EVO: Engine Mechanicals

Crankcase Ventilation

See also in the REF section of the Sportsterpedia:

A mixture of crankcase air and oil mist is produced on each piston down stroke.
This forced mixture helps to splash lube the moving parts in the engine.
It also creates unwanted pressure in the engine.
The oil mist is then separated from the crankcase air and the excess air pressure is vented out of the engine.
86-90 engines vent through a valve in the cam cover.
91 and up engines vent through a valve in each rocker box and out the hollow breather bolts in each head to the air cleaner.

Late 1984-1990 Engine Breather Valve

An important note:
Factory engine breather valves on all Evo Sportsters are inside the engine.
The lines / hoses that leave the engine and any apparatus to route the air to different places are part of the venting system.
Many aftermarket manufacturers advertise that they are selling “Engine Breathers” when they are actually only selling the vent line / tube assemblies.
See the link at the top of the page for aftermarket breather and venting solutions.

  • 1986-1990 engines continued the breather baffle tube system (implemented in L1984) incorporated into the cam cover in the area behind the oil filter.
  • The baffle tube mounts into the cam cover and the rubber umbrella installs into the center of the baffle tube. 1)
  • On each piston downstroke, crankcase pressure (air and oil mist) is routed to the breather baffle at the front of the cam case.
  • Oil is separated from the air pressure by the one-way umbrella valve.
    • The oil then drains into the cam chest through a drain hole in the breather valve compartment.
  • Exhaust air escapes past the one way umbrella valve in the baffle tube and into a vent fitting on the cam cover.
    • The air is then routed into the rear of the air cleaner via an oil hose to the cam cover.

Umbrella check valve comparison: 2)
Outside diameter:
26856-82 - 1.25“
26856-89 - 1.0”

Edge thickness (1/8“ in average): 3)
26856-82 - 0.036”
26856-89 - 0.041“

  • The pic below shows the left side (filter side) that faces into the cam box.
    • It's center inside diameter is slightly larger than the 1.25” OD (-82) umbrella valve.
    • The two 'C' shaped slots that do the breathing have an OD of 1“.
    • As a result, the (-89) umbrella valve with its diameter of 1” may not perform correctly as there is likely to be leakage.
Umbrella (check) Valves
L - (26856-82), R - (26856-89) 4)
Breather Baffle Tube (26917-84) 5) Cap, tube and umbrella 6)
89 cam cover with breather baffle tube attached (it should be tight but still pull out) 7)

Testing the L84-90 umbrella valve:
A working valve should help reduce oil mist. 8)
Since it's a one-way valve, air goes out but doesn't come back in.

To make sure the umbrella valve is still working;
Before you position the hose, use a bit of clean pipe and blow gently through the breather.
It should close the valve and stop air flow. You may have to find an aftermarket replacement if the valve is shot as OEM replacements are hard to come by now.
The 5 speed ones do not fit.

1991-2003 Engine Breather Valve

An important note:
Factory engine breather valves on all Evo Sportsters are inside the engine.
The lines / hoses that leave the engine and any apparatus to route the air to different places are part of the venting system.
Many aftermarket manufacturers advertise that they are selling “Engine Breathers” when they are actually only selling the vent line / tube assemblies.
See the link at the top of the page for aftermarket breather and venting solutions.

  • In 1991, along with the 5-speed transmission, the MoCo moved the crankcase breathing from the cam box cover outlet to breather bolts in the heads.
    Previously, these bolts were just used to mount the carb and air cleaner to the heads.
  • The new breather system uses (2) one-way umbrella valves placed in the rocker boxes (1 each placed on the intake side).
    These expel crankcase vapors through vents in the top of the cylinder head and into the carb to be burnt. 9)
    • On each piston downstroke, crankcase pressure (air and oil mist) is routed up the pushrod tubes into the rocker box (bypassing the falling oil from the rocker boxes).
    • Collected air pressure and oil mist in each rocker box is routed up into a sealed cavity in the lower portion of the box.
    • This mixture passes up from underneath a rubber one-way valve (umbrella valve) sitting over the cavity inlet.
      • The oil is designed to separate from the air by hitting the underside of the umbrella valve and dropping back down into a recessed area behind the umbrella valve in the cavity.
        • From there it should drain back into the main rocker box through a tiny hole behind the umbrella valve and then back to the lower end.
        • The separated oil flows into drain holes in the rocker boxes and back into the gearcase.
      • Air pressure is designed to continue up past the umbrella valve and exit a hole in each head on the intake valve side.
        • Air pressure escapes the head through the hollow bolts (one in each head) that hold the air cleaner mount.

