Author Topic: Searched high and low for a crankcase breather tube!  (Read 973 times)

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Offline The Gurg

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Searched high and low for a crankcase breather tube!
« on: June 17, 2021, 07:54:08 AM »
Looking for the crankcase breather tube for a 1975 CB750 K5. This is the tube that runs from the back of the crankcase to the bottom of the oil tank. Part number 15761-300-020. Looked on Yamiya, David Silver Spares, Ebay, and googled it. Not finding it.

Seeing that the crankcase breather tube takes B10 clips that go on the 5.5mm fuel line, wondering if the 5.5mm fuel line is the same exact thing?

Anyone know the dimensions on 15761-300-020 or have a source?


Offline fxef79

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Re: Searched high and low for a crankcase breather tube!
« Reply #1 on: June 17, 2021, 08:40:00 AM »
Not really a direct answer to your question, but something tells me I had to use 3/8” hose when I plumbed that fitting up to the vent on my custom oil tank - so it seems 5.5 mm would be too small. But I’m honestly just going from memory on that.

Offline fxef79

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Re: Searched high and low for a crankcase breather tube!
« Reply #2 on: June 17, 2021, 08:41:25 AM »
To add to what I said above - that was on my 73 engine. On my 78 engine that fitting on the back of the case is much larger. I had to use 1/2” hose.


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Offline Jerry Rxman Griffin aka MuthaF'er

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Re: Searched high and low for a crankcase breather tube!
« Reply #3 on: June 17, 2021, 09:49:49 AM »
95005-1200120M

12mm Order only 1. Enough for both hoses.
As of today 3/13/2012 my original owner 75 CB750F has made it through 3 wives, er EX-wives. Free at last.  ;-)

Offline PeWe

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Re: Searched high and low for a crankcase breather tube!
« Reply #4 on: June 17, 2021, 11:35:25 AM »
K5 use the thinner hose. I have a case with 1975 serial number it has the thin spigot.

 12mm inner to wide for the earlier with thinner spigot on engine case and oil tank.

I used inner 10 mm on my K2 with same spigot. 
Oil hose sold per meter or foot I guess.

My K6 has the thicker where 12mm ID work fine.

Measure before order or buy where sold at a car part shop and similar.
On oil tank spigot as well as the engine case rear spigot.
CB750 K6-76  970cc (Earlier 1005cc JMR Billet block on the shelf waiting for a comeback)
CB750 K2-75 Parts assembled to a stock K2

Updates of the CB750 K6 -1976
http://forums.sohc4.net/index.php/topic,180468.msg2092136.html#msg2092136
The billet block build thread
http://forums.sohc4.net/index.php/topic,49438.msg1863571.html#msg1863571
CB750 K2 -1975  build thread
http://forums.sohc4.net/index.php/topic,168243.msg1948381.html#msg1948381
K2 engine build thread. For a complete CB750 -75
http://forums.sohc4.net/index.php/topic,180088.msg2088008.html#msg2088008
Carb jetting, a long story Mikuni TMR32
http://forums.sohc4.net/index.php/topic,179479.msg2104967.html#msg2104967

Offline The Gurg

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Re: Searched high and low for a crankcase breather tube!
« Reply #5 on: June 17, 2021, 11:56:12 AM »
Measured the barb on the back of the case and the oil tank barb and thinnest part is 8mm. Aka didn’t measure the bulbous part of the barb.

So I think the 12mm would be too large. Saw on Yamiya that the size varies by year.

Technically can be vented to atmosphere, so don’t need the tube to run, so I could see putting the old tube on or running over to the rubber and gasket company nearby and getting a close match.

Offline fxef79

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Re: Searched high and low for a crankcase breather tube!
« Reply #6 on: June 17, 2021, 12:08:37 PM »
K5 use the thinner hose. I have a case with 1975 serial number it has the thin spigot.

 12mm inner to wide for the earlier with thinner spigot on engine case and oil tank.

I used inner 10 mm on my K2 with same spigot. 
Oil hose sold per meter or foot I guess.

My K6 has the thicker where 12mm ID work fine.

Measure before order or buy where sold at a car part shop and similar.
On oil tank spigot as well as the engine case rear spigot.

This would match up with my findings on my 73 and 78.  A 3/8" ID hose (~9.5mm) worked on the 73, and I needed a 1/2" ID (~12.5mm) hose for the 78

Offline fxef79

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Re: Searched high and low for a crankcase breather tube!
« Reply #7 on: June 17, 2021, 12:11:03 PM »
Technically can be vented to atmosphere, so don’t need the tube to run, so I could see putting the old tube on or running over to the rubber and gasket company nearby and getting a close match.

I wouldn't leave it open.  Who knows what kind of critter might decide to crawl right into the inside of your crankcase.  Around here Mud Daubers love that kind of opening.  Not to mention road dust and such. 

Offline david 750f

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Offline PeWe

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Re: Searched high and low for a crankcase breather tube!
« Reply #9 on: June 17, 2021, 12:44:13 PM »
It is very easy to connect a tube there as it should. Connect to oil tank first, case next.  Risk that oil spatter over the rear tire which is not that nice if leaving it open. Enough that ventilation from valve cover smell warm oil fumes at red light.
Early oil tank has a hose to atmosphere. Some connect that with a T with valve cover vent tube so only one tube  goes out beside kicker, behind clutch cover.
CB750 K6-76  970cc (Earlier 1005cc JMR Billet block on the shelf waiting for a comeback)
CB750 K2-75 Parts assembled to a stock K2

Updates of the CB750 K6 -1976
http://forums.sohc4.net/index.php/topic,180468.msg2092136.html#msg2092136
The billet block build thread
http://forums.sohc4.net/index.php/topic,49438.msg1863571.html#msg1863571
CB750 K2 -1975  build thread
http://forums.sohc4.net/index.php/topic,168243.msg1948381.html#msg1948381
K2 engine build thread. For a complete CB750 -75
http://forums.sohc4.net/index.php/topic,180088.msg2088008.html#msg2088008
Carb jetting, a long story Mikuni TMR32
http://forums.sohc4.net/index.php/topic,179479.msg2104967.html#msg2104967

Offline 70CB750

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Re: Searched high and low for a crankcase breather tube!
« Reply #10 on: June 18, 2021, 02:35:09 AM »
I use clear vinyl tube of proper size, got it Home Depot.
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Offline Don R

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Re: Searched high and low for a crankcase breather tube!
« Reply #11 on: June 18, 2021, 03:02:59 PM »
 Some 750's need a thin tube due to the proximity of the nipple to the case. Others can use heavy wall hose. 
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Offline Redline it

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Re: Searched high and low for a crankcase breather tube!
« Reply #12 on: June 24, 2021, 02:09:48 PM »
on a cb400f 1976, i'm having outstanding results from neighborhood dishwashers thrown out on the curb. there's usually a few quality hose clamps free for the taking as well. anything mass produced even if it's kinda crappy has specific parts that are quite higher in quality than buying the specific part alone. it's just how they get ya. but there's clear and black hoses that have specific bends that strangely fit the route of the stock hose, i think i had to trim 1 just a bit to fit. and those hoses strangely hold up to the heat and gasses produced. so far i've found 2, both work and i like the black one instead of clear. clear looks a little corny. you know aftermarket parts will be completely cheap, flimbsy junk, and for a pretty high price too. dishwasher. the last one i pulled a little pump off of it, i noticed there were 2, a bigger pump, i left that. but who knows the little one might be a high pressure washer in the next project.