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Old 28-02-2016, 12:46   #1
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Join Date: Oct 2010
Location: Vancouver
Boat: Oyster 39
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Swapping out a Volvo MD21B for a Volvo MD31A possible

Hi everyone,

As with all things in life, the seemingly most amazing things happen just after youíve spent money elsewhere! Iíve just went ahead and purchased my belovedís engagement ring only to see what appears to be an amazing deal on an engine the day after!

I donít think honey, I got you this green metal hunk here instead will go over, even if I advertise itís practicality or difference in sizeÖlook hun itís WAY bigger than that little shiny bobble.

Anyway about the engine, itís apparently an MD31-A with 51 hours on it originally (it was purchased years ago and used as a generator for 4 or 5 days before being purchased and stored for years). The price is really attractive at $3000 CAD and itís close by. I currently have a tired old MD21-B in my Oyster 39 which has been pretty much docked as a result for 4 years. I am trying to understand how practical is it for a MD21-B to MD 31-A swap out would be. Would I have spare parts etc from the 21 that would fit the 31? Is there anything alarming about a 31 sitting for so long, apparently it has all the original gauges and control box. However should I be concerned about seals etc? I am also curious about the motor mounts and the transmission mating, is that of any concern? Lastly, are parts readily available for this engine? Is the price fair? Iím really thinking this is almost too good to be true, is it?

Thanks in advance for any guidance you can offer. I am going to look at the engine this evening.
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Old 28-02-2016, 14:36   #2
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Re: Swapping out a Volvo MD21B for a Volvo MD31A possible

Comments on the MD31A engine:
4 cylinder 2.4 liter naturally aspirated engine with 62 hp@ 3500 RPM......Very robust engine for 62 hp, as the same block in turbo/aftercooled version produced 130 hp. The engine was designed and built by Volvo Penta and produced between 1986 and 1994.

Your existing MD21B engine was based on an Pugeot/Indenor block, so there will be no parts interchangeability, with perhaps the exception of the transmission.

Depending on your vessel, getting the MD31 to fit where the MD 21 currently sits might be tricky. The MD21 engine was fitted with a fairly small diameter flywheel, and that resulted in a relatively small diameter bell housing. The MD 31, on the other hand had a larger diameter flywheel and SAE 4 bell housing. I have seen instances where the MD31 flywheel housing couldn't fit down between the MD21 engine beds, and the beds had to be rebuilt.

If the engine was properly prepared and preserved when it went into storage I think it should be OK coming out of storage, but you should hear it run for awhile before making a decision. You mentioned it was used as a what if anything was done to the injection pump to allow it to run as an 1800 RPM gen set engine? Can you find out? If you hear it run it should be able to hit about 3900 RPM at hi-idle as a marine engine.

As I recall, the MD21 engine had a max. rpm of 4500, and the MD 31 has only 3500 rpm, so there will need to be either a gear ratio change, or a prop change, or maybe both. A good prop shop can help out here.

Hope this gives you something to think about.

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Old 28-02-2016, 17:36   #3
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Location: Vancouver
Boat: Oyster 39
Posts: 51
Re: Swapping out a Volvo MD21B for a Volvo MD31A possible

Thanks Doug,

It looks like I'll pass on this given all the re-jigging and such.
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