From: Dave Baker
Newsgroups: rec.crafts.metalworking
Subject: Re: Gravity die casting-problems
Date: 3 May 1999 15:18:18 GMT

>Brian Fairey wrote:
>
>> I am having problems die casting some 1" dia pistons I get a big
>> shrinkage where the piston crown is. This is illustrated at
>> http://www.sentex.net/~bfairey/piston2.jpg
>> The lh one is the original casting and the rh one is the present one
>> notice the big shrinkage where the piston crown should be.
>> The melt ,alloy 390 is at 690degC and the die at 700degF.
>> the core is vented.
>> Any suggestions.
>> Brian. Ont. Canada.


We have used a local foundry to cast our own full size aluminium pistons in the
past and made the tooling in house. High silicon piston alloys need a lot of
runner volume to avoid shrinkage and inclusions. We cast the pistons crown down
in the end with the runners feeding in to the side of the piston boss area and
got excellent results. A 1 inch piston should be easy in comparison. To be
honest it might be easier to just machine something that small from solid bar.
That is the route we are taking now with other piston designs. Your alloy
supplier ought to be able to advise you on specs for forged bar suitable for
pistons.

If you carry on casting then I suggest making the runner volume at least as big
as the piston itself and then band saw off the excess.


Dave Baker at Puma Race Engines (London - England)  - specialist flow
development and engine work. .

From: Pete Albrecht
Newsgroups: rec.crafts.metalworking
Subject: Re: Aluminum Casting Problem
Date: 30 Mar 1998 20:32:55 GMT

>>You shouldn't expect to cast aluminum around a steel core.
>>The aluminum has got to contract, and the steel won't let it.

It's done all the time. Many automotive pistons have steel skirts inside to
control thermal expansion.

Pete

From: "Ed Huntress" <mike234@bellatlantic.net>
Newsgroups: rec.crafts.metalworking
Subject: Re: Motorcycle piston/ring question
Date: Mon, 19 Feb 2001 22:28:45 GMT

>>Pistons are often designed to be not exactly round.   This accounts for
the varying rates of thermal expansion across the piston.  (The casting is
thicker where the wrist pin connects, thinner in the opposite axis.)   This
is almost certainly true in a modern motorcycle engine.<<

Just for background info, the out-of-roundness usually starts a bit below
the ring grooves. In modern car engines the shape below the rings can be
quite complex, starting as one modified ellipse and transitioning to another
one altogether, and then finishing as a third. I have some exaggerated 3D
CAD drawings around somewhere that I made from tables of Excel data that was
supplied by Ford to Federal Mogul, for making a new production piston, and
they look like squished barrels. But the ring grooves, as far as I know, are
cut cylindrical.

Ed Huntress