Technical Lifespan of a turned crank

Since its cold and I am bored and doing too much thinking I was wondering if a turned crank will last just as long as one that was not turned,I was wondering if turning goes down past the hardness making the crank a little weak. I will be needing the crank turned in my 57 235 since just polishing it did not work and now has a rod knocking,I doubt I would put a bunch of miles on it since its only went about 10,000 miles in 14 years. I never put many miles on a motor with a turned crank and was just curious.

 

A correctly turned crank with corectly fitted bearings will last a lifetime. Often new engines had cranks turned at the factory to correct machining and casting errors.
Your mileage may vary!

 

that crank can handle down to minus .030 and run ok.
I raced a 54 with a 235 in the long gone N stock with one cut .020 x.020 and a 62 belair with a 235 the crank was .030x .030 as a daily driver for 6 years,

 

They didn't harden them at the factory. Possibly some heavy duty truck engines but not ordinary car engines.

Have heard of aircraft crankshafts being nitride hardened, and having crankshafts hard chrome plated for racing but not for the street.

They make sure the bearings are made of soft babbitt material so any dirt specs will be embedded in the bearing and not chew the crankshaft. For heavy duty use, they have harder bearings with more copper and these are the ones that require a hardened crankshaft.

With modern oils and filters (air filter and oil filter and crankcase ventilation) you should be looking at 200,000 mile life span for your rebuilt IF you use the best parts, and take care of it.

A long winded way of saying that if your engine is rebuilt properly, the crank turned and polished properly, quality bearings fitted correctly, carefully broken in and the oil changed regularly, it should last a lot longer than the original which was not babied near as much as that.

 

In addition to having better bearings oils etc, a good auto machinist with modern equipment can do a more precision job than they did at the factory in the fifties. So you have that going for you too.

 

I presume the motor was original and not touched but has been about 20 years since I rebuilt it,the machine shop did all the measuring and got the parts. I now wish I would have insisted the crank be turned too and I possible would not have to take it apart again,I am almost certain it was just polished but have drank mass quanities of beer since it was rebuilt.

 

Something went wrong and will need to find out what happened.

 

This. Fillet radius is extremely important...I've seen way too many cranks turned incorrectly (with little to no regard for proper fillet radii) - leading to stress risers and breakage.

 

10 years ? 14k miles
Did it get 40 oil changes? Or 3 oil changes?
Oil gets acidic after 3 months or so,

 

Zero difference. As has been stated, stock cranks aren't hardened. Today, we build engines way better than the factories. With a precision crank regrind, block line honed and decked, it will run until you're tired of looking at it.

jack vines

 

At least 14 oil changes.

 

A properly ground and installed crank will cause you no problems.
When did the phrase "turn a crank" start? I was a automotive machinist for many years and it was always "Grind" a crank... since a crank is ground with a large grinding wheel and turning something always was done in a lathe with a cutting tool... guess Im just getting too old

 

You say it was polished, how much ? Were undersize bearings used ? They are available .001/.002 under.

 

When I was a budding young mechanic back in the late '50s I watched a guy come to our shop and set up his machine on a Chevy six still in the car and grind two journals. It didn't take long and we had the engine buttoned up and waiting to be picked up at closing. It was common back in the day.

 

If the bearings were the wrong size, I imagine it would have been noisy sometime before the last 20 years since the rebuild or 14k miles over the last 10 years. Along with lower oil pressure symptoms.

But the rod bolts could have stretched

 

I too watched this being done,, This will cause a few of the young ones on here to call foul,, can't be done. I sure wish I could find one of these machines. I have a project that needs to be done.

 

Back in the early 70's there was one automotive machine shop that still did crank grinding with engine in the car but when my friend needed the service at his shop the old fellow that did the grinding retired and the service was dropped....Bummer, another lost art...I would have liked to have seen the machine and it being done..If the OP can segregate the offending bearing an undersize bearing [as previous stated] may work or the bearing can be shimmed on backside with parting/mateing surface trimmed to account for shims and thus the bearing clearence brought into tolerence..Parts stores used to sell the shims..

