Aluminum Transfer Scuffing Removal on Outboard Cylinder Sleeves

Aluminum Transfer Removal on Mercury V6 Outboards (2.0L, 2.4L, 2.5L, 3.0L, 3.2L

If you’ve torn down a Mercury V6 and found dull, silvery streaks on the cylinder wall, you’re looking at aluminum transfer—piston material smeared over the bore after a scuff or brief overheat.

Cleaning that transfer correctly is the cheapest “machine work” you’ll ever do: once it’s gone, you can see the true condition of the wall, measure it accurately, and decide whether a hone, re-ring, sleeve, or replate is warranted.

For context: 2.0L, 2.5L, 3.0L, and 3.2L production Mercs are steel-sleeved, 2.4L are typically hard-chrome, while high-performance race 2.5L blocks are Nikasil/NiCom. The F1 2.0L can be either—verify before you treat.

Understanding what you’re working with

Start with identification. A steel or iron sleeve is unmistakably magnetic and shows a darker gray tone with a conventional stone-honed crosshatch. A hard-chrome bore (common on 2.4L) is non-magnetic and looks bright and glassy; crosshatch appears shallow and “diamond finished.”

Nikasil/NiCom (race 2.5L) is also non-magnetic but presents a finer matte silver-gray with razor-sharp crosshatch. If you’re unsure, do a tiny, timed spot test with the appropriate chemistry (10–20 seconds), neutralize, and inspect—better safe than sorry.

Once the bore type is known, the chemistry almost chooses itself. Steel sleeves respond best to sodium hydroxide (NaOH, lye) at 5–10%, because it rapidly dissolves aluminum without attacking iron.

You can use hydrochloric acid (HCl, 5–10%) on steel for stubborn spots, but only as small, timed dabs kept under ~30 seconds—acid will etch steel if you linger.

Hard-chrome bores are the opposite: HCl (5–10%) is the fast, safe way to strip aluminum transfer without touching the chrome, while NaOH remains a slower, conservative fallback.

For Nikasil/NiCom, err on the side of caution and use NaOH as your primary. If you insist on HCl, use pin-point applications of very short duration (15–20 seconds), then neutralize immediately.

Set up the job like a pro

Lay out nitrile or chemical gloves, eye protection, and ventilation. Keep two neutralizers ready: baking-soda solution for acid, white vinegar or dilute citric acid for lye.

Use cotton swabs or a small acid brush to keep applications local, and have paper towels plus a light oil (2-stroke oil or ATF) for post-rinse protection. Mask port windows and crankcase passages with painter’s tape or create small grease dams so liquid cannot run onto bare aluminum.

When mixing, remember the two rules that prevent ER visits: add lye to water, never water to lye; add acid to water, never water to acid. Never mix acid with base, and never mix either with bleach.

Degrease the bore first. A clean surface lets the chemistry touch only the transfer, not trapped oil.

The cleaning process

Work in short, controlled cycles. On steel sleeves, brush NaOH 5–10% onto the aluminum streaks and give it one to three minutes to work. You won’t see lively fizzing like with acid; instead, the transfer gels and undermines.

Wipe the slurry clean, neutralize with vinegar, rinse with water, dry thoroughly, and oil immediately to stop flash rust. Repeat until the wall is uniformly clean. If a streak laughs at you, you can spot it with HCl for ≤30 seconds, neutralize with baking soda, rinse, dry, and go back to lye for the larger areas.

On hard-chrome bores (2.4L), the routine is snappier. Dab HCl 5–10% directly onto the aluminum transfer; you’ll see bubbling within seconds. Keep each touch to 20–60 seconds, wipe clean, neutralize with baking soda, rinse, dry, and oil. If you prefer the slow-and-safe path, you can do the entire job with NaOH on chrome—it just takes more cycles.

For Nikasil/NiCom race 2.5L, stay with NaOH unless you’re extremely confident. When HCl is used at all, it should be micro-localized and very brief, followed immediately by a thorough neutralization.

Throughout all variants, let the chemistry work—avoid scraping that could gouge the base surface. A gray Scotch-Brite pad with oil at the end is fine for removing light staining; don’t chase stains so hard you remove base material.

Family-by-family nuances

On 2.5L, 3.0L, and 3.2L steel-sleeved engines, lye is king. You’ll get predictable results with minimal risk to the sleeve, provided you neutralize and oil promptly. Any black oxide that appears is superficial and will wipe out with oil or vanish during a light deglaze hone.

On 2.4L chrome-bore blocks, HCl is your friend. If the transfer clears but you start seeing dark peppering, voids, or edges that look “lifted,” that isn’t leftover aluminum—that’s plating failure. No chemical can fix it; plan for a replate or a sleeve.

On race 2.5L Nikasil, patience and NaOH preserve the expensive coating. Any sign that the matrix itself is compromised means replating is in your future.

2.0L blocks vary—verify the bore type and follow the appropriate path.

Clean, then measure—never the other way around

Aluminum transfer lies about bore size and surface finish. Once the wall is truly clean and lightly oiled, bring in the dial bore gauge and micrometer. Measure at multiple heights and axes and write down diameter, taper, and out-of-round.

Compare to your model’s specifications. On steel sleeves, plan a light deglaze hone (220–280 grit) if you’re installing fresh rings, and recheck ring end-gap in that bore. Do not attempt to hone chrome or Nikasil—both require specialized abrasive systems and procedures outside normal shop tooling.

Look closely at the ring travel zone. Shallow, smooth discoloration is usually harmless; deep scoring that catches a fingernail is not. In steel, you’ll hone or rebore; in plated bores, you’ll replate or sleeve.

Check port edges for raised lips and knock them back very lightly with a fine stone only if necessary. Finish by fogging the cylinders if the engine won’t be run right away.

When something goes sideways

If transfer refuses to move, lengthen contact time a touch within the safe window or switch chemistries (NaOH ↔ HCl) appropriate to your bore. If you see flash rust on steel after rinsing, remove it with an oiled Scotch-Brite wipe or a couple strokes of a fine hone and oil again.

If you see etched steel, the acid sat too long; a light hone generally restores the surface, but pitting in the ring path calls for a sleeve. If a plated wall looks mothy after cleanup, you’re not staring at aluminum anymore—you’re staring at damage. Budget time and money for replate.

Safety, always

Gloves, eyewear, and ventilation aren’t optional. Keep chemicals off bare aluminum crankcases and port roofs; that’s why you built dams. Neutralize tools and rags before disposal. Label, cap, and store chemicals like you plan to live a long time.

Quick recap you can tape to the toolbox

• Steel sleeves (2.0/some 2.4/2.5/3.0/3.2 production): Clean with NaOH 5–10% for 1–3 minutes, neutralize with vinegar, rinse, dry, oil. Use HCl ≤30 s only as a spot assist.

• Hard-chrome (2.4L): Clean with HCl 5–10% for 20–60 seconds per pass, neutralize with baking soda, rinse, dry, oil.

• Race 2.5L Nikasil/NiCom: Prefer NaOH; if HCl is used, make it a pinpoint 15–20-second touch and neutralize instantly.