Mercruiser 350 Mag MPI 153 Teeth Flywheel Swap
 

Hi guys

I need to fit a 153 teeth flywheel (12.75") to my Mercruiser 350 Mag MPI.
The current flywheel has a diameter of 14" which needs to be replaced due to the engine swap I'm doing.

Now questions are:

1. Is my 350 Mag MPI internal or external balanced? 12.75" flywheels are available for both.
2. Are there differences on the crankshaft flange to which the flywheel will be mounted?
3. Will I be able to fit a starter motor to the 350 Mag MPI block with a adapter bracket or do you know any smart solutions?

Many thanks
Martin

After 86-87 small block Chevy’s went to a 1-pc rear main seal.

The MAG MPI should use an externally balanced flywheel because there is no counterweight on the rear of the crankshaft.

Hi TomZ

Are you sure?
I just checked the backside of the original flywheel.
It does not have a counter weight.
I have also read that external/internal balancing does not depend on 1-piece or 2-piece seal.

So it looks like the 350 Mag MPI is internally balanced then?
If this is the case I will get a 153t flywheel without a counter weight.

Let's say it is external balanced.
Can I bolt any 153t flywheel with a counter weight to it?

Martin

They are internally balanced but there is no way to add a counterweight internally on the backend of the crank so the offset is in the flywheel.

Is the original flywheel drilled?

I’ve heard that there are differences between the 305 and 350 but I don’t mess with 305s so I can’t verify.

*edit*

I'm going to take it off tonight and upload a picture from both sides.

Martin

Why do you need the smaller flywheel?

Got a boat with a damaged engine. Had the chance to get the 350 Mag MPI for free. I think it will be an improvement compared to the 454 carb Mark 4 carb engine.

The boat has got a Volvo Penta DP280 fitted with the old school bell housing.
The 14" flywheel of the 350 Mag MPI does clash with the bell housing.
A 12.75" flywheel should work with bell housing hence i'm looking for a smaller flywheel for the 350 Mag MPI now.

Martin

I don’t know anything about Volvo but you could try looking at a parts diagram for a late 80s 5.7 to see what it uses. The base engines are all GM so if they list a small flywheel, it should work.

Interesting that your 454 flywheel worked and the 350 Mag won’t. If I’m understanding correctly.

Also, the 350 isn’t going to make the torque of the 454 even if it was a low-perf engine. You may be looking at a different prop set. Almost guaranteed.

It is a difficult one cause I read a lot that around 1987 or late 80s a few changes to the engine blocks, seal and crankshafts have been introduced. Apparently this is also a time where a split between internal and external balancing happened?!

My issue is that I'm in Germany so try to figure what parts are required and what works before ordering from the US. Sending stuff back in case ordered wrong is a pain.

The 454 was a Mark 4 big block from 1990 which to my knowledge came with a 153t flywheel as a standard. So it didn't clash to the bell housing as it was 12.752 in diameter.
After that time, I assume at the same time when gimbal housings became more popular, the bigger 168t 14" flywheels got fitted to most small block engines of which the 350 Mag MPI is one.
My 350 Mag MPI is from 2004.

Yeah I know but I wanted something more reliable and not as old. It was a pain to find parts for the mark 4 big block.
I also got the 350 Mag MPI for free so it was a no brainer.
I have a couple of propellers so should not be an issue to find the one which fits best.

Martin

This should be the correct GM part number for a factory small block Chevy - 153-teeth and listed as externally balanced.

14088650