How difficult is it to rebuild an M20?

[Rockardo]

Just wanted a few comments on how hard it is to rebuild an M20, currently Im suspecting that my head gasket is blown but still not quite sure, but apprently its been done once before and if the heads fine and the engine is cracked welp looks like im gonna need a new block. So my question is how realistic is it for me to rebuild an m20 engine for the first time

[JehTehsus]

This really depends on what you have for tools, space, time and money. But I think for most 'garage level' mechanics it is fairly doable if you set aside some money for a machinist, a few months in the garage (life always gets in the way), and maybe a couple hundred bucks for tools you might not already have.

Cheaper to buy another used M20 from somewhere though, so I would only recommend it if you are looking for both the experience and a fresh engine.

[Rockardo]

This really depends on what you have for tools, space, time and money. But I think for most 'garage level' mechanics it is fairly doable if you set aside some money for a machinist, a few months in the garage (life always gets in the way), and maybe a couple hundred bucks for tools you might not already have.

Cheaper to buy another used M20 from somewhere though, so I would only recommend it if you are looking for both the experience and a fresh engine.

Im actually a professional BMW dealer tech but im still relatively green so I still dont have the complete knowledge of building engines entirely, but i definitely have the time and the tools

[JehTehsus]

Im actually a professional BMW dealer tech but im still relatively green so I still dont have the complete knowledge of building engines entirely, but i definitely have the time and the tools

I doubt you would have any issues then - the M20 is not a very complicated motor. My build thread is kicking around here somewhere, and obviously I didn't rebuild a stock motor, but for reference this is the first motor I have ever built and it seems to have worked out fine. IMO if you take your time with it there is no big difficulty at all.

Just make sure if you are working with older parts/reusing as much as possible you have access to a decent set of micrometers, and be prepared to replace stuff if it is out of spec. I.e. maybe after tearing down and measuring you find you need replacement pistons... so build cost and time will increase accordingly. If you are able to handle that then there shouldn't be any issues.

[Rockardo]

I doubt you would have any issues then - the M20 is not a very complicated motor. My build thread is kicking around here somewhere, and obviously I didn't rebuild a stock motor, but for reference this is the first motor I have ever built and it seems to have worked out fine. IMO if you take your time with it there is no big difficulty at all. Just make sure if you are working with older parts/reusing as much as possible you have access to a decent set of micrometers, and be prepared to replace stuff if it is out of spec. I.e. maybe after tearing down and measuring you find you need replacement pistons... so build cost and time will increase accordingly. If you are able to handle that then there shouldn't be any issues.

Well that great to hear, I definitely do need to buy myself a good set of micrometers. I guess I should also be in good hands since I also got the Bentley bible on hand lol

[dvallis]

Read Das Beast for a good "how to". We knew zero about the M20 when starting and built a race engine. Some tips:

Bag and tag everything
Take lots of pictures
Clean the living crap out of everything
Get a copy of Bentleys
Add a REALOEM with your VIN to your Cell phone. Use it often
Don't buy "rebuild kits". Study REALOEM, it shows everything
Replace anything rubber
Replace the stock intermediate shaft timing gear with a sintered gear (BMW P/N 11311717398)
Plastigauge the main and rod bearings
Clay the pistons during assembly to check valve clearance
Zip tie the spring closed when installing the bitch tube, then cut the ties
Take off all the shifter bits before you pull the motor
If rod or main bearings don't move freely, they're WRONG. Check your parts
Pay close attention to the timing wheel marks on the head and crank cover
If the crank doesn't turn over easily with a socket, start over. You bent all the valves
For added fun, balance the rotating assembly with a small scale
Porting and polishing the head ports is pretty easy if you're patient with a die grinder
HD rockers and better valve springs are a good, inexpensive upgrade
As long as you're rebuilding it, why not stroke it to 2.8L? Uses off-the-shelf parts
Don't hot tank the block. Intermediate shaft bearings don't like it
Don't try and pull the intermediate shaft bearing

[Zardog]

I recently rebuilt my M20b27 into a "Stroker" motor and the biggest headache that I had was chasing down all the small vacuum leaks. This motor hates vacuum leaks. I would invest in a smoke tester to ensure everything is nice and sealed or use one from your dealership.

[Andre3127]

Read Das Beast for a good "how to". We knew zero about the M20 when starting and built a race engine. Some tips:

Bag and tag everything
Take lots of pictures
Clean the living crap out of everything
Get a copy of Bentleys
Add a REALOEM with your VIN to your Cell phone. Use it often
Don't buy "rebuild kits". Study REALOEM, it shows everything
Replace anything rubber
Replace the stock intermediate shaft timing gear with a sintered gear (BMW P/N 11311717398)
Plastigauge the main and rod bearings
Clay the pistons during assembly to check valve clearance
Zip tie the spring closed when installing the bitch tube, then cut the ties
Take off all the shifter bits before you pull the motor
If rod or main bearings don't move freely, they're WRONG. Check your parts
Pay close attention to the timing wheel marks on the head and crank cover
If the crank doesn't turn over easily with a socket, start over. You bent all the valves
For added fun, balance the rotating assembly with a small scale
Porting and polishing the head ports is pretty easy if you're patient with a die grinder
HD rockers and better valve springs are a good, inexpensive upgrade
As long as you're rebuilding it, why not stroke it to 2.8L? Uses off-the-shelf parts
Don't hot tank the block. Intermediate shaft bearings don't like it
Don't try and pull the intermediate shaft bearing

A few comments to the above.

Replace the cam gear to a sintered unit as well.

Get the head decked, check for straightness (make sure the cam spins freely in the bore with the head off the block and the rockers removed)

I did guides and a valve job on mine. Highly recommended. New guides also prolong the life of your new valve job if you chose to do one.

Getting new rocker eccentrics helps with valve adjustment later on, and is generally recommended, but is not a requirement. I reground mine using a toolroom surface grinder and a spin fixture, but these are tools many smaller machine shops don't even have.

A 2.7/2.8 build is great if you desire the goals of that engine. Understand that without a seta rotating assembly or a decked block, some undesirable conditions occur. You lose thermodynamic efficiency from a technical standpoint.

Stock head bolts may not be reused, but metric blues are a good replacement for the occasionally NLA head bolts.

Do not change the shape of the ports unless you understand the mistakes you can make easily. In many cases, you can negatively affect certain areas of your powerband if you go nuts. I ported my head, but it was composed of mostly blending the bowls, seats, and guides.

Retorque head bolts/studs after a few hours of run time.