Merc 350's, was told they put out 650'ish hp from previous owner.

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Did he replace the pistons, or over bore the cylinders? maybe a dingle hone? just trying to figure out if the rings are installed correctly File fit or throw together? go though your bill item by item to see what machine work and what parts were installed. it seems like a straight forward rebuild and hard to screw up. If I were you I would install oil dye and pinpoint it first myself, so you know what it is before you take it in and get lied to again.

Just food for thought but I seen a dumbass engine builder put a oil Gallery plug in the back of motor that had a hole drilled in it to oil a timing chain and it spilled oil in the bellhousing Non-Stop

I also worked on a jasper remanned sbc that had the rear main seal installed backwards, it leaked like a sieve hot, dont know how they missed that during their "exstensive" dyno testing !

If it's putting that much oil in the bilge and the top of the engine and engine hatch isn't coated in oily mist, I'd say it's missing something obvious as mentioned above. I've had oil leaks before but never leaks that had to be drained into a bucket, more liked wipe up with a rag.

Looking at how tight it is in there, it's probably coming out for the repair. Go put it on a dyno as is and find/fix the leak. It may have to be loaded to leak.

Yeap had the same thing happen to me !!!! plug was painted over, lasted only minutes and then huge oil leak, as mentioned before, I redid my breather system on my procharged big block once, used 3/8 lines, WAY to small, blew oil out from dipstick so bad you would have thought oil line was off, went back to 5/8 hoses, fittings etc and was fine..

Might agree a rear main seal. You can load the engines in the boat and the boat on the trailer using a propshaft dyno to help find your leak. You have to find a marine shop that has a prop dyno. I had one but sold it.

Propshaft dyno use -- I would use mirrors and oil absorbers laid down all over the place in the bilge to see what section the oil leak is coming from - then try to pinpoint it from there. Very hard to find a great engine builder in Texas for the marine side. If still in the mix for you about finding a great machine shop, the best around in Texas hands down is Reyer & Morrison in Arlington TX, not far from me. Not cheap but you will get your money worth.

I understand you have twins but this how I would go about finding your very hard to find oil leak. And who knows if and or when finding the leak(s) this way - you might even be able to fix it while engines stay in the engine compartment.

Also how about the oil filter boss adaptor down on the block if you have remote oil filters ? or its lines down there ? I have seen them leak as well.

https://www.youtube.com/watch?v=4y6SC7vaLvY

Here's my short saga of chasing an oil leak.

http://www.offshoreonly.com/forums/g...e-motor-2.html

That oil dye really works on the hard to find leaks, When I worked in the GM service dept we had to use it on any warranty oil leak complaints in order to get paid on the claim.