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Bon Ami and rings


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I was told by a local mechanic (with years of mechanical and racing experience in Italian exotics and dragsters) that when rings in a new engine just won't seat properly he runs a little Bon Ami down the throat of the carb while running the engine. He does a leak down test prior and after to determine the extent of improvement required and accomplished.

Does this sound nuts or have some of you old hands done the same thing? My rings in the new engine are not seating properly, which is why I'm interested.

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OK - Bon Ami was used by many to seat rings in the past.

That said, I would never recommend this. Bon Ami is an abrasive - Calcium Carbonate.

Aside from clogging your carb, it'll mix with the oil and find it's way into bearing journals, Cam bearing surfaces, etc.

It's true that in the past several engine manufacturers used similar 'break-in' powders such as Mack and Catepillar, but these engines were typically much lower revving and were new fresh metal, not 30 y.o. 'seasoned' metal.

It's been my experience that if your rings don't seat/seal, you're babying the engine too much in the break-in phase. That, or possibly your journal-to-rod clearances are too wide - was this checked with 'Plasti-gauge'?.

Be sure to use organic (not synthetic), non-detergent, single weight oil for the first 500 miles.

How were your rings gapped? Did you have the cylinders honed or cross-hatched?

The most accepted modern engine break-in procedure today involves running the engine at 3000-3500 rpm steadily for 20 min. after which the rings should have sealed, and you can now drive it normally.

Abrasives and other 'magic' powders will not correct improper tolerances. If the rebuild was wrong, there is no fix than to correct it.

Before that, try running the motor under load and generally give your right foot a workout. See if the oil smoke doesn't disappear within a few miles or days.


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Not sure if I agree with using it or not- but it seems to me that if your oil filter is doing its job it would never make it to the crank bearings. And I don't see how it could clog jets in the carb unless you put it in the gas tank.....

However I agree that more info in terms of what type of rings, cylinder prep and break in procedure would be a helpful addition to the post. Additionally how do you know that your rings are not seating? And I have subscribed to the break it in hard method and I achieved compression numbers- but I can't tell you how it will be doing after 250,000 miles....


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My engine is a M10B18 out of a mid-eighties 318i. It was rebuilt by an engine remanufacturer out of Canada that's no longer in business, and I never got the promised specs once the engine was shipped to me. I had the engine for a full year before we actually installed it.

The engine currently has less than 3K miles from new, and while I don't notice any smoke out the tailpipe, I do notice dirty oil and blow-by from the vent tube that's clearly excessive for a new motor. Engine runs like a top and has plenty of power, so this issue isn't significant enough to justify tearing the motor apart to fix. I've been told to just run it out for another 3-5K and hope the rings eventually seat themselves.

One red herring is that when the engine was still in the break in phase, I had an overheating incident in traffic because the temp sensor for the new electric fan got disconnected. I turned the car off before the needle got into the red zone and there was no steam, so I assumed I was in the clear, but perhaps I glazed the cylinder walls in this incident.

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lets return to the original post:

is this a question for a rebuilt motor,

with smoking or high oil consumption?

if yes - the ring caps could also be aligned instead of

offset , and 2 of the 3 ring gaps should face to

the 'up side', intake side of the block when assembling

or was this just a general history question?

i prefer my Bon Ami on the windshield for nasty road

grime removal

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What you are describing isn't necessarily indicative of rings not seating, especially if you have good power and compression.

What is your oil consumption? Have you carefully tracked this? 'Excessive' oil on the breather vent is very subjective and may in fact not involve the piston rings or even indicate anything abnormal at all.

As mentioned, if the ring gap offset is incorrect, this could be the cause as could the conrod-to-crank jounal gap previously mentioned.

The high temp condition is news and may well be a factor.

But, if you have glazed the cylinder walls, especially after 3k mi. running time, the only way to correct this is to rehone and cross-hatch the cylinders, replace the rings with new and start all over again.

Using an abrasive like Bon Ami will result in inconsistent abrasion between the ring edges and cylinder walls and may actually make the issue worse.

The likely effect will be simply an engine which doesn't have the longevity it should before requiring another bottom-end build. Unfortunately, the only remedy is to start over, but as you say, it's not worth a teardown to correct, so you will just have to live with it.


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That, or possibly your journal-to-rod clearances are too wide - was this checked with 'Plasti-gauge'?.

engine rebuilds over the years, and I'm having difficulty making the connection between excessive crank journal-to-rod bearing clearance and failure to have the rings seat in the cylinders (of course, at 63, it may just be advancing senility, too). Help me out - what am I missing here?

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Although it is non abrasive (us old timers remember Bon Ami's logo--a baby chick with the tag line "Hasn't Scratched Yet") and does a good job removing hard water spots from your glass, I still wouldn't dump any in my engine to help with a break-in.

My father-in-law, who started mechanicking back in the 20s would use it to help rings seat in a rebore. but that was in the days of iron blocks and iron pistons, along with poured rod bearings. Nowhere near the tight tolerances of a modern engine, and less than half the revs.

I nearly tried it on a Renault engine rebuild (still have the Bon Ami!) but chickened out and just kept driving it until the chrome-plated top rings seated by themselves. It just took awhile.

Leave the Bon Ami for windshield (and fiberglass tub) cleaning...


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Guest Anonymous

Worked with a guy who built competition motocross bikes. He would use a soapy bon ami concoction to lap new piston rings to the cylinder bores. He would wash and oil everything before final assembly. This would supposedly eliminate the need for lengthy break-in.

Also saw bon ami used in a Datsun 510 with lower than normal compression on a month-old rebuild using chrome moly rings. Far as I know, one teaspoon over 10 min. seemed to do the trick. Of course I would only try it as a last resort over a tear down.

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Guest Anonymous
Just drive it hard under load and see if that helps. i know what you mean by the drity oil from blowby. My old engine had that, the new rebuild does not. From what you have stated it sounds kind of like the rings but we need more info

Kind of like rings? "But we need more info?"

Why don't you specify EXACTLY what info you are looking for?

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