what i would do. i hope this helps and good luck with whatever you decide.

02-09-2010, 12:29 PM

From my experience, I have never had a rod knock only when cold. Generally that is when they are the quietest, as the oil viscosity is highest and the thicker oil layer will better cushion the play in the bearing. Your description better fits piston slap or a wrist pin. In either case as the piston warms and expands the clearance is reduced and the noise dissipates.

BTW my engine with your old heads is still sitting on the stand. Life keeps getting in the way of the fun projects.


02-09-2010, 12:54 PM

I also had that thought.. It sounds like a diesel when its really cold, but is nearly silent once warmed up. Didn't notice it when i picked the car up, as it was in the 50's.. But this morning it was about 9 degrees out and the knock is bad. I should get a sound clip of it.

Its definitly a '94-95 motor, blue injectors, new style supercharger, new style valve covers, oil pan without the sensor, etc.. But the block is painted gray, so I wonder if it is a reman'd engine that wasn't properly built?

I understand about life getting in the way. I bought this '94 SC with a bad trans to fix it and flip it, but as life happens, my girlfriend needed a car, and fell in love with this, so we now have 2 SC's..


02-09-2010, 01:28 PM
You sure it's not just a sticky lifter? That is a common problem (if you can even call it a problem) on these cars when they get higher mileage.

02-09-2010, 09:27 PM
From what you're describing, its piston knock. either the cylinder is out of round, or a wrist pin is worn.

02-09-2010, 10:24 PM
From what you're describing, its piston knock. either the cylinder is out of round, or a wrist pin is worn.

Piston slap is my vote as well....Too much clearance between the Piston and cylinder. When the pistons heat up they expand and reduce the clearance. Common problem in Early LS based small block chevy's as well....

02-11-2010, 06:58 PM

Working out a work related issue got me thinking about your knock. You mentioned that when it was 50F it was quiet on a cold start and now that it is in the single digits it knocks badly. While I was doing a stress analysis on an aluminum ceramic assembly for work got me thinking about the expansion rates of aluminum (your pistons) and the iron block. The Coefficient of Thermal Expansion (CTE) for AL averages about 23 parts per million (ppm) per degree C. Your temperature differential is 5F (-15C) to 50F (10C) or 25C. So 25 degrees multiplied by 23 ppm expansion gives a total differential from 5F to 50F of 575 parts per million. With the bore of the 3.8 V-6 at 3.81, 575 ppm is .000575, then multiplying the bore of 3810 mils by .000575 for the max piston diameter change of 2.19 mils (.00219). Now the iron block is shrinking too but the CTE of cast iron is 10.5 ppm per degree C. The block is shrinking 263 ppm over the 25C temperature change, which is .000263. The reduction is bore diameter of the block is 1.002 mils. The differential between bock shrinkage and piston shrinkage is a max of 1.19 mils (.00119). Now almost all modern pistons have a steel insert cast into the piston body to help control the piston shape change due to expansion, so the actual amount the piston shrinks may be less. The question now is, will an additional .001 clearance between the piston and the block from 5F to 50F be sufficient to create the knocking you are hearing.

Applying this same logic to the rod bearing, the fact the rods and crank are materials with similar CTE, then the clearance of the rod bearings should change very minimally over the temperature change you are talking about. The bearings cross section thickness is thin compared to the diameter of rod journal, so I dont see where that could be a contributing factor.

Could you be experiencing a cold oiling issue?

Sticking valve when its cold?


02-11-2010, 10:20 PM

02-12-2010, 02:07 AM
I vote for noisy lifters.

XR7 Dave
02-12-2010, 09:03 AM
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.


JeramieA 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. :)

XR7 Dave
02-12-2010, 09:07 AM
Oh, and forgot to mention, if they drove the car much with the wrong MAF on it, then it probably ran pretty dagone lean and detonated like a beeeotch. ;)

02-12-2010, 11:26 AM
Thanks Dave.. I think there is more damage than we think.. Crazy thing is other than the noise, the damn thing runs strong! I did replace the driver's side engine mount as it had been hitting the torque limiter, and it made the noise before the trans/converter swap, so its not that..

It also runs smooth, in fact, its the smoothest (vibration wise) 3.8L I've ever personally witnessed.

I ruled out a stuck lifter.. I very carefully listened with a mechanic's stethescope around each valve cover while it was running and it all sounds normal.

The rythm is definitly bottom end. The previous owner did admit they were not good about oil changes. She went to go look when the last time it was changed, and the sticker was completely faded from the sun. :eek:

I think I am going to let my girlfriend just drive this until we have a problem, I just can't put any more $$ into this car right now.

Thanks everyone for the input, I appreciate your time.


02-12-2010, 11:55 AM
I understand not haveing/wanting to spend the money but in the long run it could be cheaper to tear it down now.

02-12-2010, 12:30 PM
CHECK YOUR OIL FOR GLITTER. ALso if you experience cold winters try a lighter weight oil in the winter

02-12-2010, 01:15 PM
Double Deeez, is a bent rod noticable by a compression test?

Edit: Nevermind, I searched and seems like a cylinder with a bent rod should read noticably lower.


02-12-2010, 01:58 PM
Holy smokes that is some good information. Thanks Dave! I've added 'new short block' to my list for next year. :eek::eek::eek: