Combat motor oil leak

Bearwolf · Feb 14, 2011

Hey Guys

72 Combat motor. Oil leak from cyl base gasket. The poor crankcase breathing in this motor is giving me fits. Replaced the base gasket w. orig. gasket and 500 miles later it leaked again. Bought a copper gasket and installed but a fine mist is still leaking from the joint. No gasket compound was used, maybe that would have helped. If a gasket sealant is to be used on copper, what would be the best choice? Any other solutions are welcome. The gasket which I installed, is the same thickness as the orig. paper gasket but it comes in a thicker material as well. Perhaps that is a solution? Anyone out there going through the same problem?
Motor runs extremely strong. Was purchased in Canada as a total rebuild, crankcases are highly polished, head is ported, polished and fitted with slightly larger valves I was told. I didn't check them for size while I had the head and barrel of. Runs w. a single 38mm Mikuni.
It is installed in my 71 roadster, which I purchased new 40 years ago, while I'm rebuilding the orig. motor but still like to ride in the meantime.

Any clues or ideas will be greatly appreciated, Thanks.....

Johnny

Tim_S · Feb 14, 2011

I used a very light coat of Yamabond on my paper gasket. I've never seen a copper one as a base gasket though... :?: Make sure after the assembly process you double check all fasteners on the motor once it's gone through a few hot & cold cycles. Good luck ,,,,,,,,

Tim_S

DogT · Feb 14, 2011

If you don't want oil leaks, use Permatex Hylomar Blue Racing Formula. It's strange to work with, but it won't leak. I found it best to apply just out of the tube. Uses acetone as the solvent. I used it on my 69 with it's proclivity to leaking and none so far, but only 10 miles on it, but when the valve covers leaked, on went the Hylomar and leaks gone. Expensive but a tube should do a bike, like $15 for 2oz.

Plus hobot recommends it so it's got to be good.

Dave
69S

askdoctorday · Feb 14, 2011

But doesn't Hobot also recommend wheelies and crashing into goats....??

DogT · Feb 14, 2011

Wheelies and goats are good too, plus burn tracks in snow.

Dave
69S

rvich · Feb 14, 2011

The thicker base gaskets will act as a shim to lower the compression ratio. Be sure that is what you want before moving in that direction. As for copper, I really like the copper spray sealer. It is available at most NAPA stores and others I am sure. Use it at your own risk...it works great but I have no idea what it might be like to release the barrel later.

Russ

ILLF8ED · Feb 14, 2011

Hi Johnny,

Copper base gasket is a new one for me. Paper gaskets and aluminum spacer I'm familiar for cylinder base. Are you talking about cylinder base or head gasket?

Hobot · Feb 14, 2011

Hylomar can take the heat and elastic motions of cast iron barrel on Al cases. A base gasket is crush-able under these conditions and gives another unstable seam to seal. A synthetic thread I found melts, to flatten out after the grey-stuff hardens up and then can contract and leave gaps. The grey goops are like wet corn starch, pound on it fast enough and its no more a fluid-elastic medium but stiffens up and fractures to allow hot oil to be forced out. I'm looking for silk or plain cotton next time.

Need to have PCV crank vent check valve to reduce of eliminate the blow by pressure forcing oil to seek its home in the ground. Poor ring seal aggravates this.

So far a I've found a stiff artist brush clipped to ~3/8" length works best to goop on Hylomar and others. Hm, did I remove the stem plugging the case oil drain, hm.

ole crash and burn again and again hobot and out.

Bearwolf · Feb 14, 2011

Thanks All.
50 years of Brit bikes and fourty of them with the Commando. I never thought I could have help this instantaneously. What a forum this is, just absolutely great. What a world we live in. No wonder the Egyptian people wanted in on the fun. More power to them and their two-wheelers.
It is indeed a copper base gasket, available on e-bay. Loosing compression by going .015in thicker I can live with but will it work? Adding a PVC vent makes sense, I'll do that. This supprises me though. I put more than 100,000 miles on my orig. 71 motor with a couple of overhauls but never had this "breathing" problem. This is a Combat motor though. The "silk thread" method was used often on car engines with alum. heads. I guess it could work here. Where would one find the thread? The Copper Spray Sealer and Permatex Hylomar both sound like ways to go as well.
All good stuff, I'll carry on and eventually let you know how it comes out.

