Mercruiser Engine ID Help

[PugetBoat75]

Hey all, we're knee deep in swapping what we were told is a 140 in to a recently purchased boat. We were told the engine was purchased new and never run. And indeed, the internals look new, it's painted like any crate motor, etc. Looks like a brand new engine. There's no serial number plate, just a block casting number (27700) and a number next to the distributor (110071NB). From what I've read, the "NB" in that last number points to a 120 motor. HOWEVER, this engine has a harmonic balancer on the crank, which points to 140 according to research on here. Anyway, we've almost got it all back together (flywheel and coupler from the 120 swapped over, even), but now we're very confused. Can anyone help here? Thanks!

 

[Bondo]

HOWEVER, this engine has a harmonic balancer on the crank,

Ayuh,.... Welcome Aboard,.... You have a 3.0l motor there,....

 

[achris]

... a number next to the distributor (110071NB). From what I've read, the "NB" in that last number points to a 120 motor. ...

Quite correct. It appears you have a 1981 2.5 litre engine. Someone may have put a 3 litre balancer on it, but that code points to it being a 2.5 L...

Here is the Merc service bulletin with the engine codes.

View attachment 02_08 GM Eng Nos.pdf

Chris........

 

[Bondo]

Someone may have put a 3 litre balancer on it, but that code points to it being a 2.5 L...

Ayuh,.... That's gonna make for a really bad vibration situation,....

 

[PugetBoat75]

Thanks for the insight, guys. This is of course not really what we wanted to hear, but we'll move forward. looking at the internals, the engine still appears brand new and once running and in the boat will be miles ahead of what we started with. Interesting about the HB on the 120. Any recommendations on what to do about that? Why would it vibrate more than not having it?

 

[biggjimm]

I'm not sure of the difference between the 2.5 & the 3.0 but if you change the size & weight of the balancer that was used to balance the rotating assembly, you can & usually will, get vibrations that will wear out crank bearings in short order.

Some are balanced externally. Meaning the balancing is achieved by the HB & flywheel (or flex plate) instead of the crankshaft itself like an internally balanced engine.

When I was a dumb kid, I found out the hard way that the 400 small block Chevy was externally balanced unlike the 350 that is internally balanced. I ran a 350 flywheel on it. That engine grenaded Itself before I figured out what all that vibration was from.

 

[Bondo]

I'm not sure of the difference between the 2.5 & the 3.0 but

Ayuh,.... The 2.5l has Only a pulley, No balancer at all,...

The 3.0l has a balancer, 'n pulley,...