Toilet Seat Hardware

Most return trips to the counter for toilet seat hardware come down to three measurements nobody took before they left the house. Get the bowl shape, bolt spread, and mounting type right, and the rest is a ten-minute job. Miss one, and you’re driving back.

Here’s the whole game. A round bowl is roughly 16.5 inches from the center of the mounting holes to the front, while an elongated bowl is approximately 18.5 inches. Then check your bolt spread. The standard bolt hole spacing is 5.5 inches (155mm) center-to-center on virtually all North American toilets. Last, figure out how the seat anchors to the bowl — top-fix or bottom-fix. Those two systems don’t swap.

That’s it. Measure, confirm the mounting style, then pick your material.

The Three Measurements That Govern Fit

Bowl shape comes first because it dictates which seat you buy. Round or elongated — measure from the center of the mounting holes to the front edge. Seats come in standard white closed-front configurations, round and elongated wood, and a combination child/adult round seat.

Bolt spread is the second check, and it’s the one people skip. The 5.5-inch center-to-center spacing is nearly universal here, which is why most replacement hardware is sold as “universal fit.” But always verify before you tighten anything down — ensure the hinge bolt spread matches your bowl before final tightening.

Bolt dimensions themselves run in a couple of common sizes. The Moen M5677 set uses 5/16 inch x 2-1/2 inch brass bolts, while the Moen M5679 runs 3/8 inch x 2-1/2 inch plastic bolts. Danco’s universal plastic bolts come in a 7/16 inch diameter with 14 TPI threads. Plumb Pak’s plastic set is 3/8 x 2-1/2 inches. Nylon bolt kits and dedicated hinge bolt sets round out the common replacement options.

Mounting Systems and Hardware Materials

Two mounting systems exist, and a toilet built for one won’t take the other. Bottom-fixings have visible nuts under the rim, while top-fixings use expanding anchors for toilets with no underside access. Bottom-fix is the common one — bolt drops through the hinge and bowl, nut and washer snug it up from beneath. Easy to align, easy to adjust. The trade-off is that underside hardware sits in the damp, and that’s where corrosion starts.

Top-fix is what you need when there’s no access under the rim — concealed-cistern setups and skirted bowls. Replacement parts exist specifically for these: the Moen M5679 is a replacement bolt set for top-mount toilet seat hinges, used for securing the toilet seat to the toilet bowl.

Material is the real buying decision, and it’s driven by where this hardware lives — wet, splashed, exposed to cleaners. In Montana homes and cabins running hard or well water, that environment chews through cheap metal fast.

Material	Strength	Corrosion resistance	Best for
Plastic / nylon	Lower load capacity	Immune to rust, easiest to remove years later	Standard residential seats, easy future swaps
Brass	Solid, durable	Resists corrosion well	Mid-grade repairs wanting metal strength
Stainless steel	Greatest mechanical strength	Excellent	Heavy-duty and commercial seats

Plastic and nylon win on corrosion and removability. Plumb Tech plastic hinge bolts are made from durable plastic for long-lasting strength, with two white bolts and nuts per package, fitting most toilets. A nylon bolt kit fits the same logic — nylon won’t seize in the bowl-rim damp the way bare steel does.

Brass splits the difference. The Moen M5677 set includes two brass bolts that resist corrosion, plus two nuts and two washers, with a Quick Lock Nut for easier installation. Stainless steel is the call for heavy-use commercial seats where mechanical strength matters most.

Skip plain zinc-plated or galvanized steel for this job. It seizes solid in a damp bowl-rim environment, and a year or two later you’re cutting it off with a hacksaw instead of backing a nut off clean.

Soft-Close, Quick-Release, and Adjustable Options

A few upgrade categories solve real problems. Soft-close hinges use hydraulic dampers for silent operation, and quick-release hinges allow the seat to be removed for cleaning. Quick-release is the underrated one — pop the seat off without tools and you can actually clean the hinge area instead of fighting around it.

TOTO’s SoftClose line covers the range of shapes with model-specific hardware. The SS113 is the round model, SS114 elongated, and SS117 the D-shape. Mounting hardware is matched to the seat: THU689 is the under-mounting hardware set, THU651N the top-mounting seat hardware, and THU726 the mounting hardware for SS117, SS237, and SS247. Worth noting that THU651N ships with skirted toilets and is sold separately — it’s not included with the seats.

One thing about integrated hinge-bolt systems: when corrosion sets in, you can’t fix a piece. The whole seat goes. That’s the argument for keeping serviceable plastic or brass hardware on a standard seat — you replace a single bolt instead of the entire unit.

Installation Without Cracking the Bowl

This is a ten-minute job that goes sideways exactly one way — overtightening into porcelain. Align the hinges and tighten the fasteners until secure, but avoid overtightening to prevent cracking the porcelain. Work the bolts alternately, left then right, so the seat seats square instead of cocking to one side. Hand-tight plus a touch more. That’s the target.

Plastic bolts especially — don’t muscle them. They’ll snap or crush before they crack porcelain, but either way you’re back to square one.

If the seat wobbles after you’ve snugged it down, don’t just crank harder. Use rubber washers or adjustable mounting plates to fix a wobbling seat.

Pulling old corroded hardware is its own fight. Hit seized nuts with penetrating oil first and give it time. If a metal bolt is rusted solid, cut it with a hacksaw rather than forcing it — forcing a corroded bolt is exactly how a bowl ends up cracked. A white round seat with chrome hinge and the dedicated hinge bolt sets cover most straight replacement situations.

Frequently Asked Questions

How do I know if I need a round or elongated seat? Measure from the center line of the mounting bolt holes to the front edge of the bowl. Around 16.5 inches means round; about 18.5 inches means elongated. Buy the seat that matches — the shapes aren’t interchangeable.

What’s the standard bolt spacing for a toilet seat? 5.5 inches center-to-center between the two mounting holes, which is standard on nearly all North American toilets. That’s why most replacement bolt sets are labeled universal fit. Still measure yours before buying to be safe.

Should I use plastic or metal hinge bolts? Plastic and nylon resist corrosion completely and come out easily years down the road, which matters in damp bathrooms and hard- or well-water homes. Brass adds metal strength while still resisting corrosion. Stainless steel is the strongest, used in heavy-duty and commercial seats. Plain zinc or galvanized steel seizes up in the wet, so it’s the wrong pick for this job.

What’s the difference between top-fix and bottom-fix mounting? Bottom-fix runs a bolt down through the hinge and bowl, secured by a nut underneath the rim — easy to reach and adjust. Top-fix tightens from above using an expanding anchor, and it’s what you need when there’s no access under the rim, like concealed-cistern or skirted toilets. A toilet built for one system won’t accept the other.

My seat wobbles even after tightening. What’s wrong? Don’t keep cranking — that’s how porcelain cracks. Add rubber or silicone washers, or step up to an adjustable mounting plate to take out the play. If the bolts feel snug but the seat still shifts, the washers are usually the fix.

The old bolts are rusted solid. How do I get them off? Soak the seized nuts in penetrating oil and let it work. If they still won’t budge, cut the bolts with a hacksaw. Never force a corroded metal bolt — the porcelain loses that fight, and a replacement bowl is a far bigger job than a new seat.

What does soft-close actually do? Soft-close hinges use a hydraulic damper so the lid lowers itself slowly and silently instead of slamming. Pair it with quick-release hinges and you can lift the whole seat off without tools when it’s time to clean.