V-Twin Forum banner
1 - 12 of 12 Posts

·
Registered
Joined
·
148 Posts
Early model heads and late model crank= 0% crankcase ventilation. If I've read correctly s&s big inch evos want crankcase and head breathers. Your late model heads might bolt up but give your engine no provisions to breath internally. Seals will blow. Oil will leak like a sieve and you won't have 5 miles on the meter.
 

·
Make me use my bullet?
Joined
·
13,757 Posts
What are "early model heads" and what are "late model engines"? That seems pretty vague. Are we talking about Evo heads on M8 jugs?

What is the availability of parts like in Germany?
 
  • Sad
Reactions: WiSeNhEiMeR -1369

·
Super Moderator
Joined
·
22,725 Posts
You can use top breather heads on bottom breather engines. Harley did just that in 92.

Just keep the bottom breather plumbing on the bike. Remove the umbrella valves. Then use solid bolts in the breather holes at the head. Harley used threaded bushings to reduce the size of the holes so as to use the smaller bottom breather bolts.
 

·
Registered
Joined
·
1,394 Posts
You can use top breather heads on bottom breather engines. Harley did just that in 92.

Just keep the bottom breather plumbing on the bike. Remove the umbrella valves. Then use solid bolts in the breather holes at the head. Harley used threaded bushings to reduce the size of the holes so as to use the smaller bottom breather bolts.
could you do both breather systems or is it important to "cap off" the head breathers if using case breathers?
 

·
Super Moderator
Joined
·
22,725 Posts
could you do both breather systems or is it important to "cap off" the head breathers if using case breathers?
It won't head breather properly with a bottom breather cam chest and cam cover. Would it hurt anything to use both, probably not. But why bother with it, bottom breathing worked a lot better than head breathing.

Head breathing was just to appease the EPA and their demands for positive breather control.
 

·
Registered
Joined
·
1 Posts
If you are able to do some small drilling and tapping operations, it is easy to modify non breather heads to top breathers. You will also need to use the late style rocker boxes and air cleaner assembly for this to work.
Using the bottom new style rocker box gasket, locate where to drill the vertical hole that intersects the air cleaner mounting bolt hole. Drill 3/4" deep with a 7/16" drill.
Place the head on its side with the air cleaner mounting bolt hole facing up. Drill this hole 1-3/8" deep with a 27/64" drill, then tap it with a 1/2"-13 tap.
 
1 - 12 of 12 Posts
Top