Rebuilding the M25-XP

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By David Sanner, Queimada #611, San Francisco


I kept putting this project off because my engine was running ok and of course I kept hearing, 'If it ain't broke don't fix it,' but I never really felt comfortable that my engine would keep running when I needed it the most. So in December of 2006 between races I pulled out my tools and decided to see what was going on inside. It turns out that because of a botched rebuild two owners ago it really was time to rebuild.

The original plan was to just replace the head gasket as it was leaking a bit of oil and while the head was off I would get the valves done. The compression in cylinders 2 and 3 were about 15% low and there was a little extra blow-by in the crank case. After the machine shop discovered the head was in very questionable shape and had been crack repaired (not a good idea for marine use) I decided to replace the head.

Though the valve seats weren't in the best shape they probably didn't account for all of the compression loss and since I was invested in a new head I decided to really jump in and drop the oil pan, pull the pistons, and replace the rings & connecting rod bearings. Good thing that I did because, as it turned out, pistons 2 & 3 had their connecting rod caps mixed and their bearings were in bad shape... nearing end of life. I replaced the pitted piston, and installed all new rod bearings (top & bottom), wrist pins, and rings.

Fortunately the Universal/Westerbeke M25-XP is a repackaged tractor engine, the Kubota D950, so I could get most of the parts from a tractor supply for less than 1/3 of the marine price. Here are some photos that tell a bit of the story... if you've ever wondered what the inside of your engine looks like, here you go.

What did I learn?

I learned that the M25XP and similar engines are pretty robust and if you have clean fuel and reasonable compression they'll keep running even if you have a crack-repaired head, a pitted piston, and mixed up rod caps.

I also learned that these sorts of repairs/rebuilds are within reach of anyone who is reasonably handy with engine work as long as you take your time, keep the parts in order and matched, and have access to a torque wrench and a good machine shop that knows diesel engines (Kubota a plus).

For those in the SF Bay Area: Carl Stirtz (Oakland,CA) was a wonderful wealth of knowledge about my engine (and many other topics), and I highly recommend him for any machine shop work (not just diesels) including rebuilding water pumps. His fees are more than reasonable.

More than half the cost of this project was the new head ($650) but as I mentioned I ordered Kubota parts direct from tractor part dealers so the prices were 1/3 of Universal/Westerbeke. It took an afternoon to pull the head, another couple of hours to pull the pan and get the pistons out and a few afternoons to put everything back together and adjust it all.

This wasn't a complete rebuild as I still have my main crank & cam bearings but they are purported to last roughly twice as long as the parts that I replaced so I'm hoping this engine will run for another 20 years... considering the price of a repower, let’s hope so!

  • Click on thumbnails to see larger picture

  • Started with low compression in cylinders 2 & 3

  • Valve Cover, Rocker Arms & Manifold Removed

  • Head Off, Trouble in Cylinder 2

  • Pitting in the Head and had been Crack Repaired on Exhaust Valve Seat

  • Pistons 1 & 3 looked OK

  • How far will this go?

  • Misc parts removed

  • Leaking Sea Water Pump

  • Salt Dripping on Engine from Leaking Pump

  • Piston Parts (replaced 2,3,7,9)

  • Cut Away View

  • Mixed Rod Caps on Pistons 2 & 3. Whoever rebuilt this before was lucky it ran at all.

  • Connecting Rod Bearing Damage, Needed Replacing

  • Rod #1 Bearing was better

  • Piston 2

  • Cylinder Liners Look Good

  • Crank Looks Ok

  • Crank Looks Good

  • Carl Stirtz Machine Shop, Oakland CA

  • New Head (note cap on pre-chamber)

  • Rebuilt Sea Water Pump

  • Deglazing Cylinders

  • Deglazing Cylinders

  • Ready for the Pistons

  • Pistons Going Back in with Correct Rod Caps

  • First Coat of ($30/can Westerbeke) Paint

  • Head Gasket in Place

  • Head Back On

  • 18 Bolts on the Oil Pan

  • Just enough room to install the pan

  • Injectors & Valves Ready to Go

  • Sea Water Pump going back on with Globe Run-Dry Impellor

  • Back Together & Ready to Run

  • Compression tests show it's back to the factory spec, 450psi!

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