10) 11)

  • The Parts Catalog and FSM are a bit confusing regarding how many umbrellas are used and where.
    The information is there but it's just not clear.
    The parts catalog only shows an exploded diagram of one rocker box assembly on the Rocker Arms, Covers and Pushrods page.
    Yet, the description shows that 2 umbrellas are required.
    It's the same for all of the parts on that page as well as each newer parts catalog (1 pushrod, tube ,lifter and related parts).
    This is the norm mostly throughout the subsequent catalogs with multiple (same) parts.
    It's assumed this was done to save money on printing the catalogs (replicated information deleted).
    The FSM displays the same exploded diagram as the parts catalog in the front of the Cylinder Head section.
    It's only at the end of the Installation article does it say to install new gaskets on the middle rocker box (with the breather valve on the intake side).
    There is no dedicated section on the rocker box or umbrella valves (which seem to be regarded as mere gaskets).

Installation

  • The middle rocker box is symmetrically cast so that it can be used on the front or rear head.
    And, there is a spot (seemingly) for two umbrellas. 12)
  • However, only one umbrella is used in each middle section.
    (only 2 umbrella valves are used in the entire engine.) 13)
  • Each umbrella is placed on the intake side on each head (only).
  • It is not always green as in the pic below (depends on what brand you buy).
Umbrella Valve (only 1 installed per rocker box) 14) 15) Umbrella Valve Installed Locations 16)
91-03 crankcase ventilation is done through the hole with the wire tie. 17)

2004-Present Engine Breather Valve

An important note:
Factory engine breather valves on all Evo Sportsters are inside the engine.
The lines / hoses that leave the engine and any apparatus to route the air to different places are part of the venting system.
Many aftermarket manufacturers advertise that they are selling “Engine Breathers” when they are actually only selling the vent line / tube assemblies.
See the link at the top of the page for aftermarket breather and venting solutions.

  • In 2004 the MoCo made some changes to the umbrella valve configuration.
    The umbrella was retained but now inside a plastic housing with a pre-umbrella oil separating screen.
    Each 'breather valve' is assembled into a plastic fitting that is sealed over the outlet hole in the lower rocker box to the head breather bolt.
    The new breather valves were also accompanied with new style hollow air cleaner mounting bolts for the pressure to escape.
  • The new bolts are the same thread size as previous.
    But instead of a simple hex, it also has a shoulder past the hex for an O-ring to be fitted between the hex head and the air cleaner.
  • The breathing system is functionally the same as 91-03 with the one-way umbrella valves in the rocker boxes.
    These exit crankcase vapors through vents in the top of the cylinder head and into the carb mouth to be burnt. 18)
    • On each piston downstroke, crankcase pressure (air and oil mist) is routed up the pushrod tubes into the rocker box (bypassing the falling oil from the rocker boxes).
    • Collected air pressure and oil mist in each rocker box is routed up into the breather valve unit in the lower portion of the box.
    • This mixture passes up from underneath the breather unit.
      • The oil is designed to separate from the air by hitting the underside of the screen / umbrella valve and dropping back down into the rocker box.
        • From there it is routed back to the lower end.
        • The separated oil flows into drain holes in the rocker boxes and back into the gearcase.
      • Air pressure is designed to continue up past the breather unit and exit a hole in each head on the intake valve side.
        • Air pressure escapes the head through the hollow bolts (one in each head) that hold the air cleaner mount.

19)

Just as with the 91-03 umbrella valves, only 1 breather valve per side is used.
And each one is placed on the intake side of each lower rocker box.

