So finally fixed my noisy 445!

[machoneman]

Welcome Harley as I see your new. Please jump in as more is better!

Think of a piston skirt side as a tall letter T....or a quite short t where a stroker crank/rod requires a shorter t to prevent the pistons bottom edge from hitting the counterweight on the crank. I wonder if the pistons in use have a too short skirt for the combo in use, meaning the piston at BDC is rocking too much. Point is, the longest skirt (the tall T part) one can run is beneficial in minimizing rocking.

This btw is somewhat divorced from the idea of too much wall-to-piston clearance. Given ideal wall clearance for the piston design + the aluminum material its made from will minimize rocking. Yet, also having a nice long-tail T as  possible on the lower skirt in combination will give the best results.     

[My427stang]

Ross, would you mic those pistons laterally where that wave shows up?  It sure looks like they aren't round there.  I know the skirts are not round, but if you mic them in several places left to right laterally you may see a collapsed/flattened spot that causes the phenomenon.  What I am trying to suggest is twisting the piston in the micrometer to see if there is a low spot where the wave is located.   Joe-JDC

Joe, copy all, I will get some numbers tomorrow and post

This may be a dumb question but is the mark inside the bores on one side of the wall? Inner side on each left and right bank, vice versa, or on both sides of the bore? Just curious and not sure it makes a difference. I would agree it is likely in the piston like everyone else and like Joe D-C said, I would like to know the measurement around it to see if there is a variance/flat spot. I would not think short skirts would be the absolute cause. Definitely some contact going on...

Not a dumb question, that's part of the odd thing that tells me the piston is flopping around, they are on both the thrust and non thrust side.  The odd thing is how few miles on the engine, I expected clean cross hatch with just a path where it rode on the thrust side

Here is my hunch though, and it's based on nothing but a hunch.  These Probes are supposed to have a "low friction skirt"  There is nothing fancy about the skirt surface, so IMO, it either is low friction because it's smaller overall OR it's got more curve to it, both of which would reduce surface area and allow it to flop around more. 

I have nothing to compare it to, but lots and lots of 445s out there, most with 4032 pistons, but of course some with 2618 like these. Also, there are a ton of 482s, 488s, 489s, 462s, and they all run quiet with a 1.325 or 1.33 CH piston, so my hunch is this low friction design, however they got there, needed to be tighter than what they said OR somehow were cut wrong.  I haven't found anyone else using Probe pistons in a 445, which is a message too

Who really knows? I can't compare to much when the company is out of business.  I'd like to think I could either coat these or tighten the clearance in another block, but I am not sure I want to do it with this particular engine LOL

[plovett]

Be careful with using coatings to take up clearance.  I did that a long time ago and the coating flaked off partially.  I found that at disassembly, and the engine did not have a lot of miles.  I don't doubt some places are more competent at coating than others, but still...  I'd rather spend the money on new pistons. 

JMO,

paulie

[My427stang]

I am not saying instead, I am saying after this F-ing 445 is fixed, I may play with them because I cannot get myself to toss 500 dollars worth of pistons in the trash

ON EDIT:

Paulie, it's snowing and I am bored....

So, these pistons, 3.98 crank, 6.8 BBC rods and a shim head gasket....quarter inch less stroke and .003 clearance, it'd likely be a stable 9.5:1 truck motor if the skirts didn't hang out the bottom (OK, so it's a waste of money LOL, but like I said, snowing and bored)

[plovett]

I am not saying instead, I am saying after this F-ing 445 is fixed, I may play with them because I cannot get myself to toss 500 dollars worth of pistons in the trash

I hear ya.  I don't know the answer for sure, but you know that old saying about "throwing good money after bad"? 

JMO,

paulie

[Joe-JDC]

Sometimes throwing together a leftover parts engine runs the hardest, takes the most abuse, and defies logic.  Build a spare with tight .003" tolerance, and have fun.  Joe-JDC

[Heo]

 Knurl them 

[427Fastback]

I have a set of (circa 1989) Ross pistons in my 427.I don't know the alloy nor do I have a pic.They were spec'd for .007 clearance when new but like a lot of people I had trouble with the engine when new.They are now up around .0085-.009 clearance.You can hear them when the engine is cold but by 160 deg they all go quiet.Things have certainly changed with good pistons now running as tight as .003

Ken from Ross racing used to visit a shop I worked at up here in B.C..He swore I would never break one and to not worry about it..I don't worry about them but they don't make strange intermittent noise...

I have CSI'd a engine before.It can be very frustrating but at the same time you sure learn a lot..

[Katz427]

I guess I'll throw out an experience of mine. I had several small 4 cylinder engines disassembled due to noise problem. I was stumped to see wear on both the major and minor thrust of the pistons. Fairly low hours of use. I asked the lead engineer to take a look, a gent who had worked for Cadillac.
He came back with the piston pin bore was not exactly machined. He told me when you see wear on both thrust face's it is a piston pin bore that is slightly off true axis.
Not exactly sure what you have in your pictures, so I'll throw that out for some thought.

[Katz427]

I looked again at the last picture of the cylinder wall and you see the wider rub area and then as one travels down the cylinder it narrows until at bdc it flares a bit. The idea of what was happening in the engines I had was that with the pin slightly off axis that when maximum combustion force was on the piston, it would bind on the piston pin. Once this happened the damage was done and the wear on the thrust surfaces accelerated. In this case the pistons were sent back to the engine manufacturer and they got their piston supplier involved.
Maybe or maybe not. Just a case of odd experience  with piston/ engine noise.

[wowens]

Knurl them 

That brings back memories. Old school, if it rattles, knurl em. Bottom 1/8" or so of piston skirt looks to me like piston a piston issue. I think JDC is dead on.

[machoneman]

Yeah, I thought of knurling the skirts too to 'save' those pistons but didn't since say so it's not a typical machining operation found in a hi-po engine. But, it is a long used engine rebuilding practice and would work better I think than a coating.

[Lenz]

Took this photo as a visual aid to help in explaining to a friend why the old school thought that a longer stroke = slower revs due to several factors that were likely true in the 70's.  The purpose was to provide a visual picture of the weight reduction due to reduced mass versus the a "stock" 390 assembly.  These are Mahle, coated and zero noise in a 445. 

[machoneman]

Good pic that shows the big difference in the skirt length.

Obviously a stroker piston needs a shorter skirt but...I've wondered if the makers supply a shorter-than-necessary skirt to accommodate a wide range of stroker cranks. Put another way, perhaps they go a tad too far than needed.  Since I do think most makers, like Probe and others, make stroker pistons on a as-ordered basis. Maybe they could make longer skirt pistons but don't to avoid some combos interference with specific stroker crank counterweight designs. Just a guess.....

[WConley]

Curious - Why zero pin offset for this shorter skirt stroker street engine?  I think you've discovered how hard it is to stabilize a forged piston using just clearance.   

Pin offset is your friend on the street    It doesn't cost much HP and it gives you more arrows in your quiver to set up a happy engine.  I learned this from Don Sullivan at Ford when he was brought in to quiet down the noisy pilot-build 4.6 Modulars.