Another M20B30 Stroker thread

[ZoomeR]

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.

[87mtech27]

Awesome and informative thread, look forward to updates on the build. Btw what bhp figure do you hope to achieve with this build?

[bab-91]

Subscribed!

[ZoomeR]

I am hoping to be able to pull out around 220-ish hp at the wheels, but I am not sure if the cam is aggressive enough for that.
At the end of the day whatever happens. I think that it will be pretty fun engine to drive, having in mind that the pistons are the litest stock pistons and same goes for the rods. I am also planing on using lightweight flywheel (5.8kg - lightened from 8kg or a stock 6.3kg)
I will keep the thread updated. If everything goes according to plan, the engine should be on the road by the end of January.

[ZoomeR]

A quick update, just got the pistons from the machine shop:






I am pretty happy with the result. I asked them to leave more material on the lower side, since as per my measurements I had enough space between the valve and piston and decided to leave as much meat as possible on the piston.

And finally here is the weight of a piston (with the wrist pin and piston rings)
I am very pleased with the weight.
All pistons are within 1g difference



I already assembled the crank in the block, and I will put in the pistons and conrods probably tomorrow.

[ZoomeR]

Now some progress from today.

Bottom end is mostly assembled. Today I installed the pistons, also wanted to be sure for the lift and the piston shape, so I did a test with some clay inside the combustion chamber - there is at least 2.4mm between the piston and the intake valve at any time:


All the rotating assembly seems fine, but just when I was about to install the head, I checked it and it turned out that I did an epic fail - I assembled the wrong head (I am keeping this head for my turbo build), so I had to call it a day. I will assemble the proper head and install it tomorrow. And I will be just waiting form some minor stuff from the bmw dealership (gaskets mostly) to finish the engine.

Some pics of the pistons inside the block:




The piston at TDC

[RB2]

subscribed, keep the posts coming bud. I am toying with the idea of stroking a m42 I have in the shed and fitting some ITBs. it would be my first big build so your thread will be golden

[ZoomeR]

Quick update so far.
I bolted the head to the block and started to assemble some of the other stuff.
Here is a mock up how the Hartge headers will look.



Initially I was not planning to paint the block, but it looks fugly so I will take my time in the next few days and take care of that.

[ZoomeR]

some more stuff from today:

[ZoomeR]

Some progress from today (and the past few days)

Here is the engine assembled mostly:






The oil pan after a couple of hours of scrubing:




Here is my home made M50 to M20 TPS adapter (I used an old M20 TPS and cut it a bit)





Unfortunately still pointing in the wrong direction




And finally today I decided I am ready to drop the engine.




Final progress for the day (still quite a few things left to do but I am quite happy with the progress)




So tomorrow will be the big day when I should be able to do the first start.

[Gavt]

Any more updates on this

[HairyScreech]

I am surprised every one has not been jumping on this as the "wrong" way to do things, seems in the days of billet custom machined everything we have forgotten how hotrods used to be built.

Only real comments are that you may find you cannot get enough heat out of the piston crown thanks to the small top ring land.
Your right the pistons are ~6mm thick, the B30 and B28 ones are also about the same.
After speaking to a lot of people the rule of thumb is about 5mm thick on a 100mm bore. This is simply about heat build up in the centre of the crown.
At 84mm you might get away with 4.5mm but there's only one way to find out.
If this bottom end pops then perhaps a thermal barrier coat may give you the margin you need on the next bottom end.

The other thing would be that you would have been better cutting the piston crown at a 21 degree angle around it's perimeter to match the head, it would reduce the possibility of a nasty crevice volume lurking at the edge of the chamber.

Run it and see how it goes, if it pops then build another. Its not like these things are rare.