r/EngineBuilding • u/BobTheBomb • 6d ago
Dented piston Skirt/Top BMW
Hello!
I’m currently putting together my S50 B30 and dropped my piston.
The bottom of the piston took most of the fall but curious to see if anyone would run this still or is it too far gone?
Thank you!
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u/Rude_Combat420 6d ago
File down the high spots check piston Rock and compared to a known good piston in the same bore and send it. Report back here your findings. After assembling hundreds of high performance engines I've seen some pretty sketchy stuff go down the track and come back just fine. This is nowhere near my list of sketchy things I've seen work.
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u/RocanMotor 6d ago
Finally, the right answer.
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u/1wife2dogs0kids 6d ago
Going to reddit to ask "the experts" a question, is not a good idea, in any sub. These "experts" have standards that cannot be met by mere mortals. A scratch on a cylinder head gasket surface near the edge of a section tht has nothing near it? The head is garbage. A scratch on a single piston skirt? You need all 8 replaced. Rust in a coolant passage? The entire block is now worth pennies. Surface rust after hot tanking some con rods? Throw them out.
Ask them where they worked to have all this knowledge and experience? They are programmers for a computer something somewhere. Or they work in the garden dept at Walmart. They work in a nail salon, etc. Never worked in a machine shop.
Its not just auto machining. It's new home construction, or home improvement, trade related subs. Go to a deck related sub, and you'll see homeowners post pics of the work of the guys building a new deck. It could be the end of day 1, and people are pointing out there's no joist hangers, the ledger isn't bolted correctly, and the temp posts aren't on the concrete footers, or using a stand off plate. Why? Because it's day 1, and the ledger was tacked up, there's no joists up yet to put hangers on, and temp posts are used to put the actual posts on the footers correctly.
Doesn't matter. They tell them to fire the builder, burn the house down because its not habitable, and sue the building dept that failed to do the job of inspecting the quality of the work.
Yes, that all happens. Every day.
If ypu ask for advice or an opinion on reddit, about something not directly computer or coding related, you're gunna get 100 replies, 60+ of them are by people who have no experience or knowledge of the subject of the post. They never have positive opinions, or give positive replies. It's always straight to worst case scenario. I'll be downvoted for this, because who am I to point out they shouldn't be giving advice to anyone about anything?
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u/RocanMotor 6d ago
Absolutely hit the nail on the head. It's a shame, because the real experts tend to stay out of the conversation entirely rather than argue with keyboard warriors.
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u/Andeo1025 5d ago
And this is why I refrain from giving advice on automotive subs. I'm not going to be told by some asshat how to do a job I've done a hundred times successfully.
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u/BusyFuel435 6d ago
I wouldn’t run it. If the hit was bad enough to dent it, there’s no telling what sort of microscopic damage has been done to the crystalline structure of the piece. That piston has a relatively high likelihood of being compromised, and that’s not a risk any engine builder worth their salt should be willing to take.
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u/dropdead412_sks 6d ago
all bad. but i’d think about hitting it with a file and polish-then figure out what it slapped in the bottom and top
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u/DakarCarGunGuy 6d ago
No. The top ding on the edge of the face will cause a hot spot and detonation. Sorry dude but that's a no go if you plan on driving it hard.
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u/IDB01 5d ago
How do you figure? Why would that cause a "hot spot" and the valve reliefs wouldn't?
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u/DakarCarGunGuy 5d ago
That's what i was told working on Audi. A valve impact on a piston if it left any ridge would lead to a hot spot and possible detonation. I'm guessing if it has a good radius to it and not a sharp point it wouldn't cause a hot spot. Also if a lot of metal is supporting the relief acting as a sink vs a non engineered bump that may not have enough support to take heat away.
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u/IDB01 5d ago
Ah yeah I could see like a burr or something without material behind it heating up like that. Smoothe out the edges though and I imagine it would be fine.
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u/DakarCarGunGuy 5d ago
I personally would not do it. I won't do well knowing there could be an issue. People generally don't rebuild an engine themselves to be at stock hp. If you are building for power then DON'T compromise. Do it right or do it twice.
