How big of a differance between forged and cast is the thermal heat expansion? Though i heard somewhere that forged pistons expand a tad more and should have a bit bigger bore to allow for this?

TF

_________________
Even in moderation i am excessive!

I cannot answer your question directly, but can shed a little light on it. They say that forged pistons are a little noisy when cold. But mine are not, that I can hear. The rest of the car is so loud, maybe it is there and I cannot tell.

My engine is .030 over and I have Federal Mogul forged pistons for .030 over. I never measured them. I simply put them in with the recommended rings, and forgot about it. They have never been a problem. I am guessing I have 20K on this motor now and it is tight. It still feels and sound new. When I turn off the ignition it stops instantly with no coast down at all. I run Mobil-1 oil.

I hope that helps some. Your car is beautiful. What year is it. I would like to see a full photo of it.

Sam

_________________

it's a 75 not a real aar car...not that they made any...



Car ran well till it ate the cam gear....it'll have more top end when i'm done..

TF

_________________
Even in moderation i am excessive!

Nerd alert:

One would have to know the composition of said pistons to know how much they expand & contract due to thermal conditions. ME’s call this property of materials; “Coefficient of Thermal Expansionâ€￾. Here are three common materials found in engines, and their Coefficient of Thermal Expansion expressed as x (10 to the -6 per Degrees F), in other words 1/1,000,000…

Steel = 6.5
Aluminum = 11.9-13.3
Magnesium = 14.5

So for each degree change, steel expands roughly half that of aluminum, and a little less than half of magnesium. Forged substance “Aâ€￾ or cast substance “Aâ€￾ will expand & contract pretty much at the same rate, because it is the same stuff.

Hopefully I have remembered the following correctly, its been a looong time… Someone please correct this if wrong:

I have no clue as to operational temperature of a piston so I pulled 600 degrees F out of my hat.

Lets say a 4â€￾ dia. steel piston is heated 600 degrees, how much would its dia. expand. (6.5x4â€￾x600*)/1,000,000 = 0.0156â€￾

Now a 4â€￾ aluminum piston heated to 600*…
(12x4â€￾x600*)/1,000,000 = 0.0288â€￾ expansion in dia.

16 & 29 thousands of an inch seems like a lot, I may have a decimal point problem, or 600* is way off….

My mind numbing point; the aluminum piston expanded about twice the distance as the steel piston. This thermal expansion is true for length of a piston as well. The material above the wrist pin and connecting rod will elongate once heated, fractionally changing combustion chamber volume as engine warms to operating temperature.

If you are comparing a ferric based material with same in a cast iron block differences in size at temperature will be less than with cast iron block and aluminum pistons once heated. I suspect that designers of aluminum pistons allow for this difference in expansion of dissimilar materials.

Like Sam said; I built it and forgot about it. Because some engineer was good with his slide rule back at the piston company…Sam could do this.

_________________
67' Dart GT Convertible; the old Chrysler Corp.
82' LeBaron Convertible; the new Chrysler Corp
07' 300 C AWD; Now by Fiat, the old new Chrysler LLC

Here's the thing. If the pistons are +.040" then the bore should be 3.440". The correct amount of clearance should be in the piston if it's cast or forged, but the piston size should still be checked and the bores honed to match. I would expect no less than .003" clearance. It's also critical to ring seal that the bores are straight. The rings don't like to move in and out as they travel up and down which is why we have bore taper limits. Who's doing the honing?

_________________
Joshua

No one uses "slide rules" any more... the pocket calculator and then the PC made them obsolete...

Anyway.... the simple answer... the piston material determines the amount of clearance needed and for the most part, the piston manufactures adjust the piston size to allow for the needed clearence.
You still need to measure and 'do the math'... that is why I always have the pistons I want to use, prior to having the block bored / honed... we measure and "fit" each piston to the cylinder it will run in.

For basic cast pistons, with steel expansion struts, you can run those real tight, with as little as .0005 clearance. (I target .001 to .0015)

No strut pistons & Hypereutectic, usually no struts) target closer to .002 - .0025

Forged, .003 to .004 depending on final size and intended use.

For me, a little loose with some noise and / or less oil control is better then "smeering' or seizing a piston... so I run them on the loose side.

Look at the info. sent with your pistons or call the manufacturer if in doubt
DD

Funny, I have my Dad's slide rule on my desk...now where did I put that darned HP calculator that uses RPN.....



-D.Idiot

I forgot to add that I gave the new pistons to the guy machining the block. Thanks Doug.

Sam

_________________

I own three of them and do still know how to use them

Richard

_________________
Part of Tyrde-Browne Racing

Still use my 30yr old HP12C.......
the batteries last forever.

_________________
Ed
64 Valiant 225 / 904 / 42:1 manual steering / 9" drum brakes

In all of these "small" numbers, how much do a.) measurement error (human) and b.) gauge error (instrument) come into play?

Very little error in a calibrated micrometer. It's pretty easy for a man with just a little training to measure to .0001".

_________________
Joshua

I am.
The bore's are really straight and cleaned up nice, no taper just a nice 45degree cross hatch. No lip at the top of the bore to ream out, all in all the engine was in very good condition for being raced and driven all over the west coast for five years!
Only reason for the question is the other block i have the pistons are really loose and when i put the new pistons into my engine from the duster they are pretty tight in the bore. The other block has issues so it may be no wonder they are loose in that block, I'm useing it for the 104 over bore.
I'll do my math to make sure it's good to go See you at a track soon..

TF

_________________
Even in moderation i am excessive!

It is very comforting to know some of you out there in Nerd Land still have multiple slide rules at hand. I have three within reach of this PC… They kind of go with my 67 Dart, wooden cased AM-FM Panasonic table radio purchased in 67, and some other old crap from those days residing The Man Cave. Hey

Twist the Doc’s tail Department:
[quote]No one uses "slide rules" any more... the pocket calculator and then the PC made them obsolete... [/quote]

No Doc, really, pocket calculators, and PC’s??? Naw, oh go on, your pulling my leg… It’s a yoke, right? Wink-wink.

Hey, the slide rule thing was just a literary device to conjure up an image of a horn rimed, bespectacled, pocket protected white shirted dude hunched over a drafting table of yore, nit-picking the details.

The young tatted up guy with studs, sporting cut-offs, balancing a laptop while swelling a Monster drink, making a few calculations with pods screwed into his ears image didn’t make it for me… As well as the “Budweiser Engineeringâ€￾ redneck, plying his magic to a hapless racing engine out in the barn “get’en her doneâ€￾…LOL I’m sure these other two dudes will slip into my writings at some point… Stay tuned.

I don’t know about the rest of you out there, but I laid down my Picketts in 1972 when Fortran and TI took over my calculating tasks. In fact the last time I used a slide rule, and I remember it well, was to take a State of Maine Land Surveyor’s license exam in 1972. Yeah, I passed it.

Bill

_________________
67' Dart GT Convertible; the old Chrysler Corp.
82' LeBaron Convertible; the new Chrysler Corp
07' 300 C AWD; Now by Fiat, the old new Chrysler LLC