A question about "sensible" crankshaft turning

[Steve Antonucci]

Often, I find used big block forged crankshafts that have been turned
.010/.010 -or- .020/.020 -or- even .030/.030 and the question I have is over
their actual stability in a SHP engine. I am referring to the '7115 crankshaft
in particular, as the '6223's are a dime-a-dozen these days. In any event, can
any of you share your experience(s) in building an SHP engine ( L-88 or similar
performance ) using one of these "turned" cranks? What I actually would like
to know is, does the use of a crank with reduced journal diameters ( mains &
rods ) promote bearing failure faster than normal due to the increase in the
bearing wall thickness?

Having asked that, what would be an acceptable undersize crankshaft to limit
myself to? Getting a '7115 crankshaft that would be deteremental to the life
of a restored engine seems foolish, doesn't it.

Thanks,
Steve

[Clem Z.]

Re: A question about "sensible" crankshaft turning

we ground SBC cranks from 2.100 rod journal diameter to 2.000 to stroke the crank from 3.480 to 3.562 and used them in race engines with no failures. BBC cranks should be no problem .030 under

[Steve Antonucci]

Re: A question about "sensible" crankshaft turning

Clem,

If this is the case, and I'm not challenging your position, then why do cranks
that are .020/.020 for instance sell for so much cheaper than those that are
std./std. ?

Is it the perception of being less than normal diameters that drive the price?

Steve

[Jack H.]

It's a remaining life issue that affect price... *NM*

[Clem Z.]

Re: A question about "sensible" crankshaft turning

i would guess this would be reason because as far as street engines go you could go .060 under and still be safe as long as the crank passed magnaflux. the case may be the std/std ones are new. even std/std ones should be magnafluxed if they are used.

[Duke W.]

Re: A question about "sensible" crankshaft turning

All the forged SHP/HD crankshafts have a surface hardening treatment called "Tufftride", which increases durability by helping prevent the generation and propagation of surface cracks that can evenutually cause the crankshaft to fail.

These cranskshafts can be used for most racing applications with nothing more than a dimension check, including straightness, and a Magnaflux inspection to be sure they are free of cracks in critical areas. For any street engine, including SHP, these crankshafts are overkill.

Since the Tufftride treatment only penetrates a few thousanths below the surface, even grinding the journals .010" will essentially removed it, so durability is less, however, if you are restoring a SHP or even a HD engine for strickly street use, the loss of the Tufftride is probably not an issue.

If you are building and engine for serious racing, including vintage racing, I would insist on an original dimension crank or one that has been ground down no more than .010" AND THEN given a new Tufftride treatment or other surface hardening process to improve durability.

Unfortunatelly most machine shops grind any crank that comes their way "because that's the way we do them all", but as long as a Tufftrided crank has seen reasonable oil change frequency and has not been oil starved, it's common that the crankshaft journals will still be within OE machining tolerance when it comes time to overhaul the engine, including engines with well over 100K miles, so if you have a SHP engine with a Tufftride crankshaft, don't let a machine shop grind it unless visual and dimensional checks show that it is absolutely necessary to grind it undersize.

If Chevrolet had used the Tufftriding budget on better rods for early 327s a lot fewer of them would have blownn up in the sixties. The cranks were bulletproof, but the rods were junk!

Duke

[Clem Z.]

if you are into vintage racing a covette

buy a aftermarket rotating assy as they are made from even better material than the GM stuff

[Roger Legge]

Re: A question about "sensible" crankshaft turning

I think it might be worth mentioning the crankshaft fillet - the curve at the point of the journal metting the crank throw. I believe you can run into issues with the fillet radii the more you turn a crank. I don't know a lot about this, but I think possible issues are the bearing fit/clearance at the fillet radius and also the possibility of stress risers and cracks forming when this radius is ground away.

Roger

[Clem Z.]

production reground cranks do not have a larger

radius than stock cranks so std bearing inserts will fit. aftermarket cranks have a larger radius to prevent stress riser cracks in the journal so you must use a special insert that has a bevel machined into the edge that faces the radius to clear the larger radius.

[Roger Legge]

Re: production reground cranks do not have a large

Let's assume a stock Chevy cranshaft. When the crank is turned under, does the fillet radius get ground away as the crank is ground? I have heard of crank failures traced back to a crank grinder not paying close attention to the fillet radius. Or the crank grinder not having the ability to put a proper radius in place?

Roger

[Dick W.]

Re: production reground cranks do not have a large

A competent operator of the crank grinding machine will dress the edges of his stone so as to duplicate the original radius.

Years ago when "cheapie" good cranks were not available, we would arbitrarily turn cranks .010 so as to index the throws exactly. As Clem mentioned "offset" grinding was a very common way of gaining stroke at a minumun cost. Normally very reliable. Premium cranks were outragious in cost. A Kellogg crank for a Hemi was well over $1,200 35 years ago. Going way back it was very common to offset grind flathead F..d or Mercury 4" stroke cranks and gain (iffen my feeble memory ain't gone plum outta gear) .125 extra stroke by using earlier rods and smaller I.D. bearings.

No I ain't quite that old, but this is what I was told And Doc, I is still younger than you.

[Clem Z.]

Re: production reground cranks do not have a large

if the crank grinder operator is lazy and does not put the radius back on the grinding wheel after he faces it,yes the radius can disappear. i do not know how they do it now but back in the old days it was done by hand then a piece of wood was held against the radius to make a cut and then you used a radius gauge against the profile in the wood to see if you had the correct radius.

[Clem Z.]

should have used the word "dresses" not "faces" *NM*

[Joe L.]

Re: A question about "sensible" crankshaft turning

Steve-----

Personally, I'd take a NEW '6223' to a USED '7115' any day of the week (and, twice on Sunday).

[John H.]

Re: production reground cranks do not have a large

Back in '61 when I had my AA/FD, I ran a Chrysler Marine block and heads (392 Hemi) and a Moldex crank (still in business here in Detroit, BTW) with Carillo rods; I watched them finish my crank, and the fillet radii were "rolled", not cut or ground, so it didn't disturb the crank's grain flow. Put almost 100 passes on that crank, and it was still in the car when I sold it to Benny Osborn in 1963. Bought lots of sets of bearings, but never touched the crank