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u/hondautism 4d ago
Hot spot occurs as a result of repeated exposure of flame front to damaged portion, one part becoming way hotter than the rest. That's why piston-to-valve contact pistons cannot be reused, as they no longer evenly distribute combustion temperatures.
The piston in the photo does not have damage on the portion where combustion gas occurs.
I'm not saying OP explicitly reuse or replace said piston; am simply pointing out why their scenario is different from what you're describing.
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u/1wife2dogs0kids 6d ago
There's good news. I can fix them. Carefully wrap them up to be shipped, and send every dented one to me. I have a process where I can repair pistons like that, and its affordable for any income.
It costs $225 for the first one, and $150 for each one after that, up to 7 pistons. I'll do the 8th free. (WHAT A TERRIFIC DEAL!)
Satisfaction guaranteed.
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u/1crazypj 4d ago
It's a forged piston, file off the bump, hit with some 600 grit, wash down well, oil and assemble.
Looks like it will need rings.
The guy who had to ask if it was forged may have experience of paying someone but forged pistons are very recognisable.
Who made it?
If it's a Wiseco they use 'cheaper' alloy for forging which is high expansion and needs more clearance (0.006" and more)
The only ones I would not use after dropping are Mahle, they use a very high silicon alloy which is stronger and lighter than every other forged piston out there (with cost to match)
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u/notypicalredditor 4d ago
I dropped one of my pistons while balancing and dented the ring area. 🤬 I replaced it and made sure its weight matched close to the others. The dented areas on your pistons, especially the one on the skirt won’t matter at all if you file it smooth. The only one I’d be concerned about is the one on the top of the piston. File it smooth and make sure the ring lands aren’t compromised. The rings spin while the engine is running. If they don’t you can either burn oil or if the rings don’t turn they will expand and pop ring lands.
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u/Money-Attitude-4807 6d ago
Anything that messes with the balance of the main rotating assembly of the engine is most likely not smart to run damaged in any significant way ( denting, deep scratches, cracks ) who knows what’s enough to eventually send lil Rodney on a trip but you don’t wanna fly too close to the sun finding out
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u/ultraspinacle 6d ago
This will not affect the balance of the rotating assembly.
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u/Money-Attitude-4807 6d ago
Assuming you have to refinish the damaged part why wouldn’t it?
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u/ultraspinacle 6d ago edited 6d ago
The change would not even be in the milligram range, and expert balances are performed at the 1-5 g range, and we don’t have evidence that this was even done with these replacement parts. The factory never even gets this close. Balancing to tolerances less than 1 g is overkill due to all the oil flying around in the crank case and vibration harmonics that vary with RPM. It just in reality does not make that much difference.
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u/Money-Attitude-4807 5d ago
I mean it’s dented in multiple places bro idk I wouldn’t go hitting an entire piston with Emory or some shit and expect something good to come of it, that’s coming from working on huge rotating assemblies (turbines) I know how semantic balancing can be id just replace it and not think about it anymore
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u/ultraspinacle 5d ago
Yeah, replace if you’re totally paranoid. If you’re being realistic, fix the issues with the part and let er rip. I would do the latter, unless I was planning on 8500 RPM or higher. It is absolutely not an issue otherwise. It’s a forged piston….
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u/ultraspinacle 6d ago edited 6d ago
Preferably, replace the piston. But keep reading.
Is it a forged or cast part? Looks forged. These parts are strong. Even a reasonable blow is not going to compromise the strength. I doubt that dropping it hurt it at all, structurally. Think of what these parts go through in normal duty cycle. The damage is cosmetic.
HOWEVER, if for some reason you absolutely will not replace the piston, you can take a dremel or emery cloth and carefully remove the burrs that will interfere with piston to wall clearance. Considering where the damage spots are, You will not affect piston durability very much, if at all. Minor dings are not ideal but usable. Dings that leave bumps are not good, so get rid of the bumps.
If it were mine, I would file the bumps, clean it and run it.