MyE28.com

Hi guys!

i am planning on rebuilding an M30B28 for my E28. And because there is so many misleading, incomplete or simply wrong info out there on the web i decided to go and try a couple things, document it and verify it through dyno testing.

Since i have accumulated quite a bit of m30 motors and parts i want to try to play with different cam, head, bore, stroke, manifold etc combo's.

It will still be on a budget for, well, financial reasons. The main thing is avoiding having to buy custom pistons cause that skyrockets the price of a rebuild. How can we do that? Overbore the block to accept standard size pistons of the next m30 bore size.

The engine we start of with:

Euro M30B28 LE-Jetronic
86mm bore x 80mm stroke
Compression ratio 9.0:1
Head '1277358' with 58cc capacity (standard head for all M30 from mid 1981, not b35)
2.5/2.8 intake manifold (small runners)
Camshaft specs ??? If anyone can shime in on this with 100% certain information i'm all ears.
Conn rod length 135mm (same on all M30)
Piston compression height 42,5mm
Deck height supposedly 217,5mm
Head gasket thickness 1,72mm


Some more information for you so it's easier to follow:

M30 engine blocks were produced in three different sizes (please note i am only covering blocks in the E28 era, other versions do exist but we do not pay attention to those here). Standard bore sizes were 86mm for the b25/b28. 89mm for the b30/b32/b33. And 92mm for the b34. Generally speaking a block can be overbored 0.25mm and 0.50mm with oem overbore pistons. The limiting factor in overboring these engines lies in the waterjacket placement. Overbore too much and the cilinder walls get too thin or possibly even penetrated = garbage.

The plan:

Overbore the block to 89mm.
Use the same b28 crank with 80mm stroke with standard b30 pistons with new rings.
Standard '358' head with oem b28 camshaft.
B28 intake manifold.
Keep LE-jetronic for now.

About boring the standard b28 block the standard b30 bore size. In theory this should not be possible since bmw redesigned the block for a reason. I have searched tons of forums to try to find info on this. needless to say it was a dissapointing search. I did find one odd comment somewhere about someone who did it without any issues but that's it.
So i'm gonna say f it and let's do it. See what happens.

It goes without saying but all gaskets and seals will be renewed and all parts will be sandblasted and painted or coated so it looks the part. The camshaft along with the rockers i am reusing since they are in good condition. I'm still on the fence wether i should go for new valves but i guess that is overkill for now since m30 valves aren't a known failure point.

So this engine is very basic but will be our baseline for further tests, including an intake manifold swap, motronic conversion, headers, standalone etc. I will run in the engine and then dyno it to see how much power we make. First thing to check will be if it makes b30 power.

The block, crank, rods and head are at the machine shop as we speak. (29/9/22)

I will add photos soon!

First off, welcome to the group!

I haven't bored a 86MM block, and those cylinders are not siamesed in the middle of the block like larger bores, which might cause problems, but I have bored a 89 block to 92 and it came out fine. It was thin, the machinist commented on it, but when I sold the car 175K miles later, it was still running fine.

I’m looking forward seeing the results.
Are you going to test with engine dyno or classy dyno?

The engine shop should be able to sonic check the cylinder walls to verify thickness before boring anything out. That would tell you if there is enough material or not.

Mike W. wrote: Sep 28, 2022 4:38 PM First off, welcome to the group!

I haven't bored a 86MM block, and those cylinders are not siamesed in the middle of the block like larger bores, which might cause problems, but I have bored a 89 block to 92 and it came out fine. It was thin, the machinist commented on it, but when I sold the car 175K miles later, it was still running fine.

Thank you!
And cool! That's what i was hoping to hear from someone

Tiit wrote: Sep 29, 2022 1:46 AM I’m looking forward seeing the results.
Are you going to test with engine dyno or classy dyno?

Me too! And it will be a regular dyno

gadget73 wrote: Sep 29, 2022 9:36 AM The engine shop should be able to sonic check the cylinder walls to verify thickness before boring anything out. That would tell you if there is enough material or not.

I don't think they have such equipment. They have gotten the order so i guess they will check what they are dealing with before doing anything..

Experimenting is always fun.
Following to see how you make out!

Tibo wrote: Sep 29, 2022 1:30 PM

I don't think they have such equipment. They have gotten the order so i guess they will check what they are dealing with before doing anything..

hope so. If they don't have any way to measure the wall thickness, you only find out its too thin after the block is already destroyed.

The block, crank, rods and head are back from the machine shop!

So far all seems well, no problems with boring the block. looks like quality work.






Rods were checked for straightness and cleaned.


Crank checked for straightness and polished. All taps within spec for standard bearings.






The head was pressure tested for cracks. Came out good, just needs to get resurfaced now.


I started preppring the block for the rebuild with cleaning all out the threaded holes with a tap.