However, unlike the single umbrella valves in 03, each breather valve in 04 and up engines is directional.
(as in there is one for the front and a different one for the rear)

Each one consists of a poly fiber over a rubber flap. 20)
The fiber pieces are the same as the brown Scotchbrite pads that are sold in the body section of auto parts stores.
You can cut your own put them in the breather valves. They are aluminum oxide and won't hurt the engine. 21)

Breather Valves 22) View of one cut open. 23)
Oil drain hole. 24) 2006 Breather Valves 25)
2013 Breather Valves 26)

Check the breather to gasket seal

There is an anomaly when it comes to the seal around the stock breather port under the rocker box. 27)
The rocker box cover gasket is part of the seal for the breather port.
It's possible to make a mistake when installing the breathers into aftermarket gaskets and end up with oil puking out the breathers.
This could result in a leak which would bypass the umbrella (since the only seal would be down-pressure of the umbrella housing against the rubber).
The hold-down bolt for the breather is on the other end of it.
Using OEM valve cover gaskets, you would not need the orange seal.
However, using Cometic rocker box gaskets, you do need the orange seal.

Using OEM Rocker Box Gaskets:
The pictures shown with the brown breather and no orange seal are of the original configuration. There are no known sealing issues here.
There is a thicker area on the gasket that fits down into the rocker-box hole.
Also there is a bump to take up extra space next to the umbrella housing and it fits nicely in the hole of the gasket.

28) 29)

Using Cometic Rocker Box Gaskets:
The pictures with the white-ish (newer) breather which comes with the orange seal included.
The Cometic gasket has a hole sized to fit the rocker-box port which is much larger than the umbrella valve housing that fits in it.
To make this seal properly you would need the orange seal but you would only have the orange seal if you bought new breathers also.

30) 31)

Head Breather Bolts

Sub Documents

See also Evo Crankcase Pressure and Engine Breathing in the REF section of the Sportsterpedia.

Since 86-90 engines breath through the cam chest cover instead of the heads, the air cleaner bolts are solid (not hollowed).
91 and up engines breath out the heads through hollowed out air cleaner mounting bolts (or breather bolts).
Without the hole through the bolts, the engine can't breathe.

The size of the orifice in the hollow head breather bolts makes the crankcase pressure rise higher (or lower) in the process of expelling the gas. 32)

91-03

91-03 breather bolts.
Outside threads are 1/2“ x 13 (hole through the middle is 5/6” nominal).
33) 34)

04 and Up

Crankcase pressure was increased in 2004 by several means.
One of which was the reduction of the hole size in the breather bolts on the engine side (more restriction).
The smaller hole creates higher pressure in the crankcase on piston downstroke which aides in oil scavenging.
They also allow the air to leave the vents at a faster rate.

04+ breather bolts.
Outside threads are 1/2“ x 13 (2 different sizes thru the middle - nominally 1/16” (1.5875 mm) in the rear and 5/16“ in the front). 35) 36) 37)
38) 39) 40)

If you know someone with a lathe, you can have some stainless steel breather bolts made. 41)

Symptoms of Breather problems

Read more here on the Evo Crankcase Pressure and Engine Breathing in the REF section of the Sportsterpedia.
The most noticeable signs of breather valve problems are weeping gaskets or oil excessively leaking out the breather or puking oil out of it.
When the umbrella(s) gets hard, it doesn't flex well to allow the engine to breath out on exhale.
Then the trapped in air is compounded on the next stroke. The excess air contributes to more vacuum created and implodes the gaskets.
Or it contributes to too high of oil density and slings excess oil out.
Other factors are involved so results will vary.



7)
photos by Bryan350 of the XLFORUM http://xlforum.net/forums/showthread.php?t=1712011
10)
drawing by Hippysmack
11) , 16) , 19) , 33) , 34)
photo by Hippysmack
17)
photos by Hippysmack
25)
photos by JackAttack of the XLFORUM http://xlforum.net/forums/showthread.php?p=4435889
This website uses cookies for visitor traffic analysis. By using the website, you agree with storing the cookies on your computer.More information