Can someone tell the difference between top breathing heads and bottom breathing heads for evo motors and how to identify the two? Thanks.

 

Top breather heads have the same casting number as non-breather. However, breather heads have a drilled port - on the top of the pushtube side forward of the rockerbox mounting threads closest to the intake port. That drilling continues the port thru the side of the head where the air filter backing plate mounts.

Generally, the mounting threads for the air cleaner are larger on breather heads, but easiest way, if looking at a photo - look for an extra hole near the 5/16" tapped hole on the intake end. Breather type heads also require a different rockerbox for those ports, but they can be used on older engines, either with the later rockerboxes or without plugging the outlet on the engine case. The earlier rocker boxes will simply cover the port on top.

In '92 1/2 they did away with the fitting on the right case, under and behind the oil pump where the breather was located and began allowing the crankcase to breath thru the top and back into the intake... for EPA I reckon. Though I have seen a few 92's with breather heads AND the case fitting for a breather. Mid-year changes - ya gotta luv em

 

Very helpful and informative t150vej. Just to clarify, if a man was building an evo, the older style rockerboxes will not work with the newer heads (post 92 1/2) like the one you posted the picture of? Also, can you explain the fitting on the right case under and behind the oil pump? Im interested in building an evo motor and dont want to buy wrong parts be it cases, jugs, heads, ect ect. Thanks again.

 

The rockerboxes will directly interchange. Older "bottom breather" rocker boxes would simply cover the vent hole in the top of the head. Conversely, "top breather" rocker boxes would simply have no where to vent because of the absence of the vent port in the head.

Bottom breather cases have a 1/8" NPT thread tapped into the right case. It's behind the oil pump and nearer to the bottom.

Another thing to remember is the nose cones are different. On a bottom breather, the vapor goes thru a baffle in the nose cone, and into a tube that loosely slips into the case leading to the NPT port in the case for the breather hose. That hose was originally routed to the air filter housing. Unfortunately, they had a habit of slobering and making a big mess with this arrangement, so most guys (and dealers) re-routed that hose to the ground, most times with a tiny air-type filter on the end.

So, if you can find a case with the pipe thread I mentioned (84 1/2 - 92 1/2), you can use either head/rockerbox combination. If you're by chance going to build a modified engine, it works really well to utilize the bottom and top breathing arrangements. The oil pump is different also but only by the cover outlets and bolt pattern. And there's so little difference in that pattern, it's real easy to try to use the wrong gaskets and naturally they won't bolt-up.

There is a difference in the pinion (gear) shafts after 89 or 90 where the shaft is made to the right flywheel. Instead of replacing a bad pinion shaft, you replace a sleeve on that shaft for the bearing. I don't like that setup because it takes a machinist to grind the replacement sleeve (like an automotive crank) and the tip of that shaft can wear and score, causing some loss in oil flow to the crankpin. The later pinion bearings are one-piece and work fine but I recommend using the newer race with them. You can still buy the older ones, but not at a dealer - it's all upgraded now.

So, with the exception of the oil pump to block, all the Evo stuff will interchange throughout the model years, provided pieces are matched in certain areas like I've mentioned. Jugs, flywheels (except fuel injected), pistons, rods, heads, cases, cam, lifters, rocker arms, etc. is all the same.

And I'm sure before it's over, a lot of guys here will jump you to buy an Ultima or S&S engine and you can still buy brand new Evo engines for about 2800. - 3500. But if you want to actually put an Evo together yourself out of parts, I'll help you any way I can. I don't know everything, but I do know my stock Evo engines

 

either style heads will work,non head breathers can be machined for head breather holes & head breather holes plugged for non breathers (S&S has inserts),very simple operation,in`92 the pumps changed to,make sure pump is right for year cases,iwould personally get bottom breather cases & head breather heads & rocker boxes, and run both,especialy if your going big inch
kirby

 

Thanks for all the really informative info. To t150vej, Im going to send you a request as a friend so it will be easy for me to contact you for further assistance.

The bike I will be dealing with at least at first is a 95 FXSTC which you can see under my profile. Im wanting a high hp motor for this bike and I know, I know, torque is the thing with daily riders but this bike is not for that. I want the rumpety rump for this particular bike. Here http://www.nightrider.com/biketech/hdbuildstage3_95.htm is basically the motor I want to build. I do want to be able to run this motor off high octane pump gas. I was just thinking maybe I could pick up a set of heads to have the headwork already done and a set of jugs to have them matched to the pistons so everything would be ready to go together instead of tearing the motor down and waiting on the work to be done with it torn down. Then eventually I would like to be able to do evos for friends or possibly even for profit. Im 33 years old and Im pretty new to some of this stuff but Im eager to learn. Ive already got the SE-57 cam, crane adjustable pushrods and a 42mm mikuni HSR carb that are all brand new for this application. I plan to buy a crane single fire ignition, a set of SE pistons and a barnett clutch. Is there anything else you might suggest or do you see any problems with any of the components or direction Im going with this? Thanks again.

 

Mine '92 is just like this, bottom breather with also top breather heads and rockers, just tapped with solid bolts instead of breathers bolts.
Hate this.

 

Don`t be a Hater......

 

Especially 11 years later and with no explanation as to why..

 

If you noticed all these multiple year dead threads coming back to life, new member, 1 or 2 blunt posts, never heard from again = forum administration