crankcase oil leaks from 850 commando

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Hi, having just stripped my commando (to do amongst other things) to stop an oil leak from the rear top of the 'case joint (it can be seen when the engine is being revved) I expected to see something obvious but I can't. As far as I know it has only done about 17k miles after being a new build in 91, it looks like Hylomar was used for jointing.I will prob use a silicone sealer(sparingly of course), I don't think it is down to pressure as it has a large bore breather from the timing cover. Anyone care to comment please?
Thanks for your comments in advance, Paul.
 
Do you have a reed valve in the breather line? If not, hang one there and I'll bet your oil leak goes away.
 
Large bore breather is a bit misleading. No, I take that back. Its very misleading, in fact the hose goes to a stepped down connector before it empties into the oil tank. In addition the crankcase to timing cover internal breather holes are inadequate but unless you're prepared to do a major engine out job, the best choices are as Jim mentioned is to fit a one way reed valve breather close to the timing cover on the present outlet hose. The most popular reed valve product is known as the XS650, because its from some industrial engine that was successfully adapted to the Yamaha 650 and sold by Mikes XS shop in Florida USA. The reed valve blows fumes out but creates a vacuum on the up stroke, with the resulting low pressure keeping oil from penetrating the case joints. The 2nd alternative is to add more breather outlets, such as from the top of the inlet rocker cover. May or may not be successful depending on how bad your case joints have become at this stage. Worth doing a torque up on the motor bolts.

Mick
 
When I rebuilt, I used hylomar on the joint betwen the case halves and Loctite Plastic Gasket in lieu of a paper base gasket and have had no leaks on the motor. Solid copper head gasket sprayed with Permatex Copper Gasket Spray. Hylomar again on the primary chaincase halves, High-tack red gasket spray on the paper gaskets. Honda seal on the drilled out tach drive, reusable teflon gaskets on the valve covers. So far, the only leak has been the outer gearbox cover. Apparently the paper gasket has turned to mush over time. No special breather set-up. The last time I used a paper base gasket, it leaked like a sieve.
 
if your engine leaks it obviously means there is a gap where there shouldnt be one . When taking the engine apart look at the joining faces for differences in colour (shiny or dull )You do not say whether the leak is from the cylinderbase or at the top of the cases themselves .The second possibility is not likely but then one never knows . If you have a leak at the base it probably is the base joint so next time do not fit one .Assemble the 2 cases but do not fully tighten the bolts put the barell on to align the 2 cases in a horizontal plane then tighten the cases .take barell off and continue I use loctite 574 (or pematex )
For all the other joints ;piece of glass and emery paper ,I never use silicon I consider that as a bodge (althou an easy one )
As for that reed valve ,.................
People race them without reed valves and make them leakfree.
decide for yourself
 
Thanks for the replies, it is coming from the crankcase joint and there does not seem to be any scores on the joint, however, it does look a bit discoloured in that area. The breather hose vents down by the rear mudguard as in Norvil style
 
This is a common problem on Nortons which my '72 had since new. Fixed mine completely with $3.98 Motormite valve in breather line where the diameter steps down. Has been working for about 4 years now. Yes there are more elegant solutions.
 
There is only one cure. Nail one crankcase half to your bench, & lap them together with fine lapping paste.
Don't like Hylamar & even worse, silicon. Welseal is good.
 
The old NOC Service notes make mention of this being a problem with 750s. It states that the shuffling is shown by a dull grey area on the joint faces.

Do you know who last put your engine together ? A likely cause on 850s would be failure to 'dummy clamp' the cases up tight when freshly jointed. This means bolts in all lugs with spacers as necessary.

If you're able to post photographs of the joint faces, that may help.

My inclination for a problem metal to metal joint would be to try Yamabond (Three Bond 1194).

http://www.threebond.co.jp/en/product/s ... /1194.html
 
Dental floss!

An old trick to joining crankcase halves is to lay a thread of dental floss in your compound (Yamabond, Permatex or what-have-you) before bolting together.
 
BillT said:
Dental floss!

An old trick to joining crankcase halves is to lay a thread of dental floss in your compound (Yamabond, Permatex or what-have-you) before bolting together.


Raise it up, wax it down.
Put it in a box and sell it in town. Frank Zappa "Dental Floss Tycoon"
 
Danno said:
BillT said:
Dental floss!

An old trick to joining crankcase halves is to lay a thread of dental floss in your compound (Yamabond, Permatex or what-have-you) before bolting together.


Raise it up, wax it down.
Put it in a box and sell it in town. Frank Zappa "Dental Floss Tycoon"

MONTANA

I might be movin' to Montana soon
Just to raise me up a crop of
Dental Floss

Raisin' it up
Waxen it down
In a little white box
That I can sell uptown

By myself I wouldn't
Have no boss,
But I'd be raisin' my lonely
Dental Floss

Raisin' my lonely
Dental Floss

Well I just might grow me some bees
But I'd leave the sweet stuff
To somebody else . . . but then, on the other hand I would

Keep the wax
'N melt it down
Pluck some Floss
'N swish it aroun'

I'd have me a crop
An' it'd be on top (that's why I'm movin' to Montana)

Movin' to Montana soon
Gonna be a Dental Floss tycoon (yes I am)
Movin' to Montana soon
Gonna be a mennil-toss flykune

I'm pluckin' the ol'
Dennil Floss
That's growin' on the prairie
Pluckin' the floss!
I plucked all day an' all nite an' all
Afternoon . . .

I'm ridin' a small tiny hoss
(His name is MIGHTY LITTLE)
He's a good hoss
Even though
He's a bit dinky to strap a big saddle or
Blanket on anyway
He's a bit dinky to strap a big saddle or
Blanket on anyway
Any way

I'm pluckin' the ol'
Dennil Floss
Even if you think it is a little silly, folks
I don't care if you think it's silly, folks
I don't care if you think it's silly, folks

I'm gonna find me a horse
Just about this big,
An' ride him all along the border line

With a
Pair of heavy-duty
Zircon-encrusted tweezers in my hand
Every other wrangler would say
I was mighty grand

By myself I wouldn't
Have no boss,
But I'd be raisin' my lonely
Dental Floss

Raisin' my lonely
Dental Floss
Raisin' my lonely
Dental Floss

Well I might
Ride along the border
With my tweezers gleamin'
In the moon-lighty night

And then I'd
Get a cuppa cawfee
'N give my foot a push . . .
Just me 'n the pygmy pony
Over by the Dennil Floss Bush

'N then I might just
Jump back on
An' ride
Like a cowboy
Into the dawn to Montana

Movin' to Montana soon
(Yippy-Ty-O-Ty-Ay)
Movin' to Montana soon
(Yippy-Ty-O-Ty-Ay)
Movin' to Montana soon
(Yippy-Ty-O-Ty-Ay)
Movin' to Montana soon
(Yippy-Ty-O-Ty-Ay)....


Zappa was a GENIUS!!
 
BillT said:
Dental floss!

An old trick to joining crankcase halves is to lay a thread of dental floss in your compound (Yamabond, Permatex or what-have-you) before bolting together.

That's an old Jame Bond trick. If someone messes with your cases while your not around, you'll know if they've beed disturbed.
 
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