Telling forged from cast?

Telling forged from cast?

Author
Discussion

funkihamsta

Original Poster:

1,261 posts

262 months

Wednesday 5th March 2003
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OK so how can you tell forged from cast when it comes to finished items like pistons etc..? For example would you be able to tell from just looking and touching that a crank was made from billet steel?

deltaf

1,384 posts

256 months

Wednesday 5th March 2003
quotequote all
Forged items like cranks and rods generally tend to have a wide flat dividing line down their centrelines.
It tends to look somewhat rough and about 1/4 inch in width.
Cast cranks and rods have the same dividing line, but its usually very much sharper in definition and only a couple of millimeters wide.

A billet crank is machined directly for a "billet" of chrome moly or similar steel, theres no dividing line on these as theyre machined.

Forged pistons are generally heavier in weight than an equivalent cast piston, with a closed surface.
Cast pistons can seem to look somewhat "grainy" in appearance, especially on the underside.
Pressure cast pistons are cheaper than forged and almost as strong.
It all depends on application and budget!

andytk

1,553 posts

265 months

Wednesday 5th March 2003
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Yeah but what about finished items.
ie. things that have been cast and then the surface has been machined to get to tolerance.

After all how the hell could you use a piston that was sand cast? It wouldn't have the required surface finish.

Andy

deltaf

1,384 posts

256 months

Wednesday 5th March 2003
quotequote all
Dont think theyre sand cast andy.
Ive never seen a machined cast iron rod yet.
The ones in my golf are machined from chrome moly so they dont have any marks on them.
Generally to improve the rods they remove the dividing line (known as casting flash) and polish the beams of the rod before shot peening them.
So theres not really any marks on a set thats been worked on like that.
Pistons especially forged ones have a very dense grain structure, the cast ones from underneath moreso, look grainy.

Boosted LS1

21,167 posts

259 months

Wednesday 5th March 2003
quotequote all

andytk said: Yeah but what about finished items.
ie. things that have been cast and then the surface has been machined to get to tolerance.

After all how the hell could you use a piston that was sand cast? It wouldn't have the required surface finish.

Andy


You can still tell. A forged piston will be very smooth on the inside and under the crown, you won't see any ridges or latticing under the crown which you would find on a cast item. The ridges are where the moulds are joined together (usually 2-3 piece) and the latticing stabilises everything during casting. A cast piston may have latticing on the outboard edges of the pin bosses. This area would be machined on a forged piston. In fact, just about everywhere will be machined on the outside of a forged piston. As for rods, I reckon they're all forged nowadays. With the cranks, if it's cast you will see a hairline parting line running the whole length of the crank if it's a casting. This line disappears where the machining of the journals has taken place but on the rough stuff it's clearly visible. If your still stuck you can phone me if you like to discuss it.

zefarelly

229 posts

256 months

Thursday 13th March 2003
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slightly different problem . . . .crankshaft, Ive just bought a 2nd hand crank which is apparently made from billet . . . .it has a very slight casting line on it but generally looks much better than standard cast iron . . . .it has 2731.E on one web . . . .its a 6 bolt Ford Kent type . . . .anyone know If Ive been having my plonker pulled? its definitely not standard, but what exactly it is I dont know. . . . .

anyway, it looks useable and it was cheap

Boosted LS1

21,167 posts

259 months

Thursday 13th March 2003
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Sounds like its a casting. A billet crank comes from a round length of steel. Like a tube but with the centre filled in. I have only seen a few billet cranks but can't recall there being a parting line or anything like it. Are you sure your descrption is good? Send me a pic if you can and I shall try to tell you what it is. The part number seems like the grinders number.

zefarelly

229 posts

256 months

Friday 14th March 2003
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I stripped the donor engine last night . . .it has the same crank in it, so Im now convinced its a cast iron 6 bolt pre crossflow crank (early MK2 Cortina). . .the donor has 125E lotus rods in too . . .which is nice