M20 questions

[tjclemens2]

Hello,
I’m looking to buy a used m20b25 for a swap and I have a few questions. First off, is there a big difference in performance between the 12v and 24v. I was told 24v came in the 325i and z1 and the 12v came of the 5 series 525i. Also if I buy a used m20, what should I go through and check? Should I tear the whole engine down and do a complete check? I want this thing to look nice and to be reliable but I don’t want to drop thousands to rebuild it if it’s not necessary. Finally what are the major fail points of the M20 just so that when I have it out on the engine stand I can do all of the work that needs to be done to make it super reliable! Thanks!!

[Name]

it is my understanding that all m20b25 engines are 12v. if you are looking for a 24v engine, the M50/52 is where you may want to look. There are plenty of reasons to change to either, but if you are swapping I would recommend m50/52. Not for any particular reason other than they are more modern and easier to find in in better condition.

[tjclemens2]

it is my understanding that all m20b25 engines are 12v. if you are looking for a 24v engine, the M50/52 is where you may want to look. There are plenty of reasons to change to either, but if you are swapping I would recommend m50/52. Not for any particular reason other than they are more modern and easier to find in in better condition.

yah only thing is, I live in California where it is super hard to do engine swaps. Plus I’m 16 and don’t rly have the money for a newer engine like a m50/m52

[Chilezen]

Alright, so why are you looking to buy an M20? What car do you have? Is something wrong with it?

The M20 is a pretty stout engine with few shortcomings. One thing to "replace" (not necessary but useful) are the rocker arms, with a heavy duty set. Ireland Engineering is one vendor of them.
Just get a set of all gaskets and seals from a reliable vendor (I like FCP Euro) and change out all the ones in the kit. That will cover your bases. Buy a book like Chilton/Haynes for most DIY tasks, or spend some bucks for a Bentley manual.

Should you tear the whole engine down? That's up to you and some other trustworthy source to physically determine that.
It's great that you want to be prepared and do things right. I would say to complete the basic maintenance (as you would with any car) and just run it. Parts aren't as cheap as a Honda, but for its age, parts are still available pretty easily. Good luck!

[rudy]

This is weird! I also got my e30 at 16 and I'm also in California, I'm 18 now though and I'm moving to Pittsburgh in a month for college. Mine is a 1986 325e, you can look through my build thread but I basically resealed the engine, refreshed the suspension, and have been beating on it since then. It's treated me amazing and I haven't had any problems with it since I did the work to it. You can definitely rebuild an m20, but honestly you can also just reseal a known healthy one if you're on a budget.

You should look through some other build threads on here though, plenty of people have gotten their 24v swaps legal and they documented the entire process so people like us could do it too. It may give you the confidence you need to do an m50/m52 swap instead of an m20.

Btw, I was at the Chula Vista West LKQ last weekend and there was a 1988 528e there that still had its super ETA m20. ICV, AFM, harness, and everything was still there too, no transmission though. There was also an e30 at Chula Vista East down the street which had an m20b25 block left which you could take the engine arms off of. All of that stuff would give you a good start to the swap if you're set on an m20 instead of anything else, just sayin. Or just check OfferUp, Marketplace, and Craigslist a bunch till you find a cheap m20. It's still possible nowadays!

[McGyver]

Welcome, and good on you for taking on a project!

Definitely buy the Bentley manual (or download it from they DIY section). That is going to have all the technical information and instructions to do pretty much everything. You're also going to need to read a bunch to learn how stuff works. Build threads are a good source of information.

As asked, why do you need an m20? What car do you have?

Being 16 and short on cash, it would make the most sense to either keep your current engine running or to replace it with the same thing. The 6 cylinder cars came with m20b27 engines in the early 325 and the 325e, the 325es had a m20b27 with a fancier cylinder head, and all 325i cars came with an m20b25.

These old engines make sufficient power, are easy to work on, and are very reliable. Unless you know the engine has a problem that should be fixed, just reseal it and change the water pump & timing belt.

As far as engine swaps go, it is possible in CA. The engine has to be from a car that is newer than your car and you have to run all the original emissions equipment (intake to catalytic converter and ECU).

