There are traces of water in the engine oil (compression data is in order - see below, engine has about 500 hours) we are not sure where this water may be coming from.
Recently the engine has been emitting black smoke from the exhaust.

The mechanic told me about a possible crack in the engine block, but honestly I'm a little skeptical.

Can anyone tell me what parts are prone to cracks that cause water to come in? During the winter the boat is in the yard indoors.

Mercruiser MCM 260. 350 CID
Serial number A609 695
Year 1986 (not 100% sure)

Boat: Chris Craft Stinger 260 (1986)

Compression data:
Cylinder 1. / compression (Bar) 10.8
Cylinder 2. / compression (Bar) 10.2
Cylinder 3. / compression (Bar) 10
Cylinder 4. / compression (Bar) 10.8
Cylinder 5. / compression (Bar) 9.9
Cylinder 6. / compression (Bar) 10.3
Cylinder 7. / compression (Bar) 9.7
8. cylinder / compression (Bar) 10.0

Thank you in advance for your help.

1) Crack in engine block or cylinder head
2) Defective oil cooler
3) failed head gasket
4) possible intake manifold gasket/crack

I'm looking for a replacement of the block. Anyone can provide me some hint where to purchase it? Thanks much in advance

Thats basically a small block chevy 350. really common, (at least here) at the price point between a block and a complete motor, find a complete rebuilt unit

X 2 ^^^^
and i’ll add
#1) exhaust. Ie: water leak inside them.
#2) reversion. If an engine has a cylinder or so not firing, that can pull water back while running.

If has oil cooler, ck that first. 5.7’s usually don’t have engine oil coolers though. If not…I would check the exhaust first. Also find/fix why engine running badly. Then ck intake . With intake off for that you can also look for crack in lifter valley, where typ from freeze issue. Then next prob headgaskets.
My 02.

https://www.michiganmotorz.com/5-7l-...engine-1987-95

between the highest and lowest compression readings you are within 10% which is normal.....when you say there are traces of water in the oil....what is that based off of?
If you are seeing the milkshake in the valve cover area but the dipstick is clean then i'd say you just need to heat up the engine and burn off the moisture as its normal.
If you have a light brown chocolate milkshake on the dipstick then yes there is some thing wrong.
Could be something simple like an intake manifold gasket.
Could be a bad riser gasket letting reversion back into the cylinders.
Could be as bad as a cracked block too....lots of variables.
You can take exh manifolds off and pressure test them to see if any air leaks.
Block off the cooling system on the engine and put some pressure in there....10psi is more then enough and see if it leaks down and see if you can pinpoint where the leak is. With the spark plugs out and system under pressure, if there's a leak you should be able to hear where its coming from hopefully. If you cant hear it but its leaking then its a hairline crack and the motor will probably need to come apart anyway to find it.

And black smoke out of the exh is just telling me that its running rich...possible carb adjustment needed...

I was also going to say https://www.michiganmotorz.com/

I bought 2 new base engine 350s from them, fall 2019, for my Formula 292. Delivered right to my door on a pallet/wood crate.

I've been very happy with the results. Only issues I had was what was carried over from the old engines (fuel pump, alternator).

Once I resolved that, I now have 200 hours (2 seasons) on them, and they run very well.

I should have replaced my intake manifold when I did them, I replaced anything hard to get to, like new flywheel, new coupler, new starter.
I should have done new fuel pump at the time as well.

Anyway, I'm happy with them, did all the work myself. I also did new exhaust manifolds, elbows, and thermostat housings.

Winter Work

1. pull risers and check,
2. then pull intake manifold and check for cracks in lifter valley. and check intake gasket.
3. cylinder heads rust out to a point where they eat into the head gasket compression ring area.
good luck

Thank you!