M20B25 Teardown

Written By Ricardo David

Having made the decision to swap out the anemic BMW M10 4-cylinder stock engine in my 320i with a newer and more powerful donor engine it was time to find a M20 6-cylinder engine. If you recall in my last post the M20 engine can drop into my 320i with the right factory E21 parts from a 323i. The M20B25 (2.5L) was the last iteration of the M20 engine put into the BMW 3-series, and more specifically into the USA BMW 325i from 1987 to 1991. My research into the various forums indicated this was the motor to get because of:

  • last generation M20 motor

  • Bosch 1.3 engine management

  • free flowing “885” head

  • the E30 tuning community has a long established history of hot-rodding this motor with tried & true recipes for success

  • stock power at 168 HP / 167 lb-ft of torque @ 4,300rpm (vs 99HP / 100 lb-ft of torque in my 4-cylinder M10)

I first started my hunt searching for a 1987-1991 BMW 325i running donor parts car via Facebook marketplace. However, I quickly established that the going rate for a car like that was $3 to $5k. Getting an entire car would also help me document all the engine systems & harness routing in the process of engine removal. I would be a great educational & documentation experience. However, trying to find a $3k parts car proved very difficult and eventually I realized spending $2 to $4k more than I needed to was foolish. I could simply buy the motor and the extra bits I need and figure out how it all goes together later. That extra $2 to $4k could be better spent hot rodding the motor upon rebuild!

Facebook marketplace is filled with a strange group of people but the sane folks with actual communication skills clearly stand apart from the numerous clowns I encountered. Alex S, with his clear and prompt replies to my questions, quickly established himself as a fellow auto enthusiast who parts out E30s. Thankfully he was only a couple hours away from me in Elk Grove, CA and was able to set me up with a donor M20B25 as well as my complete parts list per Todor’s excellent M20 into E21 swamp guide: http://www.todor.info/repair/e21swap/

M20B25 engine parts for E21 transplant per Todor:

  • M20B25 engine (valve cover, head, block, oil pan)

  • Everything that's attached to the M20 engine, including:

    • Wiring harness and Motronic ECU, along w/ the 3 engine relays (main, fuel pump, O2 sensor) that are attached to the wiring harness

    • Intake manifold, intake airbox, MAF, sensors, ICV, all vacuum hoses

    • Exhaust manifold

    • All fuel injection

    • All ignition parts, coil

    • Starter

    • Water pump thermostat

    • Alternator

    • A/C compressor

    • Do NOT need power steering pump but if easy to keep on, ok

    • The Belt-driven fan from the M20B25 engine

    • Entire throttle cable & throttle cable bracket/assembly connected on top of intake

  • Oil cooler from E30 325i

  • Exhaust downpipe (where the O2 sensor located)

  • Do NOT need transmission, flywheel, clutch

  • Do NOT need radiator, expansion tank

  • Do NOT need fuse box

I shared this list with Alex S and he was like “no problem, when can you stop by?” It was great to hook up with Alex, talk BMWs and he patiently showed me all the parts I requested and was helpful with additional suggestions about my project. A true Restoration Gearhead and he kindly shared his personal contact information if I wanted to follow up with any questions. I had zero issue paying him his fair asking price for everything and in return he carefully selected the best parts from his stash for some of the items on my list.

The next step was engine tear down and documentation. Each part is photographed in place prior to removal with my iPhone, then bagged with its fasteners, and labelled with part name and the date of the photo so I can return to it later for reference in my Photo library on my Mac. Since most of the parts are grimey (oil + dirt), I clean them prior to bagging in my parts washer and then super-cleaned in my 5 gallon Ultrasonic parts washer. (I use ZEP Citrus Degreaser diluted to a 4:1 ratio).

There is a long history of M20 hot-rodding / restoring on the Internet and I simply followed the disassembly procedure in the following outstanding YouTube video (hat tip to Restore-It channel):

So that is how I spent my holiday break after Xmas but before New Year’s and I could not have been happier slowly removing each part, cleaning it, understanding its function, and uploading that knowledge into my brain.

Ricardo David
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320i -> 323i parts