M50 block dilemma. Run it as is?

[make it easy]

Hello friends. I am facing a dilemma. My M50 block is in a condition that has been characterized as not very good by the specialized engine shop where I decked my head. The guy I spoke with has been very honest with me after I told him my "story", trying to bring some context about the car. He genuinely suggested to clean the block to the best of my abilities and run it as is, because fixing it the right way will be expensive.

I appreciated his honesty, and I wish could have discussed details with him, but I didn't want to steal more of his time (that I must say he kindly spent with me at no charge). And he's a very busy guy, work orders scheduled for 2.5 months ahead, people want him personally to work on their engines (but it's all american engines I must say).

So why did he suggest to run the block as is? Well, because I told him that I got this car (1994 E36 325ci) in a very good body shape, and not bad mechanically either, low mileage (110K miles as per odometer), brand new roof for... $400 USD. Honestly, this was just a symbolic price. I bought it from my boss/friend who is 50+.
So first thing I did is store the car and prepare for Auto-Manual swap. Well, it's been stored for over 5 years. As life happened, I did not get a chance to work on it, until last year.
As I started to work on the swap, I got so much into it, that the transmission swap turned into a complete engine pull, replacement of all engine gaskets, other bad items as I go, vanos fix etc. I've never done it myself before, I've never gone this far, but my hands are growing from the right place, and I am enjoying the process.
Last thing I committed to (recently) was head gasket replacement, head machining, cleaning valves, replacing guide seals. The whole extended project was still within my budget, all transmission parts + $2000 for all seal and gasket kits, belts, accessory components, etc,etc,etc.

But then I wasn't expecting to see this...

(Continued in the comments section below...)

[gdavid]

I find your screen name ironic

[make it easy]

First thing I noticed was pitting corrosion in cylinders. Very light spots that I can't even feel with my finger, and then 3 other spots that were sticking out. I cleaned those 3 spots (see the pic with the cylinder with shiny spots), nothing sticks out anymore, but I feel the very light protrusions (in depth, not humps).
- The shop guy said to use Scotch-Brite to clean it all. This would not remove metal and would not affect cross-hatching. The black marks left in protrusions will actually help seal, but there's still a chance of oil consumption because of that.

Proper way to fix that would be honing, if it wasn't for these factors:
- The top edge of the bore gas a ridge (area that never comes in contact with the piston rings). You can feel it with the fingernail pretty well. If the block is disassembled for honing you'd most probably require new piston rings anyway. The problem is that new piston rings are new and have more material. If the ridge is not removed (honing will not remove it) then the new ring (top one) will come in contact with the ridge and break. The old top ring is worn on the other hand, it developed that ridge in first place so now their edges match perfectly and there is no contact.
- So to remove the ridge, we must re-bore. Removing that ridge and matching to existing cylinder width is next to impossible. Plus he told me that the cylinders seem to have developed an elliptical shape by 0.003 of an inch (3 thou) on the exhaust side. So if we re-bore, then we must address this as well. This means new pistons, rings, and me effectively selling one of my kidneys on the black market.
- There is no guarantee that going down the rabbit hole will reveal other issues that will require attention and might as well be fixed once the block is disassembled.

I need your opinions on the best and worst case scenario with this block if I clean the hell out of it and run it as is.
What can be done to this block to get as close to the best case scenario as possible?


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I find your screen name ironic

This is my nickname from IT world, which is my professional life. Working on cars is just my hobby, noob here. But I've always felt that a good context and details are key to proper troubleshooting. That being said, I can't deny that sometimes I get carried over with my storytelling lol)). Thanks for the comment, it made me laugh!

[gdavid]

For the level of effort you have in, it is probably worth it to replace the block. This is a pretty rare failure, you should be able to find a m50 or m52 with a warped head for 250 or less, strip the block, hit it with a dingle berry hone and keep moving.

[make it easy]

For the level of effort you have in, it is probably worth it to replace the block. This is a pretty rare failure, you should be able to find a m50 or m52 with a warped head for 250 or less, strip the block, hit it with a dingle berry hone and keep moving.

Thanks for the input. Are you referring to the ridge, elliptic deviation by 3 thou, or rust as rare failure?

[gdavid]

The pitting, but the ridge is more pronounced than one would expect at 110k miles.

[make it easy]

The pitting, but the ridge is more pronounced than one would expect at 110k miles.

I am suspecting that the pitting is due to storage for such a prolonged time, 5 years. Never ran the engine during this time, the condensation might have formed on the cylinders that had valves in opened positions.
As for the ridge, I am not entirely sure how it's possible, since the bore was out of spec on the exhaust side bu 3 thou only, so 84mm + 3 thou. Wondering what's the diameter of the top "circle belt" then...

[spyderg0d]

Nonsense. The ridge can be reamed, but isnt a nessessity if you dont remove the pistons. The 3 thou of the bore shape, is on the edge of acceptable, if the engine held compression, its fine, use a 10 40 oil. There was an ebay seller that sells bmw engine pistons standard or oversized with wrist pins and rings for about $400. I think they are m54 pistons, but the difference between m54 and m50tu piston top dimension wont create any negative effects in a obdl systems car. I use them on my stroker engine, people use them as cheaper more readily available replacement for m50 parts within circumstances. Thats if indeed you wanted all the block work done. But realistically, you probably couldve boaght 2 used running engines from auto recyclers with warrenties for 2 grand.

[make it easy]

The 3 thou of the bore shape, is on the edge of acceptable, if the engine held compression, its fine, use a 10 40 oil.

Can you help me find the specs for M50, including maximum out-of-round, max taper, exact bore diameter? Thanks.

On a side note, is it possible to test the block for possible leaks when the engine is out and almost apart? I would assume that least I need installed is the cylinder head with all valves closed and then blow some air through the spark plug port?

[Eric93se]

Now that I can see the images, I would definitely leave it alone. You can polish the piston tops removing the coke that's on there, get a non-metal polishing wheel something by 3m.

Don't waste you money trying to make it perfect.

[Eric93se]

Something like this, but do some more research to find even better:
https://www.amazon.com/Goodson-3M-Ro.../dp/B0009RMAFM

[spyderg0d]

Brass brushes to clean pistons. Just take your time and dont dig into the aluminum pistons,. The brass wont harm the piston pr the iron block.

[tjm3]

Can you help me find the specs for M50, including maximum out-of-round, max taper, exact bore diameter? Thanks.

Try this. Same engine as the 525i of the same year.

[make it easy]

Thanks gentlemen!

Try this. Same engine as the 525i of the same year.

Holy crap, 0.005mm roundness deviation??? Is this a typo?? That translates into 0,2 thou!

[tjm3]

That's right out of the BMW docs. The thin, low tension rings and ring materials require accurate machine work and don't break in same as our granddaddy's Chevy.