"OM NOM valves," said the pistons

[jfj707]

I bought a wrecked E36 (M50B25tu) to part... got some great performance stuff from it, but really wanted the engine since I knew it had been rebuilt.

I also knew it had been running well enough to wrap it around a tree.

Well, I took the head off yesterday, and I got a nice surprise - the block had been bored to 86mm, with what I assume to be S50 pistons. Well, too bad the pistons apparently kissed the valves, and I am not sure when or why. Apparently the compression was good enough for it to run OK, but there are several bent valves.

What could have caused this? Timing wrong? connecting rods too long (140mm) for the S50 pistons? Could the chain have skipped at all during a wreck?

Is there any value or usability in the head with a few bent valves or the short block, or are these pistons trashed? I know the intake side has machined indents, but the exhaust side of the piston's indents are pretty jagged, as you can see in the pics.

http://www.e34pickandpull.com/IMG_1403.JPG
http://www.e34pickandpull.com/IMG_1404.JPG

[strad]

An overrev can do that. That's why they call it a "money shift" when you downshift from 5th into, say, 2nd instead of 4th. It's caused by valve float, which is basically the valves springs are not able to close the valves quick enough at high rpms to avoid the piston.

edit: I think the pistons will be fine. People re-use M20 bottom ends all the time with evidence that the valves and pistons have made contact, usually from a timing belt failure. The head could be another story, but a good inspection will tell you if the casting has suffered any damage.

[jfj707]

What could be done to the pistons to ensure no "hot spots"? That is really what I would be worried about.


Thanks - I did not realize that about valve float.

[HE53]

It was an over rev.. or money shift, but it was a lucky one that it wasn't over revved too much and it didn't break a valve. Shouldn't be anything to worry about on the pistons, but I'd replace your rod bearings as they probably took some abuse when that happened.

[jfj707]

Can the pistons be machined at all at the impact points?

[kalevera]

Clean them up and take another pic? I'm not seeing much of anything to worry about on the pistons, but it's hard to tell with all of the junk in there.

[Stir Fry A Lot]

Marvel Mystery Oil is great for cleaning piston heads. Just pour enough into each cylinder to barely cover the entire surface and let it sit for about 5-10 minutes. Then just clean off the tops with a mildly abrasive scrub pad or air tool.

[jfj707]

http://www.e34pickandpull.com/IMG_1406.JPG

[ducky328i]

yeah, those are definitely not supposed to be there. i don't see where they would hurt anything to be used as long as their structural integrity wasn't compromised ie...cracks in the piston mainly...maybe jsut smooth out the top of them a little...

[Stir Fry A Lot]

I personally don't think it looks too bad but my knowledge is limited. Gl with trying to salvage it.

[PDXBMW]

I've seen worse pistons reused in engines. The main thing you want to make sure of is that all of the high points around the indentations are ground flat. The high points are what would cause a hot spot on the piston resulting in detonation. In that last pic it looks like the valve possibly may have contacted the sealing surface also. Also make sure that there was no damage to the skirts or side of the piston which could score the cylinder. You might have gained 1cc of displacement! :D

[jfj707]

Cylinder walls look fine (and I think the engine was running like this for a while), but if I build this engine I will have them honed and deridged anyway.

Should I have a machine shop grind them, or can they be very carefully filed by hand? This is piston #6, and it is the worst one.

[T444E]

Well was the money shift what caused the accident?

And the pistons look like a smiley face, an Asian one ^o^

[jfj707]

Well was the money shift what caused the accident?

And the pistons look like a smiley face, an Asian one ^o^

Alcohol caused the accident. This dude's twin brother got loaded and wrapped the car around a tree.

[Stir Fry A Lot]

Its always the twin :rofl

[Binjammin]

From the looks of those pistons, they've been double struck. Once when the valve was open and the stem was bent, then again at least a second time. I'd say considering it's just an m50tu, you're going to spend way too much money rebuilding that engine and not get anything out of it. You can score a good, running m50 for ~$400 these days, the machine work and parts to rebuild that wrecked thing will far exceed what you'll put into it.

[jfj707]

M3 86mm pistons - that's what I was contemplating what to do with the short block. Pistons look no good to you, Ben?

[PorscheTech911]

Them hits on top of the cylinders= more displacement. Haha more CC's now

[nmp411]

To combat what everyone is saying about the pistons, it is NOT a good idea to run them as is.
The nooks and crannies can cause ignition hot spots which can lead to pre-ignition and detonating. Since you are that far now, I think you should just get them fixed.
But nice find!

[miekedmr]

It's like any other metal surface. You can remove the sharp spots with a dremel, sandpaper, whatever. The very minimal change in displacement and chamber geometry will not cause any issues, as long as the damage areas are smoothed out so they don't cause hot spots.
I'd be more worried about the condition of the bearings, really.

[Binjammin]

Clean them up and take another pic? I'm not seeing much of anything to worry about on the pistons, but it's hard to tell with all of the junk in there.

Pretty sure the intake valve reliefs are meant to be there, but the exhaust valve reliefs are not.


M3 86mm pistons - that's what I was contemplating what to do with the short block. Pistons look no good to you, Ben?

I've reused impacted pistons before, but those look like they've had some serious impact. I'd say it all depends on what you're looking to use the motor for, but for the time and effort, just get another motor.


It's like any other metal surface. You can remove the sharp spots with a dremel, sandpaper, whatever. The very minimal change in displacement and chamber geometry will not cause any issues, as long as the damage areas are smoothed out so they don't cause hot spots.
I'd be more worried about the condition of the bearings, really.

You're hardly going to impact chamber geometry and volume, but it can seriously affect balance. I'd hardly worry about the bearings, you're talking about a few small hits on the face of the piston, then all the valves are bent and no more impacting. It wouldn't hurt to pull a few caps to inspect, but I wouldn't wager on there being a huge amount of damage from it.

[miekedmr]

Pretty sure the intake valve reliefs are meant to be there, but the exhaust valve reliefs are not.

Oh wow, I thought all four reliefs were SUPPOSED to be there, and they were just a little misshapen from the impact.
I guess I should have looked closer!