73 Mercruiser 3.0 engine, rebuild or swap, compatible years?

I have a 72 SeaSwirl with a bad Mercruiser 3.0L engine and trying to decide if I should rebuild or replace it with a complete late model 3.0L. I haven't been into the engine yet, but it sounds like it threw/broke a rod, clunk/knocking noise coming from the bottom of the engine. I was idling and took a 3' wave into the stern which caused the engine to die. The engine never ran the same after that and blew about 5-6 hours later. I guess the exhaust flapper was burned out, swamping the engine and bending a rod which eventually broke. That's my best guess

I'm looking at $1000+ to rebuild the engine, but if the crank or block are bad it could get pretty expensive, so I'm also considering a newer used engine if I can get one for a decent price.

I have a few questions about Mercruiser 3.0L compatibility

-If I replace the long block with a newer engine what years are compatible, 14" flywheel, large intake port head, engine coupler & crank? Am I missing anything important?

- Can I install a complete late model engine (engine, manifold, carb, ignition, flywheel, starter, alternator, coupler, engine mounts) into the 72 SeaSwirl without modifications? If not what modifications would be required? Do newer intake/exhaust manifolds have the mounting hole for the old style (72) throttle linkage?

The boat is pretty reliable since the outdrive was completely rebuilt 10 years ago then resealed along with bellows 2 years ago, head and manifold replaced 4-5 years ago, new alt, new starter, carb runs fine,,, $500 of new tires/rims week before the engine blew. I'm kind of perplexed as what to do since I don't want to dump a ton of money into a 73 boat, but also don't want to cheap out on the repair.

Re: 73 Mercruiser 3.0 engine, rebuild or swap, compatible years?

Re: 73 Mercruiser 3.0 engine, rebuild or swap, compatible years?

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Re: 73 Mercruiser 3.0 engine, rebuild or swap, compatible years?

I just went through a rebuild on a 3.0 and all I can say is "I shoulda got a 4.3"

If I were in yur place I d go to a 4.3 I don t know how difficult it would be considering your outdrive and all but it would be money well spent. I have nothing against the 3.0 except it is strictly marine and is difficult and expensive to get parts for.

The pros here can help you either way,3.0 or 4.3.

Re: 73 Mercruiser 3.0 engine, rebuild or swap, compatible years?

Thanks for the welcome and replies,

Yeah, it's pretty much the same engine, but the little things have me concerned; coupler, head, flywheel, shifter/carb linkage mounting points. I don't want to buy a used engine only to turn around and dump another $500 into extra parts to make it fit my old boat. A brand spankin new 3.0 sounds great, but the boat isn't worth the $3K, I'd be looking for another boat at that point. Unless I can find a used 3.0 that I know is a sure thing, I'll probably rebuild my 3.0 if it isn't too thrashed. Anyone know a source for rebuilt 3.0 shortblocks? Everything on the top of my engine has been recently rebuilt and should be good to go.

The 3.0 is a pretty good match for my 16.5' SeaSwirl, only wished it had more power on a few rare occasions, 4 adults and 2 kids or up at 9000' elevation. Other than that I'm really happy with the power of the engine. If it turns out I dump the SeaSwirl, I'm going to be looking for a 18' - 20' deepV with a V6,,, maybe a V8...

Re: 73 Mercruiser 3.0 engine, rebuild or swap, compatible years?

Keep an eye on ebay they have a few every now and then

Re: 73 Mercruiser 3.0 engine, rebuild or swap, compatible years?

Hey watermellonI,

I saw your post on another thread about rebuilding the 3.0 and getting +35 HP out of it, what parts did you upgrade on the build and what do you think netted you the most power?

I've decided to rebuild my engine and want to do some HP tweeks while I have it apart. A few years ago I polish/ported the head, installed under cut valves (stock size), new springs and retainers, hardened seats, port matched the head and manifold. It should be good to go, but I'll have a valve job done to be safe.

How did you build the lower end? I'm going to bore it 0.20 - 0.30 over, check the mains/rods (machine if needed) and have it balanced. I'm not sure what stock compression is for a 73 3.0 (9.0-1, 9.5-1? I run 87 octane without pinging) but wouldn't mind bumping the compression a bit (10.0-1, 10.5-1 ?) and running 93 octane fuel. Even if I don't bump the compression ratio, I'll probably get better performance from the rebuild since the last compression test showed 115-122# dry and 135-140# wet, not too good...

While rebuilding the engine I want to maximize torque and maintain smooth idle, basically do a balance/blue print on the engine without moving the power band. I'm not looking for top end power since I troll a lot and normally cruise at 3300 RPM,,, every once in a blue moon up to 4500 RPM at WOT

Re: 73 Mercruiser 3.0 engine, rebuild or swap, compatible years?

4 adults and 2 kids or up at 9000' elevation

Click to expand...

35 extra HP at best will help you get 1 mph at sea level at top end. If you want to boat at 9000' with 4Adults+2kids then you need a 4.3. You need the low end torque of cubin inches, not max horse power. Along with gear ratio changes and prop changes carb changes etc.

You will never make one setup work great at both altitudes.

Re: 73 Mercruiser 3.0 engine, rebuild or swap, compatible years?

The only times I have a lack of power is at 9000' (1 adult, 1 kid)... or... while heavily loaded with 4 adults and 2 kids (at 1000'), not both at the same time.

When at 9000' altitude I lean down the carb (.065 > .057 or .055 if I remember correctly) and change from 17 pitch to a 15 pitch prop. I haven't bumped up the timing yet but going to try it on the next trip, 2-3 degrees I guess? I'm also considering a 13 pitch prop, but waiting to see how the boat planes with the new engine. Can't swap gears for altitude since I'm only up there 1-2 times a year.

I actually wonder how much power will be gained with the rebuilt engine compared to the old engine and poor compression, 160-170 psi on a rebuilt engine compared to 120 psi on the old engine?

Since I'm rebuilding the engine, I figure it wouldn't be a bad idea to bump up the compression to gain a bit of power. The only problem I can see is that I will have to run high octane fuel, but an extra $3-4/trip isn't that much of a problem.

Re: 73 Mercruiser 3.0 engine, rebuild or swap, compatible years?

I suppose your gonna stick with this thing so I ll tell ya I had to have mine bored due to scoring in the cylinder walls, 1 piston fried then I found some sort of ring marks in the bore of 2 3 4, the shop bored it to 30 over to get her cleaned up. The whole motor had recently been rbuilt new gaskets everywhere as I tore it down I had no idea it had been rebuilt so I probably contributed to the piston meltdown.

The head appears to be a 140 (larger valves) I m not really into all the tech aspects of this. I haven t built a motor before this (fixed a few though) I havent really been able to get it out and test it much, carb problems, aside from the carb issues I ve been having, it runs great. I can t recall the pistons I put in off the top of my head so I ll get back to you on that.

I was also thinkin I might be able go a little stronger in the prop area too.

Sorry it took me so long to catch up with you, not sure if I ve been much help.