 

The in-car grinders were focused on the earlier splash-rod Chevy sixes, which frequently had difficulties. These things recut the rod throw, and rebuilt/rebabbited rods could be had in semi-finished to be reamed to whatever size produced a clean crank surface. The purpose was to allow the job to be done by just dropping the pan instead of removing the engine and taking everything apart.

 

I always wondered where all the grinding debris went ? And how the hell could you clean it all out?

 

There was a lot of faith in gravity in those days... but when grinding a single throw, a few sections of newspaper jammed in there could block most of the bad pathways for particles with inadequate allegiance to gravitation. And, of course, the rod throws had no oil passage to collect debris. These engines did so well wearing themselves out unassisted there wasn't much need to worry about external influences.

 

I wish I could write like Bruce.

 

the crank in my avatar 283 was turned in 1961. with over 200,000 miles it is now in my custom 58 impala.

 

Many years ago I recond a Falcon 250 that came out of a Taxi with 350K on it, the crank showed no wear at all and I clocked up another 370k. Change the filter with good oil every change.
In regards to machining in place, I once saw a mechanic honing bores with the block still in the car and the crank unprotected. I have always wondered how long that motor lasted. JW

 

Valve jobs on L-heads and quickee hone jobs for new rings in bad bores with engine still in car were once routine...!
On the actual first question...there have been some cranks with special surfacing, like the old Chevy 302 cranks with "tufftrided" surface...that was some sort of very shallow hardening, lost when crank was turned.

 

this is correct , on some engines they roll the fillet in a special press to make the fillet actually under cut of the surface to compact the metal around the fillet and some so called machinists would grind them out and the crank would crack and fail right at the intersection of planes often it would look like someone chiseled right thru the intersection and it would break clean
, Pontiacs were known for there rolled fillets ,
we used to chrome cranks to save them if they were damaged and undercutting was too risky or a new crank was too expensive , mostly we done it on forged or steel cranks . nice thing about a chromed crank was if the oil pump picked up some air the bearings wouldn't see immediate damage.

as for that in frame crank grinder we used it on tractors on the farm too so you wouldn't have to fracture the frame on some . they were slow speed engines so it wasn't too critical . and the oil was changed very often too .

 

I had a guy grind IN THE CAR one journal of a friends Japanese car in the 80s. It was his cheap daily driver. When I pulled the pan the numbered rod caps were mixed up. Someone was in there before and thru it together and sold it. The car owner didn't want to spend for a rebuilt motor. It cost at the time about the same as getting a crank ground out of the car $100. We took the starter out and he put an electric variable speed AC motor there. The grinder on the journal looked like a heavy duty Dremel with a micrometer adjustment. As the AC motor turned slowly, the grinder took minimal cuts. the one bad journal took a .040 undersize insert. I warned the owner of the car that it might not last too long as it was not the best type of repair. Anyway the car ran Ok for years.



Ago

 

I believe the life expectancy of a turned crank is directly proportional to the amount of rpm you expect it to turn, and the amount of care you give the motor.
The only crankshaft I ever broke was one that happened to be turned. I'm positive that missing 2nd gear didn't have any effect on the failure of that crank, it all had to be because it had been turned. Gene

 

Its just in a cruiser and will never see 4000 rpms,its been so long since it was apart but I do not remember if it had been turned before or was the way it came from the factory. I am curious to see why its knocking and still not sure its the rod but it rattles when I let off the gas when shifting and was told its the rod,I know its on cylinder #2 only and thinking about dropping the pan to see if I did something stupid like not fully torquing the rod after checking the clearance before pulling the motor,

 

Got another question and can too much be taken off,I can not remember if it had been turned before I got to it.

 

Only if they don't make bearings to fit.