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Old 12-30-2016, 07:10 PM   #1
Ding250SX
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Join Date: Jul 2016
Location: Little Elm, Texas
Posts: 14
M20B25 refresh/rebuild

ok so my m20 is just about toast. i bought my 325i about a year ago. it started and ran but coolant was mixing with oil. i bought the car anyway thinking it was just a head gasket. i took the engine out to reseal all the gaskets. that's when i found a cracked head and 2 popped out freeze plugs. turns out the PO had a cracked heater core and they drove it until the head was toast.freeze plugs popped out because they topped it off with straight water 600 dollars in parts later i put the engine back in the car but it had some kind of piston slap on cold start under light load that mostly went away at operating temp. a few thousand miles later my head gasket starts leaking oil between cylinders 3 and 4(i actually made a post about it not too long ago). fast forward to present day and the head gasket is basically puking and that noise is getting a little bit louder. lucky for me i found someone selling a B25 for pretty cheap ( i don't want to say how much because someone will try to buy it out from under me lol). anyway the engine supposedly has 120xxx miles with recent water pump and timing belt replacements. do you think it's worth the trouble to tear it down and inspect if compression and leak down tests are good? maybe just throw some rings and bearings in there and call it good?
i was also thinking of going turbo one day but that's way down on my priority list
sorry for the long post. Cheers if you read through the whole thing.
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Old 01-01-2017, 04:01 PM   #2
redlightpete
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Join Date: Mar 2012
Location: Greenville, SC
Posts: 222
If you're pretty sure the used motor is good, you might want to leave it alone. 120k isn't that much mileage for these motors, and it is tough to replicate the original assembly quality. Maybe just change the pan gasket and the rear crank seal if it is leaking.
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Old 01-09-2017, 07:32 PM   #3
Ding250SX
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Join Date: Jul 2016
Location: Little Elm, Texas
Posts: 14
UPDATE

I picked up the engine a few days after the original post. it came almost complete. i also got a spare dme, coolant tank, AFM, starter, and a brand new alternator. $200 for everything, but i did have to drive 240 miles one way to get it. engine looked good from the oustdel. the thermostat housing is still all there in one piece. During the tear down i only found a few minor things. #5 rod bearing is showing some wear but the rod journal on the crank is ok. all other bearings have very little wear. cylinder bores also show minimal wear. ring end gap was a little bit on the excessive side. spec is .008in to .016 (yea i know German car should be metric i only had SAE feeler gauges) i consistently had .017 to .018. the oil rings also have a good amount of oil sludge on them.
I will be taking the cylinder head apart and inspecting it within the next few days.









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