Good luck!

[tjclemens2]

This is weird! I also got my e30 at 16 and I'm also in California, I'm 18 now though and I'm moving to Pittsburgh in a month for college. Mine is a 1986 325e, you can look through my build thread but I basically resealed the engine, refreshed the suspension, and have been beating on it since then. It's treated me amazing and I haven't had any problems with it since I did the work to it. You can definitely rebuild an m20, but honestly you can also just reseal a known healthy one if you're on a budget.

You should look through some other build threads on here though, plenty of people have gotten their 24v swaps legal and they documented the entire process so people like us could do it too. It may give you the confidence you need to do an m50/m52 swap instead of an m20.

Btw, I was at the Chula Vista West LKQ last weekend and there was a 1988 528e there that still had its super ETA m20. ICV, AFM, harness, and everything was still there too, no transmission though. There was also an e30 at Chula Vista East down the street which had an m20b25 block left which you could take the engine arms off of. All of that stuff would give you a good start to the swap if you're set on an m20 instead of anything else, just sayin. Or just check OfferUp, Marketplace, and Craigslist a bunch till you find a cheap m20. It's still possible nowadays!

Are you from SD??? I’m in OB and have been looking for some people in the community. Also I will def check on the pick your part, I have been dying to get out there!

I think I misspoke when saying rebuild the engine, I just want to pull it apart and go through everything and make sure things are working properly. Just pull the heads, have a shop plain and clean everything, then put new gaskets and seals in it and know it’s pretty rock solid! My thought was just to find a m20b25 off Craigslist or somewhere and from there just go through it because it looks like I can pick one up for around 500 dollars!

[tjclemens2]

Welcome, and good on you for taking on a project!

Definitely buy the Bentley manual (or download it from they DIY section). That is going to have all the technical information and instructions to do pretty much everything. You're also going to need to read a bunch to learn how stuff works. Build threads are a good source of information.

As asked, why do you need an m20? What car do you have?

Being 16 and short on cash, it would make the most sense to either keep your current engine running or to replace it with the same thing. The 6 cylinder cars came with m20b27 engines in the early 325 and the 325e, the 325es had a m20b27 with a fancier cylinder head, and all 325i cars came with an m20b25.

These old engines make sufficient power, are easy to work on, and are very reliable. Unless you know the engine has a problem that should be fixed, just reseal it and change the water pump & timing belt.

As far as engine swaps go, it is possible in CA. The engine has to be from a car that is newer than your car and you have to run all the original emissions equipment (intake to catalytic converter and ECU).

Good luck!

just bought a Bentley manual!

The reason for me wanting to swap the m20 is because I have a 85 318i with the m10 and I’m just getting tired of how little power it has. I want something that can get up and go but not too much power. I have a getrag 240 too so I feel pairing that transmission with a solid m20 will be a fun driver.

I also misspoke, I don’t mean fully rebuild the motor with new pistons and everything, I meant re seal. So pull heads and everything and makes sure it’s all working correctly. Having a shop plain all the mailing surfaces and just put in new seals and gaskets. Just so that I will have confidence in it for future use!

[tjclemens2]

Alright, so why are you looking to buy an M20? What car do you have? Is something wrong with it?

The M20 is a pretty stout engine with few shortcomings. One thing to "replace" (not necessary but useful) are the rocker arms, with a heavy duty set. Ireland Engineering is one vendor of them.
Just get a set of all gaskets and seals from a reliable vendor (I like FCP Euro) and change out all the ones in the kit. That will cover your bases. Buy a book like Chilton/Haynes for most DIY tasks, or spend some bucks for a Bentley manual.

Should you tear the whole engine down? That's up to you and some other trustworthy source to physically determine that.
It's great that you want to be prepared and do things right. I would say to complete the basic maintenance (as you would with any car) and just run it. Parts aren't as cheap as a Honda, but for its age, parts are still available pretty easily. Good luck!

Hey, so I have a 85 318i with a m10 and I’m just a little tired of the lack of power especially on the highway. I feeel like just trying to speed up around a car means me having to put pedal to the floor just to gain 5mph.

