What kind of damage would result in the failing of the crank pulley breaking off the key way and falling off? Worst case I would assume from being here and reading minimal threads on this topic replace the whole engine... Best case, replace crank pulley and install new key way.... Need more input to this situation as it seems to be a dark void around here and it always results in someone arguing faulty install/bad crank pulley. I'm not here to try to figure out why crank pulleys sometimes fall off... I'm here to gauge some input into what can result after a crank pulley comes off our cars and obviously ruins the key way. Thanks in advance, please post some input.

From my experience even at a slow speed if the crank bolt backs out the keyway is most likely going to shear off and score the crank. AT this point IF no other damage is done to the front end housing, timing belts etc. a new crank is an option. However a new crank is several thousand dollars plus all the work to disassemble the motor etc. That is why most opt for a low mileage used 55 engine. I used an S55 engine which required on the rework of the oil sump as the E55 is double sump and the S55 is not.

BUT before you get to even being in this situation if you are doing a pulley install my advice is get someone who has done it before and KNOWS what they are doing. My INDY shops had done 0 before mine and I paid the price. I was was fortunate in that their insurance cover the replacement so in the end i had a 70K mile engine replaced with a 7k mile engine. It did take nearly 2 months and LOTS of stress to get it all worked out though.

So you're saying at a minimum in your experience... The crank is no good if the key way "fell into the oil pan" and broke off.

Here is a search result of similiar situation.

https://mbworld.org/forums/w211-amg/...ed-keyway.html

Read all those... So on the other end of the spectrum he got lucky that just the key way fell into the oil pan and no damage to his crank.

The ones I've seen as the pulley was walking off the end of the crank shaft the keyway was sheard in some one with the result being the crank is scored as the pulley works it's way off the end of the shaft. This situation will require a new crank in my opinon as welding or sleeving the end will always leave questions and when things go wrong again it's not good!

IF you are saying the keyway was intact I'd still have the crank looked at very carefully.

+1 on what Grantwoodtx is saying