Tomorrow i will brush off the outside some more and paint it.
Going to order bearings soon. I will be reusing standard b30 pistons but they will get vaporblasted to clean them up. Put new rings on them and then i can build the bottom end.

The pics don't work for me. Goes to the imgbb page, and has a number by the top that is presumably the pic number, but no image.

Mike W. wrote: Oct 26, 2022 4:10 PM The pics don't work for me. Goes to the imgbb page, and has a number by the top that is presumably the pic number, but no image.

Same here, pictures aren't working.
Even tried to disable my adblocker, but it didn't help.

Mike W. wrote: Oct 26, 2022 4:10 PM The pics don't work for me. Goes to the imgbb page, and has a number by the top that is presumably the pic number, but no image.

oh that's strange for me they work. What's a better image host then? I'm new to this forum thing:)

Tibo wrote: Oct 29, 2022 3:59 AM

Mike W. wrote: Oct 26, 2022 4:10 PM The pics don't work for me. Goes to the imgbb page, and has a number by the top that is presumably the pic number, but no image.

oh that's strange for me they work. What's a better image host then? I'm new to this forum thing:)

https://imgur.com/ Seems to work pretty well, although they will shrink down large images. But it's free and you can post the image, not just the link by right clicking and using "copy image location" then using the forum button for images.

Mike W. wrote: Oct 29, 2022 1:08 PM

Tibo wrote: Oct 29, 2022 3:59 AM

Mike W. wrote: Oct 26, 2022 4:10 PM The pics don't work for me. Goes to the imgbb page, and has a number by the top that is presumably the pic number, but no image.

oh that's strange for me they work. What's a better image host then? I'm new to this forum thing:)

https://imgur.com/ Seems to work pretty well, although they will shrink down large images. But it's free and you can post the image, not just the link by right clicking and using "copy image location" then using the forum button for images.

Updated my post, hopefully you can see the pics now. Please let me know!

Pics now workie.

Tibo wrote: Oct 29, 2022 3:17 PM

Mike W. wrote: Oct 29, 2022 1:08 PM

Tibo wrote: Oct 29, 2022 3:59 AM

Mike W. wrote: Oct 26, 2022 4:10 PM The pics don't work for me. Goes to the imgbb page, and has a number by the top that is presumably the pic number, but no image.

oh that's strange for me they work. What's a better image host then? I'm new to this forum thing:)

https://imgur.com/ Seems to work pretty well, although they will shrink down large images. But it's free and you can post the image, not just the link by right clicking and using "copy image location" then using the forum button for images.

Updated my post, hopefully you can see the pics now. Please let me know!

Cool, now they work!

You're now speaking of stock B30, 3.0L, not boring or stroking it?

You're now speaking of stock B30, 3.0L, not boring or stroking it?

I bored out a b28 block to accept stock b30 pistons. So the bottom end technically will be stock b30. Not entirely sure what i'm going to choose for cam, intake and injectors.

Thanks for the pics.
I like that engine stand.

Tibo wrote: Oct 29, 2022 5:53 PM

You're now speaking of stock B30, 3.0L, not boring or stroking it?

I bored out a b28 block to accept stock b30 pistons. So the bottom end technically will be stock b30. Not entirely sure what i'm going to choose for cam, intake and injectors.

I forgot you bored it. You do realize you can put an 86MM crank out of a B32 or B34 or B35 in there and use B32 pistons for a nominal 3.3? Or are those things just too hard to get where you're at?

In my experience with mostly stock stuff, displacement makes a big difference, but compression does too. With the B30 pistons can you find the 9.5 compression ones or just 9.0? Or late, like E34 3.0 pistons? I would guess they like the B35 pistons are set up for a larger combustion chamber, which would mean higher compression with a B34 head.

Regarding a cam, Devinder did some measuring years ago and found the stock B35 cam to be close to the Hartge 292 cam, and much less cost. He also posted a pic with the cam profiles that might still be here, if you look and can't find it ping me, I have it saved somewhere. In case you're unaware, you can run a Motronic cam in an L jet car with a conventional distributor, you just need the adapter/repair nut that goes on the front of the cam, which the last I knew was still available. That alone might open up more options for you.

Mike W. wrote: Oct 30, 2022 4:32 PM
I forgot you bored it. You do realize you can put an 86MM crank out of a B32 or B34 or B35 in there and use B32 pistons for a nominal 3.3? Or are those things just too hard to get where you're at?

In my experience with mostly stock stuff, displacement makes a big difference, but compression does too. With the B30 pistons can you find the 9.5 compression ones or just 9.0? Or late, like E34 3.0 pistons? I would guess they like the B35 pistons are set up for a larger combustion chamber, which would mean higher compression with a B34 head.