Thanks, Johnny

bmwbob · Feb 14, 2011

I've always been told to not use a base gasket! Just a thin layer of your favorite gasket goop!

rvich · Feb 15, 2011

As for silk thread, I would check with fly fishing shops (online for that matter) that cater to traditionalists.

Russ

Foxy · Feb 15, 2011

Gday All, just wondering if anyone has used used the Hylomar in a spray can? I see some on shelves at auto stores.
Foxy

xbacksideslider · Feb 15, 2011

For me, it's a choice between silk thread and hylomar or Yamabond, and thin paper gooped with anhydrous lanolin.

For applying any gasket sealer, I trim those cheap tin handled brushes made for applying soldering acid down to 3/8"

Hobot · Feb 15, 2011

Keeping a close squish band is helpful to Combats anti-detonation.

Spray on Hylomar is same stuff just either easier or messier to apply depending on conditions used. Can combine with a thread to back up/take up space in seams.
But Hylomar can take up as much gap as the thicker grey goops and easier to remove.

Copper or silver pain or made fo it copper sprays are standard good practive, but I'm going to try just Hylomar around the bores and push rod tunnels and oil drain hole. Only full race WOT heads see into 400+F temps and Racer Blue stands 600'F.

About all engine and gear box fasteners need more torque than manual shows while about all the nuts on drive sprockets/clutch need rather less.

Lockite has a tape version out now, may try that for less mess stcky-ness.

There is a special cheap brake valve PVC that someone can ID or $100 fancy ones made my Krank that a few venders handle. Cheap one needs renewal yearly but is good fit and function.

At hi rpm Combat throw oil back so the case is modified to move oil sump to rear. But the low down breather pumps excess sump oil back to tank faster than the oil pump can, so many just extra oil mess on hot rod playing.

mikegray660 · Feb 15, 2011

illf8ed said:
Hi Johnny,

Copper base gasket is a new one for me. Paper gaskets and aluminum spacer I'm familiar for cylinder base. Are you talking about cylinder base or head gasket?

i've always used the copper base gaskets but do spread a thin swipe of Loctite 518 - never had a problem leaking and done 3 commando's w/ this

loctite 518 gasket maker is a non-corrosive, anaerobic sealer designed primarily for use on aluminum, iron, and steel flanged mating surfaces. I 518 gasket maker fills gaps of up to 0.050 in. and forms a flexible, solvent-resistant seal that will not tear or decay. Parts will disassemble easily even after extended service, and old gasket material can be removed with a putty knife.

i'm sure some one will or has asked- what about a PCV valve

JimC · Feb 15, 2011

The most effective oil leak stopper I've used is a PCV valve. There is no way in hell to seal up a crankcase that has positive pressure other than welding it shut. The PCV valve, no doubt, creates a negative (vacuum) pressure. Much easier to keep oil in that way.

mikegray660 · Feb 15, 2011

JimC said:
The most effective oil leak stopper I've used is a PCV valve. There is no way in hell to seal up a crankcase that has positive pressure other than welding it shut. The PCV valve, no doubt, creates a negative (vacuum) pressure. Much easier to keep oil in that way.

Jim
I knew you'd say that! (and i agree)

JimC · Feb 15, 2011

Ya', Mike, I do love the PCV valve. All this B.S. about special sealants, trick crankshaft seals and special techniques is pretty much peeing into the wind if the crankcase is pressurized. I speak from experience there. The other one that gets to me is trying to get a better front disc brake with no regard to caliper to master cylinder ratio. You can have a Brembo Moto GP master cylinder, but if it's not properly matched (correct ratio) to the caliper(s) you've spent a lot of money for naught.

pete.v · Feb 15, 2011

Check for burrs and raise metal around thread holes.
You can put nothing, something, everything or a combination of all the above but if the mating surfaces are not as perfect as posible, things will leak. Maybe not as much with a PCV, but they will leak.

JimC · Feb 15, 2011

pvisseriii said:
Check for burrs and raise metal around thread holes.
You can put nothing, something, everything or a combination of all the above but if the mating surfaces are not as perfect as posible, things will leak. Maybe not as much with a PCV, but they will leak.

And the converse is true. Everything properly mated and sealed, but with crankcase pressure the oil WILL find a way out.

Combat motor oil leak

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