Rebuild M20 -- keep the block or get a new one?

[aeos42]

I have a 325i with the original motor (M20B25) with 220k miles. It runs okay for now but I'm looking to do a full rebuild/refresh of the motor, with maybe some mild upgrades.

For convenience I was thinking it might make sense to buy another M20 (maybe with lower miles?), rebuild it and then swap it in. But does this make sense? Mostly I'm wondering if I get any advantage of getting a lower-ish mileage M20.

Does the condition of the motor generally matter if you're going to rebuild it anyways? I might consider keeping the one I have if there's zero advantage to going out and finding a lower mileage block.

Thanks!

[e30davie]

Its always nice to start with a known entity for a rebuild. If you know yours works and isnt catastrophically broken i would rebuild that. On the flip side, you can get a second motor rebuild that, and just swap it over in a weekend so you can take your time on the rebuild and keep driving your car. But back on the flip side (the original side?), some people might like the idea of keeping the original motor in the car. I don't care too much for that, but some people do.

[digger]

if you are going to overbore then it doesnt matter much. if you are going to rehone only then the one with bores in the best condition will be best ( that might not be the lower mileage one but it usually is).

[zaq123]

Rebuild M20 -- keep the block or get a new one?

Don’t get “new” used block ...especially since you won’t know what you end up getting until the disassembly anyway. Unless one is already stripped and you can measure it before the purchase. Plus it won’t make a financial sense IMHO anyway...to bore your block shouldn’t cost as much.
Bore your block to + size pistons and call it a day. I think I’ve seen brand new set of OEM +size pistons going for around $400.


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[TobyB]

Gimme those $400 pistons! Last I priced they were about twice that...

Me, when spending the money to properly rebuild an engine vs. a 'rering' or other redneck 'build'
I almost always start with a spare engine. It's worth the nominal extra costs
of having the head checked and block magged.
And I've taken several 'known good running' engines apart to find obvious issues anyway..

Then, when it's ready to go in, I make sure
the old one's got all the bugs out of it so
that its exterior parts can go onto its replacement.
(that usually includes carbs, fwiw)

Then if there are problems, I stand some chance of figuring out where they are before
the fresh engine's compromised by trying to get it to run with leaky injectors, sinky floats,
etc etc etc.

works for me
t

[zaq123]

Gimme those $400 pistons! Last I priced they were about twice that...

+0.5mm are $87/each....just checked.

[r-mm]

The chief advantage that comes to mind is being able to work on a motor that is not presently powering your vehicle. I debated this as I was considering either resealing my m20 or doing a swap the advantage of the latter being having all the time in the world to rebuild the donor motor


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[Cayoncrusier93]

+0.5mm are $87/each....just checked.

Link?

[TobyB]

Link?

t

[zaq123]

Link?

t

laziness will cost you a finders fee. deal? 41 in stock

[aeos42]

Thanks for all the info guys. Would love the link to the $400 OEM+ pistons...I saw OEM+ cast pistons from Ireland Engineering for $675, that's the cheapest I saw.

Was also thinking about getting the head from BimmerHeads -- seems like you get a lot of the work done in one package.

In terms of different block vs. my current block -- by the time I'm rebuilding this motor I hope to have another car...but it may not be the case. I'm hoping the motor lasts through next winter and I'm able to make decisions next summer.

Again thanks for the opinions!

[zaq123]

Rebuild M20 -- keep the block or get a new one?

https://www.rmeuropean.com/Products/11251714810-MFG62.aspx

Ebay got them too. I’ve seen them as low as just over 400 at some point. Note that the ring set, wrist pins and c clips included. At $87 a piece....still a deal


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[r-mm]

Mahle loosies...