Regarding a cam, Devinder did some measuring years ago and found the stock B35 cam to be close to the Hartge 292 cam, and much less cost. He also posted a pic with the cam profiles that might still be here, if you look and can't find it ping me, I have it saved somewhere. In case you're unaware, you can run a Motronic cam in an L jet car with a conventional distributor, you just need the adapter/repair nut that goes on the front of the cam, which the last I knew was still available. That alone might open up more options for you.

Yes i am aware of the 3.2 option but like you said getting hold of such pistons here is difficult. There are some used sets on ebay but i find that a bit sketchy. Also i have acces to used b30 pistons for free (leftovers from an e9 restoration) and since i'm on a budget that comes in handy. That being said i just took a look at those pistons (e9 3.0csi) and they have a very big dome. Bigger than i have come across so far. I will posta pic of them here tomorrow. So not sure if they will be useable. If they aren't i also have acces to a younger gen b30.

The b34/b35 combo is something i will be looking into later down the line. For now i'll stick to the 3.0.

Regarding the cam i will definetely check that out, hard to come by solid info regarding the cams on m30s. I am aware of the adapter. I am planning on using it so i can start of on L-Jet. Later switch to Motronic to see what gains this would make. Would make for a pretty interesting test if you ask me!

The block is painted. The red plugs looked better in my head than they actually do so i will repaint those black. The whole color scheme will be exclusively black and red.



The reason is it has to match the car it's going into:


Regarding the 89mm pistons i encountered a bit of an orange problem..


They are all seized in the block. At first sight the pistons themselves don't look too bad. I have soaked everything in penetrating fluid. Next i'm gonna try getting some movement i them with a blow torch. If they come out without too much harm and they aren't missing any bits i think they will still be usable. After thorough cleaning and inspection offcourse!

Good looking car, but that block is sure ugly. Not always of course, but pistons can be pretty forgiving, you might come out ok. I'm not sure you want to heat up the pistons, at least not when you're trying to get them out, heat expands things of course and the aluminum of the pistons will expand more than the cast iron of the block. Good luck with it regardless.

this is going to sound strange, but try vinegar instead. It will dissolve the rust and will likely do a better job of freeing it up than oil will. Stronger acids may eat the rust faster, but they may also eat the aluminum pistons.

I believe it may be worth to investigate using the late BMW M30B30 pistons (used in 5er E34 530i and 7er E32 730i - both with M30 engine, note that the same models also came with M60 V8 engine).
Such pistons are easily available in used condition, pretty much anywhere in Europe. More, they are cheap, because nobody wants them - all the enthusiasts want to go with the B35 pistons.
Just for example, have a look at the current Ebay item# 175130458602 (no affiliation).

You must check if there's no interference between the above mentioned M30B30 pistons and the "traditional" (casting# 1277358) M30 cylinder head combustion chamber. The odds are very good for it to work. If that confirms, you would be getting a compression ration of about 9.8:1.
An alternative is to use the late M30B30 cylinder head casting# 1708497, that was installed on M30 engined 5er E34 530i and 7er E32 730i - but in that case the compression ratio will be 9:1.

RobertRO wrote: Nov 10, 2022 4:21 AM I believe it may be worth to investigate using the late BMW M30B30 pistons (used in 5er E34 530i and 7er E32 730i - both with M30 engine, note that the same models also came with M60 V8 engine).
Such pistons are easily available in used condition, pretty much anywhere in Europe. More, they are cheap, because nobody wants them - all the enthusiasts want to go with the B35 pistons.
Just for example, have a look at the current Ebay item# 175130458602 (no affiliation).

You must check if there's no interference between the above mentioned M30B30 pistons and the "traditional" (casting# 1277358) M30 cylinder head combustion chamber. The odds are very good for it to work. If that confirms, you would be getting a compression ration of about 9.8:1.
An alternative is to use the late M30B30 cylinder head casting# 1708497, that was installed on M30 engined 5er E34 530i and 7er E32 730i - but in that case the compression ratio will be 9:1.

Hi Robert, thank you for your insight. It is something i will look into, who knows what i can end up with!

Small update:

I had to give up on the rusty donor block for the pistons. This is how it looks inside, as if someone deliberately degreased it so it could rust.



After spending a couple hours just to get the crank out i finaly realised it was a waste of time. If someone happens to have a set of B32 pistons they would let go for a reasonable price i'd be happy to hear it!

And here is a pic of the surfaced head.



Other parts like bearings and some parts of the timing system came in. But pistons are priority right now so i can at least start building the bottom end..

that bottom end is nasty. Sometimes leaving it gooey and greasy has it's advantages. At work our process generates a lot of ozone, but we also get this disgusting brown sticky gunk that out-gasses from spices. Ozone destroys steel, but thanks to that coating of brown gunk, none of the steel in the exhaust fan or it's ductwork has any deterioration.