Hi Guys,
I am building a 3.0 liter M20 and I decided to share my experience so far. I would like to note that I am no expert in engine building so if someone has good (or not so good) advice I am opened to opinions.
The target: M20B30 engine using only (or at least mostly) BMW parts, with least modifications possible. I want to achieve high comp engnine (10.5-11:1) that would be with internals that are as light as possible so that it can rev as high as possible.
So far the internals I am using are the following:
M20B25 IX block (I had this one lying arround)
M54B30 Crank (89.6 stroke)
M54B30 Conrods (135mm)
M54B25 Pistons (KH:27.8 - these are the shortest bmw pistons I could find - 0.5mm shorter KH than M54B30 pistons)
After measuring the deck height it turned out that this one is the tall one, however due to previous overheat the block surface was not flat, so I had to shorten it a bit.
The result is 206.2 deck height. As a result the above mentioned internals is the pistons sticking 1.5mm over the deck.
The piston is 6mm thick at its thinnest place (the center), so I figured that it can be shaved with 1mm so that the piston will be only 0.5mm over the deck (since the HG is 1.75mm thick it should be able to handle it without a problem).
At the first assembly the crank could turn without any issues in the block (crank only)
I have seen some people mentioning that the oil pump drive needs to be shaved a bit, but in my case there were absolutely no issues. Go figure.
The first issue was when I assembled one piston and a conrod and connected them to the crank.
The conrod was hitting the side of the cylinder, it was barely touching it but when the engine is warmed up there will be issues for sure. So here is the solution I cam up with:
I took off some from the cylinder wall, roughly 2mm in each direction. After measuring with some clay now it has 2mm+ on each side from the conrod to the cylinder wall, so I am happy.
I had 2 options for the piston modification:
here they are: (the 2 sides of one test piston)
This option is the cleaner and better looking in my opinion, the downside is that there are 3mm distance from the top of the piston to the first compression ring so it will transfer alot of heat to the piston rings.
The alternative design is the following:
This way there is more metal to the piston ring, however this design turned out to be worse for 2 reasons:
1 - more sharp edges, and in my case since this will be high compression engine it will be an issue.
2- the piece of metal at the end of the piston is too near the valve and it might cause a problem at high RPM.
So bottom line is that I decided to go with the first design for both sides of the piston (intake and exhaust)
Here is how the piston looks like at TDC:
One more issue that I faced, is that the spline on the M54 cranshaft is a bit thicker than the M20 one and I had to machine the spocket that holds the timing belt, nothing too major.
Regarding the head, I am using stock 885 head, with no modifications so far with catcams springs, vac motorsport retainers and reground camshaft to 280 degrees with 10.7 lift
Nothing interesting there.
This is my progress so far. I am waiting for the machine shop to do all the pistons so that I can start assembling the bottom end.
I am building a 3.0 liter M20 and I decided to share my experience so far. I would like to note that I am no expert in engine building so if someone has good (or not so good) advice I am opened to opinions.
The target: M20B30 engine using only (or at least mostly) BMW parts, with least modifications possible. I want to achieve high comp engnine (10.5-11:1) that would be with internals that are as light as possible so that it can rev as high as possible.
So far the internals I am using are the following:
M20B25 IX block (I had this one lying arround)
M54B30 Crank (89.6 stroke)
M54B30 Conrods (135mm)
M54B25 Pistons (KH:27.8 - these are the shortest bmw pistons I could find - 0.5mm shorter KH than M54B30 pistons)
After measuring the deck height it turned out that this one is the tall one, however due to previous overheat the block surface was not flat, so I had to shorten it a bit.
The result is 206.2 deck height. As a result the above mentioned internals is the pistons sticking 1.5mm over the deck.
The piston is 6mm thick at its thinnest place (the center), so I figured that it can be shaved with 1mm so that the piston will be only 0.5mm over the deck (since the HG is 1.75mm thick it should be able to handle it without a problem).
At the first assembly the crank could turn without any issues in the block (crank only)
I have seen some people mentioning that the oil pump drive needs to be shaved a bit, but in my case there were absolutely no issues. Go figure.
The first issue was when I assembled one piston and a conrod and connected them to the crank.
The conrod was hitting the side of the cylinder, it was barely touching it but when the engine is warmed up there will be issues for sure. So here is the solution I cam up with:
I took off some from the cylinder wall, roughly 2mm in each direction. After measuring with some clay now it has 2mm+ on each side from the conrod to the cylinder wall, so I am happy.
I had 2 options for the piston modification:
here they are: (the 2 sides of one test piston)
This option is the cleaner and better looking in my opinion, the downside is that there are 3mm distance from the top of the piston to the first compression ring so it will transfer alot of heat to the piston rings.
The alternative design is the following:
This way there is more metal to the piston ring, however this design turned out to be worse for 2 reasons:
1 - more sharp edges, and in my case since this will be high compression engine it will be an issue.
2- the piece of metal at the end of the piston is too near the valve and it might cause a problem at high RPM.
So bottom line is that I decided to go with the first design for both sides of the piston (intake and exhaust)
Here is how the piston looks like at TDC:
One more issue that I faced, is that the spline on the M54 cranshaft is a bit thicker than the M20 one and I had to machine the spocket that holds the timing belt, nothing too major.
Regarding the head, I am using stock 885 head, with no modifications so far with catcams springs, vac motorsport retainers and reground camshaft to 280 degrees with 10.7 lift
Nothing interesting there.
This is my progress so far. I am waiting for the machine shop to do all the pistons so that I can start assembling the bottom end.
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