tominohio

What was the Factory installed forged crank casting number for a 1966 L79 Corvette? I know what ones are considered OK to use.....4577 or 2680.
I have been told all 66-67 Corvettes left the factory with a casting number of 4577. Is that correct????

Thanks, Tom

Jackfit

My original engine with 300,000 miles has that crank.

Jack

tominohio

Thanks Jack! Going thru my L79 rebuild now and the crank can still be turned but would be at the limits. I'm going to get a better #4577 so the search is on! Beautiful Black Vette by the way!

tbarb

FWIW, I believe the crankshaft for 1963-65 is the #2680 and for 66-67 the #4577. I also believe it's the same part except the balancing because the connecting rods changed in 1966 to a better design and were heavier.

Nowhere Man

Would they also change becouse of them drilling and tapping for the balancer bolt?

BADBIRDCAGE

Keith

Drilling and tapping a hole into the snout of a forged crankshaft is a machining operation and would not affect either crankshaft. What I don’t know is if all the later forgings were drilled and tapped for the balancer bolt.

Rich

SWCDuke

SHP/FI 327s used a different crank forging than the 250/300 HP engines (both are forged) The former has slightly more end balance mass due to the heavier forged pistons.

I think that the later 327 SHP engines used a different forging, but not sure. I know one of them is 2680, but not sure if that is early or late. The additional material added adjacent to the rod bolt seat in '66 was only a few grams, and this could have been compensated by grinding a little more material off the bottom balance pad. Then again, maybe this was the reason for the different later forging number.

SHP cranks were Tufftrided (250/300 HP were not), which is why they should NEVER be turned unless absolutely necessary. Even reducing journal size .010" will remove the hardened surface, which reduces durability.

All SHP/FI crank noses were drilled for the 8" damper center bolt; 250/300 HP cranks were not drilled by GM, but it's common to drill them in the field for a center bolt. Without the center bolt the damper can be tough to install without damage.

Duke

Critter1

I agree. I've tried to explain that to several folks but not many agree. Cast crankshafts work harden in use so no special surface treatment was necessary, but that's not the case for forged shafts. The hard surface from this process was less than .010". Probably closer to .003-.004".

Scott Marzahl

You can have a turned crank Nitrided if you feel it is warranted for high performance use. I think the last one I had done ran around $125 by Henry Velasco crankshafts.

Westlotorn

If the crank has already been turned he is already past the factory hardness.
I would not hesitate to run a crankshaft that had to be turned 10-20-30 or 40 under.
If your crank is still good and bearings are available turn it and run it.
There used to be 327 bearings available all the way to 50 under. I have not checked in a long time so the list of available sizes may have changed. Clevite and Federal-Mogul both OEM bearing manufacturers had these available.

Jackfit

My original crank was abused by me for 220,000 miles and 4 bad rebuilds, and one great one 73,000 miles ago...they can be turned and fitted by a good builder

Jack

BADBIRDCAGE

Used to be able (late 60’s and early 70’s) to get small block connecting rod bearings at +.060 from Vandervell.

Don’t know where to get it done anymore but used to get crankshaft journals hard chromed to be able to finish at standard size.

Pop Chevy

The factory nitriding is nice, but it's not the end of the crank if it's gone. There are many cut cranks going down and around racetracks all over the world.

johngandersonjr

I just had my original 67 327 rebuild and the snout was not drilled. I think that started in 68. I thought my crank was shot and found a standard 4577 for $50 locally. Turns out crank was fine so I have a spare now.

I had mine taped so I could use a ratchet to turn motor over

tbarb

In a previous post you mentioned that all forged crankshaft have some sort of hardening treatment, can you expand on that because I always thought the journal hardening was just for the shp cranks.

I have seen cranks with the full part # and some with just 2680 (last four), I think the same for the 4577 cranks but I don't know if that means anything or just a different machining person.

Critter1

Somewhere, I have some excellent GM paperwork on the tufftride and nitride processes. I'll dig it out. In the meantime, here's a link that explains more about the processes.
One process treats the entire crankshaft surface while another process (nitride?) only hardens the surface of the journals. It's easy to tell which process was used as the machined surfaces of the counterweights of the crankshaft will be a dull grey if the shaft was tufftried treated but those machining surfaces will be bright/shiny if nitride treated.

https://www.highpowermedia.com/blog/...of-crankshafts

tominohio

Excellent Link that does a great job of explaining the process! Thanks to this Thread a member has contacted me who has a crank that is Perfect for my L79 rebuild.