I also misspoke when talking about my goals, I don’t want to fully rebuild it. I want to reseal, pull heads and everything to make sure the rocker assembly is all good. Then take it to a shop and have them surface all the mating areas then just put new seals and gaskets in the areas that matter. Just so I have peace of mind for future driving

[laidback93]

I think you're on the right track. Find a decent M20b25, address/reseal anything that needs attention and slap that bad boy in.

I will say this. Try to confirm compression is good if possible. I recently swapped in a supposedly low mileage M20 into my E30 and all 6 cylinders are even, but low in compression. It's kind of hard to believe as it runs and pulls strong.

It goes without saying that you'll want to install a fresh water pump and timing belt.

[TobyB]

Apart from the 'bad boy' you're going to want an engine harness, battery harness, and quite likely a C101 kit, since most M20B25 harnesses will have the larger, round
C101, and an 85 is likely to have the early square version.
A 'good girl' AFM, and the plumbing to attach it to the 'bad boy'
Oh, and you want some 'bad boy' M20 engine mounts, a 'bad boy' radiator, an expansion tank (doesn't have to be 'bad boy') and the appropriate hoses.

www. realoem.com is good for figuring out all the differences- stick your VIN in, and then stick the VIN of your donor engine, and compare.

It's a relatively straightforward conversion, as it all bolts together, but there are a few quirks.
Which are all pretty well documented here.

With a 240, be gentle in 1st gear. If you get the drift urge, you'll want to swap to a later diff, halfshafts, and maybe a G370.
For now, it'll be fine.

t

[McGyver]

just bought a Bentley manual!

The reason for me wanting to swap the m20 is because I have a 85 318i with the m10 and I’m just getting tired of how little power it has. I want something that can get up and go but not too much power. I have a getrag 240 too so I feel pairing that transmission with a solid m20 will be a fun driver.

I also misspoke, I don’t mean fully rebuild the motor with new pistons and everything, I meant re seal. So pull heads and everything and makes sure it’s all working correctly. Having a shop plain all the mailing surfaces and just put in new seals and gaskets. Just so that I will have confidence in it for future use!

Well, an engine swap is kind of a big deal and way more involved than just replacing an engine. Your best option would be to buy a crashed car to use as a donor for all the things you're going to need. As mentioned, it's not just the engine, but all the accessories, wiring harness, control unit, instrument cluster, cooling system, etc.

That's a ton of work for ~150whp. Also, a g240 will not bolt up to an m20 because m20's have a different bolt pattern.

If I were you, I would try to find a decent m30b35. That'll get you around 190whp and SIGNIFICANTLY more torque than an m20. Also, your g240 will bolt up to the m30. There's a ton of info on this swap, so you'll need to do a bunch of reading. I built an m30 car and it was an absolute blast to drive.

[It's Soda Not Pop]

Well, an engine swap is kind of a big deal and way more involved than just replacing an engine. Your best option would be to buy a crashed car to use as a donor for all the things you're going to need. As mentioned, it's not just the engine, but all the accessories, wiring harness, control unit, instrument cluster, cooling system, etc.

That's a ton of work for ~150whp. Also, a g240 will not bolt up to an m20 because m20's have a different bolt pattern.

If I were you, I would try to find a decent m30b35. That'll get you around 190whp and SIGNIFICANTLY more torque than an m20. Also, your g240 will bolt up to the m30. There's a ton of info on this swap, so you'll need to do a bunch of reading. I built an m30 car and it was an absolute blast to drive.

I think getting a M20 will be more straight forward for him. He is new at this and I assume not a lot of cash to throw at it.

[McGyver]

I think getting a M20 will be more straight forward for him. He is new at this and I assume not a lot of cash to throw at it.

Having done an m30b35 swap, I don't think putting an m20b25 into an m10 car is going to be significantly easier or cheaper than putting an m30b35 into the same car. Both require you to change literally everything. In fact, putting an m30b34 may be easier than an m20b25. It just has less power and runs an older version of Motronic.