FXR Bottom Breathers vs Top Breathers

Every Evo makes pressure under the pistons. Each down stroke shoves air and oil mist into the crankcase, and it has to go somewhere. Where it goes is the whole difference between a bottom breather and a top breather. Most of this is about the Evo. Shovelhead FXRs come first.

Shovelhead FXRs: 1982 to 1983

The 1982 and 1983 FXRs ran the 80 inch Shovelhead, and a few very early 1984s may have too. They breathe the old Harley way: a breather gear in the gearcase, timed to the pistons, vents the flywheel cavity on the down stroke. The air leaves through a 1/4 inch pipe fitting on the right case, inboard of the oil pump. Later Shovels, about 1979 on, ran that hose to the back of the air cleaner; earlier ones aimed it at the chain.

It's the same idea as the early Evo: timed gear, vent on the case, nothing through the heads. Don't confuse the breather with the cam chest to oil tank hose. That one just balances pressure. And don't plug the breather. Old hands run it over the oil tank and down by the chain, or back to the air cleaner.

When the Evo took over in 1984, the bottom end layout carried on. Head breathing waited until 1993.

Bottom breather: 1984 to 1992

A gear-driven rotary breather valve, timed off the crank, lets pressure out into the cam chest on the down stroke and closes on the up stroke. The mist drops oil in the cam chest and in a breather trap in the cam cover, then the air leaves through a fitting on the right side of the case and out a hose. Stock routing varied, and most got rerouted to drip at the frame anyway, so check yours.

Top breather: 1993 on

Same timed breather gear, new exit. Pressure goes up the pushrod tubes into the rocker boxes, past a rubber one-way umbrella valve in each head, out a drilled port in the head and through a hollow breather bolt into the air cleaner backing plate. The engine eats its own vapor. There's no vent fitting on the case. The 1999 to 2000 CVO FXR2, FXR3 and FXR4 are late Evos, so they're top breathers too.

The years

1992 is the odd one. The heads are top breather castings, but the factory plugged the ports with threaded inserts and ran solid bolts on a bottom breather bottom end. Riders report some 1992s left with parts of both. We can't confirm that from factory paper, so treat any 1992 as unknown until you look.

Why Harley changed it

Emissions. Harley's own patent for the head breather says pollution rules require crankcase vapor to be cleaned before it hits the air, so the vapor goes into the intake and gets burned. A hose you can point at the ground doesn't pass that test. Which rule and which year drove it, EPA or California, we haven't pinned down.

Pros and cons

Bottom breather. Vapor stays out of the intake, so the air filter stays dry. The catch: rerouted to the ground it drips, it isn't emissions legal, and a kinked or plugged hose pushes oil out the gaskets.

Top breather. Clean looking, no hose, legal. The catch: oil mist ends up in the air cleaner and can weep down the right side after a hard run, and the umbrella valves harden with heat and age.

Common problems

Solid bolts in top breather heads. That plugs the only exit, and the oil comes out wherever it can. A top breather needs the hollow breather bolts.

Plumbing a 1992 both ways. Owners who opened the head ports on a 1992 bottom end report oil blowing out the bolts. Pick one.

Oil in the air cleaner on a top breather. Some is normal. A lot means look at ring seal, an overfilled oil tank or tired umbrella valves before you blame the system.

Worn breather gear. The stock gear is plastic, and a chipped one or one timed wrong upsets breathing on either system.

How to tell at a glance

Look at the right side of the case, low and behind the oil pump. A vent fitting with a hose on it: bottom breather. Smooth case, no fitting: top breather.

Pull the air cleaner cover. Hollow bolts into the heads with a passage into the backing plate: top breather. Solid bolts: bottom breather, or a 1992 with plugged heads.

Opened up, a bottom breather cam cover has an oil separator and a partition the gasket seals. A top breather cam cover gasket is just a perimeter.

Mods that work

Breather manifold or catch can. On a top breather, a crossover tube joins both heads into one line. Run it to the backing plate or a catch can and the filter stays dry.

Reed breather valve. S&S makes one for 1993 to 1999 top breathers that replaces the breather gear and opens on pressure. Builders warn against it on a bottom breather because it can't clear the breather pocket.

Steel breather gear. Replaces the plastic one on either system.

Harley's fix for early Evo oil carry-over

Early Evos are known for pushing oil out the breather. Harley's answer shows up in its Screamin' Eagle 1340 cylinder head instructions (sheet J01243, dated January 2007), under "Additional Crankcase Ventilation Parts (1984-1992 Models)." It isn't a recall or a numbered service bulletin. It's a recommended update "to improve breathing and reduce oil carry-over": later rocker boxes, a fresh breather gear, a separator in the cam cover trap and a head breathing backplate. In short, it turns an early bottom end into a top breather.

Part numbers are as Harley listed them; several are now obsolete. Check with a dealer before you order.

Read the vent notes carefully, because there are two cases. With the new backplate, the heads do the breathing, so you plug the case vent port. With an early style backplate or similar air cleaner, you fit the threaded head inserts (29304-92) and leave the case vent alone. Harley's words: "do not plug or reroute the crankcase vent." Never block both.

More Evo know-how: FXR known issues and the year-by-year guide.