Thunderbolt 850 - Crankshaft Replacement

Replacing a crankshaft that has flattened inner splines aparently from someone running the motor onto a beach or something. The driveshaft and motor are in very good shape and we taken apart the entire motor and gone through it. If I just replace the crankshaft with a new one...can I get away with just replacing this one part? or should I add a set of rod bushings?
I really don't want to drop more cash into motor...just want to get it running for a couple more years until I purchase a newer motor.

Thanks

There are no "rod bushings"--just needle bearings.

That's a major job for someone unexperienced with Merc overhauls.

Jeff

If you don't want to drop cash into this (and I wouldn't given it has a toasted crank) you would be best to part it out and recover what cash you can to be put towards another motor.

A new crank is out of the question - discontinued and close to 2 grand when they were available.

That leaves you looking for a used or reman - and anyone who knows what they have will be looking for close to 1000 bucks for it if it's been ground/cleaned up.

Plus, since you have her cracked you would be foolish not to do all the bearings, since the crank was toasted, plus you need all the gaskets and seals.

And if you are going to do all that (and not make it a waste of money), at least replace the piston rings and get the cylinders honed.

If you have the tools, skills and manual you might get her back together for 2000 bucks if you don't find anything else wrong.

Given that the value of a running 850 in the used market is somewhat under 1000 bucks, it's probably not worth it....

I got a brand new Mercury Quicksilver crankshaft from marineengine.com for $238.45/+shipping $8.00

Part Number
440-4979A 2

A powerhead rebuild kit online sells for $715.00 and included all the needed parts, but the price was too steep for me at this time. New and rebuilt powerheads are ranging from $1k to $2k and this would be totally ridiculous to shell out unless the motor had some kind of sentimental value, though it definitely doesn't .

This was their last one at that price. There are parts for these older motors all over the internet as well. I guess we will examine the old crankshaft for strange and excesive wear and proceed from there. The compression on all cylinders was 115-120 so I don't think the motor is dead yet. After repairs we'll probably double up the oil in the fuel mixing for a couple tanks and just putt-putt around the lake until the motor has a chance to prove itself.
I do plan on upgrading the motor and also selling the existing motor for parts like you said.

Awesome deal on the crank - makes the build worthwhile.

And you have done your own reasoning on whether or not it's "worthwhile" - many people initially don't analyze it properly.

You point out the cost of a reman/rebuilt powerhead - and yes, generally for about 2500 bucks or less (depending on the model), you can get one with "all the work done".

But that is "just the powerhead" - you would still require the ignition system, carbs and a good lower unit.

Often a toasted powerhead is the result of a fuel issue which must be corrected but is not factored into the cost.

So while you can get a powerhead for say 2K once you add in the 600 bucks for carb service (at a shop), plus maybe 300 or 400 for ignition parts, the rebuild becomes "not worth it" for a particular motor.

If you can do all/most of the work yourself you could have this "refurbished" for maybe 1500 bucks or so (plus a good few hours of your time).

Only you can answer whether that is worth it or not.

I personally have a model year cutoff of 1986 (one of the years the model lines changed) - anything built before that I would not rebuild/refurb "for myself".

But having said that, even if you ended up dropping 2K "making it like new", that's a pretty cheap outboard in today's market - and barring toasting a lower unit, it could easily last you 15 or 20 years without any trouble....