Cast or Forged...

[Wayne 962]

I think this is forged:

http://www.pelicanparts.com/swapmeet_pics/Pomona_June_02/HTML/Page-156.htm

It's early 911S:

I think that this is cast:

http://www.pelicanparts.com/swapmeet_pics/Pomona_June_02/HTML/Page-150.htm

It's supposedly a 906 piston, although it's slightly different than the one in Bruce's book...

Any thoughts? I have a $50 bet over this...

-Wayne

[pwd72s]

I have no desire to settle your bet. But..hyereutectic in the 2nd pic? Very possible.

[Early_S_Man]

Nope ... both are forged, slipper-skirt designs! Not early on #1, though ... none other than RS 2.7 piston! Mine have identical 90R27 marking under the crown ==> 90 mm!

What was the diameter on #2? Could be any of the early-mid racing pistons ... lots of odd sizes to choose from! An 85 mm, for example could be 908 or early 4.5 917!!!

[Wayne 962]

Okay Warren, if both are forged, then how do you get the material out of the center of the piston with the forging process? Everyone I have taken it to says it was cast, as they see the parting lines, and the rougher surface in the bottom of the piston.

To forge a part, you need a straight shot down with a stamp. These have an undercut, which makes the traditional process of forging impossible. (the same process that they use to make rods).

However, there are processes, as I recently learned, that are really pressure casting, yet they are called forged by the piston manufacturers. Perhaps these are pressure cast.

They are not forged in the traditional manner that rods and Fuchs alloys are forged...

I still need some definitive proof though either way. If you have any old, books that will tell, I'd love to know. So far, this has sparked a big debate...

-Wayne

[5axis]

How about a multi piece forging that is electron beam welded.* IMSA 962* pg 72 BA's book.* Oil cooling in the dome.*

Wayne undercuts may be possible with a collapsing core system. You would see PL's where the cores meet.

Very cool. probably very expensive
david 89 turbo cab

[Early_S_Man]

Nice try, David, but Wayne is right about the undercut pin boss and the roughness. I didn't notice the roughness in my browser ... it wasn't until I saved pics 149-57 and enlarged them that I noticed the roughness, and I missed the undercut at quick glances through the sequence. Cast it IS!

[5axis]

Agreed. Cast. I followed your idea and was able to zoom in a bit.
The splotchyness looks like what we would have attributed to too much moisture in the sand. I worked as a pattern maker at a foundry to pay my way though school. Being the new guy meant I was mr. sample. The furnas and pour areas are scary and hot. The senior pattern makers did not like to leave the air conditioned tool room. (when hell freezes over or the melt deck is under 100)
I remember some really big pistons for CAT. I got to cut them up to evaluate gating problems.

Funny all that work for a education and I am still a fancy machinist.
david

[J P Stein]

Waynes evualation is spot on.
Disregarding a welded structure, it's nearly impossible to create a forging with the internal shape of pic # 2......draft angles are necessary to remove the die from the piece. Pic #1 has em', pic #2 doesn't. Note the excess material above the pin boss of #1 vs the lack of same in #2.

If the identity of the peices are correct, this is 60/70s technology.
One would have to use 3 peices and weld them together to create the internal shape of pic#2...not likely.

I suppose it would be *possible* to create a collapsing core forging die, but why? It's much easier to remove excess material by machining.

The company I work for goes to great lengths NOT to use forgings. The lead time for a forging die is now about 3 years, I'm told. 5 axis CNC mills now work on "forged blocks" of 7050. There has to be a trade off in ultimate strength here, but I'm assuming the engineering wonks have their sh.....ah.. stuff togehter

Easy 50 bucks, eh?

[Wayne 962]

All excellent comments. I can't still prove it though and collect my $50.

Sorry, I also have more pics on this page:

http://www.pelicanparts.com/swapmeet_pics/Pomona_June_02/Page1.htm

Let me throw another monkey wrench into the fray here. Here is a photo from page 147 of Bruce Anderson's 911 Performance Handbook:



Taking a closer look at:

- The 962 piston. This one would appear to have a forged profile (no undercuts), however, it has the hollow ring through the inside that is used to get oil in to cool. I'm not sure how you would forge this piston with the inside hollow ring

- The short-stroke 2.5L engine appears to have undercuts under where the piston pin bosses mount. This would be cast?

- The 2.8L RSR piston appears to have no undercuts, except for that center hole section. This piston looks forged to me from the profile, however, how did they get that section hollowed out? From the pattern of the material, I'd say that it was machined out - it looks like a flycutter could get in there and make that pattern.

- The 906 piston looks clearly cast to me. You can't easily machine that piston in that pattern.

Okay, one of my experts on the book is telling me that all the cast pistons had that steel ring cast into them. Obviously this is not true.

Any more thoughts on this?

-Wayne

[Wayne 962]

Okay, just saw the bit about the electron-beam welding on the 962 piston. But I don't think that they had that technology for the 906 piston...

-Wayne

[J P Stein]

The pistons from my 2.4L "T" motor had steel imbedded in them.
As I recall, it was visible around the pin bosses.

They are the ONLY pistions I have ever seen built in such a manner.......it's a fair bet that they were cast.
Heavy SOBs, too.