Hello Ted,

I hesitate to make a reply since you aimed your inquiry towards those people who have rebuilt red blocks or one that have extensive knowledge and experience with them.
I consider myself as not having those qualities to offer as I was never an automotive machinist even though I'm familiar with the machining principles used.

Since I have six of the engines in my possession (one on a stand) ranging in years from the B21 of '78 to the present '92 I will give you observation and hope it helps.
I too, have notice an increase of piston noise starting with the 1986 after about 250K where as my '91&'92 have slightly more noise with less mileage.

I want to blame the increase with the B230 "design changers." It is my belief that, Somewhere, in the late eighties they started trying to lighten the internal components for a quicker response for revving up.
It is my understanding they narrowed the width of the rods on the big end or the whole thing for that matter. I don't know about the wrist pin end in the piston.

The next thing they did, like many other manufacturers, is they reduced the piston skirt length to reduce weight.
From what I knew or know about the purpose of the piston skirt it is necessary to have one for piston and rod support in the cylinder.
The piston is actually a bearing support for the rods lateral or radial thrust down the cylinder on the power stroke.
The rings seal and support the upper part but the skirt also guides and aligns.
The piston or its skirt is actually made "out of round" in relation to the top of the piston by a few thousandths of an inch at a right angle to the wrist pin.
The skirt is made to fit the cylinder with minimal clearance when new.
They get away with this because tha area expands at a slower rate than the rest of the piston due to it being farther away from the combustion zone. So now, when cold, it does not rattle or clack like a soup can when it's being pushed down.

If a piston runs colder than the others due to consistent misfires or mixture issues for various reasons, it will wear considerable faster.

In this cars case, with low milage, it is likely that it had lots of around town "cold" short runs.
I suspect it's possible that you are hearing two Pistons at the same time alternating together since there are always two up and two down. That would make for the greater noise if they were real bad.

What I believe happens is the piston beats or wear themselves round over time because they do not have that length of oil and fit support.
The nineties cars seem to have this issue a little worse than earlier engines due to "upping the ante."
In some racing circles this is done routinely because they only care about a few hundred miles of going in stupid circles.
So what we gain in quickness, we lose in longevity or quietness. The battle of the "Q's" for easier thinking along with the "quest" for greater sales against competitors. All kinds games were at play here.

I have always been told that piston slap goes away with warmth and piston swell. On my air cooled motorcycles I alway warmed the cylinders until I felt some healthy heat from the fins.
A water cooled engine has less piston clearance than air cooled engines because they can, so for longevity, it was a good practice. Never had a piston slap sound from them.


A rod noise begins just as small and sounds similar, that is, until it can't take it anymore then gets louder and sooner rather than later it can go away permanently.
60 pounds sounds like a lot for an idle pressure to me, but what do I know, I have never check one of my engines as I should have.
There might be an oil flow blockage to rods? That would come on suddenly with bad maintenance.
I believe you are saying some of the same things with your experiences.

Have you tried killing any cylinders, on purpose, with a rod tipped stethoscope on the blocks exhaust side?
I have also tried listening to the pan for a bad rod. I'm not sure if it's possible to be conclusive? I stick with the slight piston slap idea and drive them.

Now that I got that out there. You brought up spark knock and if "variances" can cause these sounds, I assume at idle mostly?

In this area we also do not know what kind of gas is used by her but the knock sensor is suppose to take care of that. If is bad, you may be on to something.
The knock sensor works off the CPS and we know they can act weird at times.
A really rough idling engine, that's almost trying to die, sure can knock loud but it's a really clack when it's the timing.


These systems can and do adjust very quickly and mask what they are doing to keep it running.
With that said, I see your point of thought very quickly.
I also understand the point of the other poster, that thing are not being said, but then, maybe it is not the right questions being asked from her.

Did she bring the car, I assume new to you, after this just started or has this knock been going on for quite sometime? Is she a regular customer?

I have found out, over my time of fixing things, is that the owner that uses the equipment that usually has experienced changes. They have not thought about them in awhile and get use to them.
If questioned correctly, they might have some clues that they don't know they have.

Ask how she uses the car from day to day and who has cared for the car or drove it the most?
Your best source for reasoning some of this stuff out might be standing right in front of you.
You know it's about being detective. (:-)

I hope this was not guesswork and definitely not voodoo on this night! (:-)

Phil