Modifying Crankcase Breather Timing
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From: NYC
Modifying Crankcase Breather Timing
I was reading the S&S installation guide for big bore kits and it mentioned
I read the same in William Denish's book, "Big Twin High Performance Guide". This is talking about the window to the breather gear.
I don't know which years it relates most to, it seems old Big Twins had smaller oval crankcase breather windows (I've never seen that) because on later Big Twins, they already have a rectangle cut but it's a simple cut, probably just done through the front tappet block orifice as it has quite a high lip to it and is not finished off.
Has anyone ever played around or done work in this area. May be it's more of a Shovelhead question?
Yes, obviously, 1,000s of bikes have done 100,000s of miles without even popping the cover open but I'm interested in how much of an effect it has and how much folk opened it up. I don't know of anyone who has.
To maximize efficiency when scavenging oil from flywheel cavity in Big Twin engines, it is necessary to check and usually modify crankcase breather timing.
I don't know which years it relates most to, it seems old Big Twins had smaller oval crankcase breather windows (I've never seen that) because on later Big Twins, they already have a rectangle cut but it's a simple cut, probably just done through the front tappet block orifice as it has quite a high lip to it and is not finished off.
Has anyone ever played around or done work in this area. May be it's more of a Shovelhead question?
Yes, obviously, 1,000s of bikes have done 100,000s of miles without even popping the cover open but I'm interested in how much of an effect it has and how much folk opened it up. I don't know of anyone who has.
o check and modify breather timing perform following steps:
A. Bolt degree wheel pointer to drive side crankcase half using center case stud and nut. Position pointer towards sprocket shaft.
B. Install degree wheel assembly on sprocket shaft. Rotate flywheel assembly so front piston is at TDC, top dead center. Position degree wheel on sprocket shaft so pointer indicates 0° TC, top center, and tighten set screw.
C. Rotate flywheels in normal direction of travel to 10° before TC as in Picture 6. Observe breather gear and hole in crankcase through tappet block opening to determine hole in breather gear and hole in case relationship.
D. Rotate flywheels to 10° after TC. Again note relationship. If edge of hole in gear appears in hole in crankcases during this span of degrees, then cases have proper opening specs. If not, then opening side of hole in cases must be ground. Grind hole to 10° before TC, maximum timing specifications.
E. Rotate flywheels to 55° after BC, bottom center. Observe breather gear and hole in crankcase to note their relationship.
F. Rotate flywheels to 75° after BC and note relationship. See Picture 7. If back edge of hole in gear disappears from view during this span of degrees then cases have proper closing specs. If not, then closing side of hole in cases must be ground. Grind hole to 75° after BC, maximum timing specifications.
G. Note location of hole in gear as opposed to hole in crankcases. Grind hole in cases so location matches holes in gear with respect to where toothed portion of gear hits cases. Picture 8 shows finished breather gear opening.
H. Optional Step NOTE - This procedure increases area that air and oil have to pass from breather gear cavity to gear side.
To modify screen:
a. Early gears - Remove clip that holds screen in
place.
b. Early gears - Turn screen so it covers hole made
by removing clip.
c. Early gears - Spot weld screen in place.
d. All gears - Drill all holes in screen as large as
possible without weakening screen.
A. Bolt degree wheel pointer to drive side crankcase half using center case stud and nut. Position pointer towards sprocket shaft.
B. Install degree wheel assembly on sprocket shaft. Rotate flywheel assembly so front piston is at TDC, top dead center. Position degree wheel on sprocket shaft so pointer indicates 0° TC, top center, and tighten set screw.
C. Rotate flywheels in normal direction of travel to 10° before TC as in Picture 6. Observe breather gear and hole in crankcase through tappet block opening to determine hole in breather gear and hole in case relationship.
D. Rotate flywheels to 10° after TC. Again note relationship. If edge of hole in gear appears in hole in crankcases during this span of degrees, then cases have proper opening specs. If not, then opening side of hole in cases must be ground. Grind hole to 10° before TC, maximum timing specifications.
