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bad piston


slowpoke

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Bought a used set of pistons on this forum recently  (looking for a set for a rebuild i'm preparing for) and learned a couple things and i have a question.  What i learned:

 

1) before you buy pistons ask that they be cleaned up with good pics faces and skirts

2) before you buy ask that all the rings be removed and get pics of the ring lands

 

Three of the pistons looked good, rings i could get off, no excessive carbon deposits, etc. etc.  The fourth, however, clearly the offending piston which is why they were removed from the donor engine, had frozen rings, and lots of carbon on the face.  Well, after a bunch of soaking w/ PB Blaster i've gotten 2 of the three rings out (the middle ring is still being stubborn), but more importantly, the carbon on the face came off and this defect, divot, (see below) was exposed.  the question:  is this piston usable in any way?  the knick's clearly been there a while, notice the embedded carbon deposit in the interior of the divot matching the surrounding piston. 

 

This is my first rebuild, but i'm thinking this is a goner.  Too bad cuz new pistons seem expensive. But if this is bad, this is my second bad set that i've bought on the forum.  the first set was good but got banged up in shipping (lots of skirt scuffs due to insecure packaging rendering them useless.  Thankfully the seller refunded me the base cost on the first set.

 

any other solutions?  do people sell single pistons?  I called IE and they said minimum order was 4....  so i could buy 5 but not one.  ;)

 

thanks!

 

piston big.jpg

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Eh, feather out the sharp edges and run it.

The head would have taken the brunt of that!

 

I have yet to find a used set of BMW pistons that have the 

ring lands in decent shape, but I DO buy junk...

 

t

now working on a 408 stroker

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"I learn best through painful, expensive experience, so I feel like I've gotten my money's worth." MattL

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When I dismantled my 73's engine at 204k miles, I discovered something (it's not metallic, and I think it's a piece of spark plug insulator) firmly embedded in one piston.  Based on the carbon deposits around it, that piece had been living in its piston for some years--with no detrimental effects.  I dismantled the engine because the head gasket blew in three of the four cylinders; other than that it was running fine, just an unnatural thirst for coolant.

 

+ 1 on Toby's suggestion, with the additional suggestion that since you'll be removing some metal from the damaged piston, weigh all four pistons to make sure their weights are as identical as possible.  You can even weights out by removing a little bit from the piston skirt.

 

mike

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'69 Nevada sunroof-Wolfgang-bought new
'73 Sahara sunroof-Ludwig-since '78
'91 Brillantrot 318is sunroof-Georg Friederich 
Fiat Topolini (Benito & Luigi), Renault 4CVs (Anatole, Lucky Pierre, Brigette) & Kermit, the Bugeye Sprite

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Ouch, you can see the valve outline on the right side of that crater. Cant tell if its a piano or bathtub top from the pic but i suppose that doesn't matter really.

I don't think I would be comfortable with that damage but if it has some thickness there some folks might.

There are other issues with used pistons that most cases, make them a risky proposition to buy.

First and foremost is wear in the ring grooves from the hard rings. They tend to wear beyond limits there and they just can't be renewed.

Recommend you check clearance there on all of them before going any farther.

Got a Haynes manual?

Engines are like onions, many layers that can cost $$$$ and bring tears to your eyes.

Edited by tech71
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  • Haha 1

76 2002 Survivor

71 2002 Franzi

85 318i  Doris

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If the ring lands are slightly oversized total seal piston rings will custom make rings to return to the original clearance the only question is how much wear can be off set and still retain the original strength of the land?  

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If everybody in the room is thinking the same thing, then someone is not thinking.

 

George S Patton 

Planning the Normandy Break out 1944

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If pistons get scrubbed to clean them up it might hide the scuffing that takes place on the skirts if a piston has been overheated and too much cleaning may not make sense.  I though the first pic was a piano top.

Edited by HBChris

HBChris

`73 3.0CS Chamonix, `69 2000 NK Atlantik

`70 2800 Polaris, `79 528i Chamonix

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On 2/16/2020 at 8:50 PM, Son of Marty said:

If the ring lands are slightly oversized total seal piston rings will custom make rings to return to the original clearance the only question is how much wear can be off set and still retain the original strength of the land?  

Super cool I didn't know that was a thing. ...   Thanks

 

"Goosed" 1975 BMW 2002

 

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12 hours ago, HBChris said:

If pistons get scrubbed to clean them up it might hide the scuffing that takes place on the skirts if a piston has been overheated and too much cleaning may not make sense.  I though the first pic was a piano top.

It may be just the angle of the picture.

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