Quick Answer: For most residential wells with drop pipe up to 2 inches, a slide-type no-lead brass adapter like the Merrill MBNL50 (about $60-$70) is the standard pick — it goes through one drilled hole in the casing and lets the pump be pulled straight up for service later. Wells with aggressive or acidic water should step up to a stainless steel version like the Merrill SSMB (about $88-$140). The trap: a pitless adapter isn’t one universal part — it has to match your well casing’s outer diameter, your drop pipe size, and your discharge size all at once, and a mismatch on any single dimension means the wrong part showed up.
A pitless adapter is the fitting buried below the frost line that connects the vertical drop pipe inside the well casing to the horizontal service line running to the house, without needing a person to climb down into a pit to make that connection. Per the Wisconsin DNR, pitless adapters were invented in the early 1950s specifically to eliminate well pits — below-grade concrete or wood chambers, commonly 3 to 8 feet deep, that installers once had to physically enter to join the pipes — and most well construction codes today no longer allow well pits at all. Getting the adapter itself right matters just as much as having one: a mismatched or poorly sealed adapter defeats the sanitary purpose it exists for.
Pitless adapters compared
| Adapter | Type | Material | Best for | Price |
|---|---|---|---|---|
| Merrill MBNL50 | Slide | No-lead brass | Most residential wells (best overall) | ~$60-$70 |
| Merrill SSMB | Slide | 304 stainless steel | Corrosive / acidic well water | ~$88-$140 |
| Baker Monitor 1BTLF | Slide | Lead-free brass | Alternative brand, 1" drop pipe | ~$85-$90 |
1. Merrill MBNL50 — Best Overall
Merrill MBNL50 No-Lead Brass Pitless Adapter
- Slide-type design installs through a single drilled hole in the casing, with compression gaskets on both sides of the wall and an O-ring sealing the two mating halves together.
- No-lead brass construction meets current drinking-water standards and holds up to typical residential well chemistry without the added cost of stainless steel.
- 1" FPT drop pipe and discharge covers the size most residential submersible installations use; a 1-1/4" version (MBNL225) exists for larger setups.
- Lets the pump and drop pipe be pulled straight up for service without digging up or disturbing the female half left in the casing wall.
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The MBNL50 is the safe default for a standard residential well: the common slide-type design, a material that handles ordinary water chemistry, and the 1” sizing that fits most existing installations without hunting for an adapter.
2. Merrill SSMB — Best for Corrosive Well Water
Merrill SSMB Stainless Steel Pitless Adapter
- 304 stainless steel resists corrosion from acidic, high-iron, or sulfur-heavy well water far longer than a brass or bronze body in the same conditions.
- The male portion can be installed facing either direction, giving some flexibility on casings where clearance or existing plumbing limits orientation.
- Priced noticeably above the standard brass MBNL line -- Merrill's own specialty stainless variants (SSMBP, SSMBI) run $124-$140, while the base SSMB runs closer to $88, so confirm which specific series a listing refers to.
- Worth the premium specifically where water chemistry is the driver, not as a general "better" upgrade for a well with ordinary water.
Stepping up to stainless only pays off once the well’s own water chemistry calls for it — a water test showing low pH, high iron, or sulfur content is the signal to spend the extra money, not a hunch.
3. Baker Monitor 1BTLF — Best Alternative Brand
Baker Monitor 1BTLF Lead-Free Slide Pitless Adapter
- Same slide-type mechanism and sizing category as the Merrill MBNL50, giving buyers a second major-brand option when a local supplier stocks Baker Monitor instead of Merrill.
- Listed on multiple state approved-equipment registries (including Illinois and Michigan) alongside Merrill's slide-type line, confirming it meets the same sanitary-seal standard.
- Lead-free brass construction matches current drinking-water material requirements.
- Not a drop-in swap for every Merrill-specific accessory or test-tap fitting -- confirm thread compatibility before mixing brands on the same wellhead.
Brand choice between Merrill and Baker Monitor at this level mostly comes down to local parts availability — both use the same slide-type approach and meet the same approved-equipment standards.
The sizing trap: three dimensions, not one
Most buyers search for a pitless adapter the same way they’d search for a hose fitting — expecting one size to fit all. It doesn’t work that way. A correctly ordered adapter has to match three separate measurements at once: the well casing’s outer diameter (common residential sizes run 4”, 5”, 5-5/8”, and 6”), the drop pipe size descending to the pump (typically 1” or 1-1/4”), and the discharge size of the service line running to the house. Manufacturers publish distinct model numbers for each combination rather than one universal part, and per Merrill’s own installation guidance, a well casing of 5 inches or larger lets the pump and drop pipe be pulled for service without digging up the buried female half at all — a detail worth confirming before assuming a smaller casing works the same way. Measuring the casing’s actual outer diameter, not guessing from the well’s age or depth, is the only reliable starting point.
Slide vs spool: the other adapter decision
Beyond sizing, there’s a second choice buyers often don’t know exists. A slide-type adapter — the kind used in all three picks above — installs through a single hole drilled in the casing wall and is the dominant choice for residential and light commercial wells with drop pipe up to about 2”, per Boshart’s own comparison of the two designs. A spool-type adapter instead sits between the casing and a wider upper barrel, using a two-piece spool-and-housing assembly sealed with O-rings against machined seats, leaving the full casing bore clear and unobstructed end to end. That open bore matters specifically for well rehabilitation or cleaning equipment that needs to pass all the way down the casing — some local well codes require it for exactly that reason, so checking with the local permitting office before ordering is worth the five-minute phone call on any new installation.
Why the connection point matters more than it looks
A pitless adapter sits right at the junction where current well codes replaced an older, riskier design. Per Wisconsin and Michigan’s own well-construction criteria, the connection to the casing itself has to be threaded or welded — a plain compression joint directly against the casing wall is explicitly not permitted. That’s a holdover lesson from the earliest quick-connect fittings of the 1950s, which used simple compression couplings that health authorities rejected almost immediately; modern slide and spool adapters satisfy the rule by sealing with gaskets and O-rings through a properly fabricated hole or barrel rather than a loose compression fit to the casing itself. It’s a code detail installers already account for, but it’s also why a generic “pitless fitting” bought without checking its listing status on a state’s approved-equipment registry is a real risk, not just a technicality.
How it fits with the rest of the pump system
The pitless adapter sits at the same depth-driven boundary as a couple of other components on this site. Our well pump check valve guide covers the valve that’s often mounted just above or just below this same connection point to hold pressure when the pump cycles off, and our well pump control box guide covers the above-ground electrical component for 3-wire pumps that share the same wellhead. If the casing itself — not just the adapter seal — shows damage near this connection, that’s a bigger job than an adapter swap; see our well casing repair guide for how that scope differs. And since pulling the pump is part of most adapter work regardless of type, our best submersible well pump guide covers the 2-wire vs 3-wire tradeoff that affects how much of that job is a DIY-feasible swap versus licensed contractor work.
The bottom line
Measure before ordering, not after: confirm casing outer diameter, drop pipe size, and discharge size as three separate numbers, then decide between a slide-type adapter (the standard choice for most residential wells, drop pipe up to 2”) and a spool-type adapter (worth it only where full-bore casing access is required). The Merrill MBNL50 covers the common residential case, the Merrill SSMB earns its higher price on corrosive well water, and the Baker Monitor 1BTLF is a solid alternative when Merrill isn’t what the local supply house stocks.
Prices are approximate 2026 figures — tap the buttons above to check the current Amazon price.