E. Rotate flywheels to 55° after BC, bottom center. Observe breather gear and hole in crankcase to note their relationship.
F. Rotate flywheels to 75° after BC and note relationship. See Picture 7. If back edge of hole in gear disappears from view during this span of degrees then cases have proper closing specs. If not, then closing side of hole in cases must be ground. Grind hole to 75° after BC, maximum timing specifications.
G. Note location of hole in gear as opposed to hole in crankcases. Grind hole in cases so location matches holes in gear with respect to where toothed portion of gear hits cases. Picture 8 shows finished breather gear opening.
H. Optional Step NOTE - This procedure increases area that air and oil have to pass from breather gear cavity to gear side.
To modify screen:
a. Early gears - Remove clip that holds screen in
place.
b. Early gears - Turn screen so it covers hole made
by removing clip.
c. Early gears - Spot weld screen in place.
d. All gears - Drill all holes in screen as large as
possible without weakening screen.
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Joined: Mar 2010
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From: Honah Lee
Learn all you can and make sure you understand how it works and the thought that went into it. This goes for the whole oil system.
I think if I were you, I would try it as is. So if it pukes a little oil. Just wipe the drool off the old girls chin and keep getting up. That is a pretty valuable part of the engine you are messing with. That is some cool thoughts you posted however.
My 2004 pukes a little oil. There is a lot of incorrect info out there. Make sure it's not coming from a tin man. Take the twin cam. Lot of people drill the drain hole in the bottom of the baffle box in the head larger thinking that helps. All that does is open up the bottom for more crank pressure to blow up thru it from the bottom.
Once you start grinding, you cannot put it back. Make sure the check ball seats in the oil pump. Make sure your piston to cylinder clearance and rings are to spec. Drive it like a Harley and not a Sportbike You wont have excessive oil. You will have some.
Even my 2005 Yamaha Star has it. More than my 04 Softail. Big difference there is Yamaha catches it it a box in the filter and has a drain tube with a cap on it.
I think if I were you, I would try it as is. So if it pukes a little oil. Just wipe the drool off the old girls chin and keep getting up. That is a pretty valuable part of the engine you are messing with. That is some cool thoughts you posted however.
My 2004 pukes a little oil. There is a lot of incorrect info out there. Make sure it's not coming from a tin man. Take the twin cam. Lot of people drill the drain hole in the bottom of the baffle box in the head larger thinking that helps. All that does is open up the bottom for more crank pressure to blow up thru it from the bottom.
Once you start grinding, you cannot put it back. Make sure the check ball seats in the oil pump. Make sure your piston to cylinder clearance and rings are to spec. Drive it like a Harley and not a Sportbike You wont have excessive oil. You will have some.
Even my 2005 Yamaha Star has it. More than my 04 Softail. Big difference there is Yamaha catches it it a box in the filter and has a drain tube with a cap on it.
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From: NYC
Thank for the reply.
I can't say I have any specific problems. I just view it on a par with cleaning up the castings on the intake and exhaust ports. Without a doubt work can be done there and makes a difference, "porting" your oil system wont make it go faster but will it pump more oil?
Denish actually talks of (people/racers?) polishing the cam chest walls to encourage oil flow ... obsessive stuff for a long winter's day perhaps?
I mentioned in another thread how in my castings the oil galley at the top/back of the cases don't line up by about 1/4" (LHS side is narrower). Again, I don't think that is by design, just a cost saving in manufacture.
Should they match up ... what are they like in aftermarket cases?
I can't say I have any specific problems. I just view it on a par with cleaning up the castings on the intake and exhaust ports. Without a doubt work can be done there and makes a difference, "porting" your oil system wont make it go faster but will it pump more oil?
Denish actually talks of (people/racers?) polishing the cam chest walls to encourage oil flow ... obsessive stuff for a long winter's day perhaps?
I mentioned in another thread how in my castings the oil galley at the top/back of the cases don't line up by about 1/4" (LHS side is narrower). Again, I don't think that is by design, just a cost saving in manufacture.
Should they match up ... what are they like in aftermarket cases?
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