Topic: Dang oil leak-'78 CB750K

[denward17]

Went out today for about ~30 mile ride, after getting home spotted wind blown oil weeps on both sides of cylinders.

Looks like it is originating somewhere in the head between 1-2 and between 3-4.

After cooling I went out and cleaned the areas the best I could.  Plan to take the tank off next week, remove plugs to see if cam tower stud holes are weeping.

After that, who knows......

Top end has been together now for about 2 years or just a little more, with about 3k miles since top end rebuild.

Doesn't appear to be head gasket or valve cover gasket, but will investigate further.

Did I mention I hate oil leaks......

[bryanj]

There were a batch of undersize pucks out there a while ago

[MauiK3]

dang, too bad, might have to remove the head. Post a photo if you get time.

[Stev-o]

Doesn't appear to be head gasket or valve cover gasket, but will investigate further.

Did I mention I hate oil leaks......

If you have difficulty finding the leak, there is always the baby powder trick.

I hate oil leaks too but on classic bikes [and cars], they are inevitable. 

[denward17]

Indeed, planning on spraying it down this afternoon with foot powder and running it in the garage for about 20-30 min. to see if I can spot something.

[denward17]

Sprayed foot powder and ran it for about 35-30 mins, it got hot, oil psi was down to ~18 psi at idle.  I did have a fan in front.

It appears I have some weeping in multiple places on the front of the valve cover.

Head gasket did not show an oil leak, but just under #1 and #4 exhaust showed some past evidence of leaking.


Will take plugs out in a couple of days to see if it looks like pucks are leaking.

Could be the thrust washers on the head bolts?  Or the o-rings between head and cylinder( of the cylinder studs?

I did take the plugs out before starting and the plugs looked ok.

[Stev-o]

The third pic clearly shows valve cover weeping, unsure of the other two? 

I'm no expert of oil leaks, I typically just wipe them clean and ride on.

[BenelliSEI]

Den…. The cb750K0 I’m working on now has a drop hanging off the fin, just under #1 ex. pipe…… The new owner say it leaves one drop on the ground after a long ride. I suspect a “puck”. Like Steve, my advice to him is “wipe it off”. It will probably stop before it gets worse…..

[denward17]

The best I could tell the pucks are not leaking, using a sharp focused light and a small mirror.

Couldn't really see #2 and 3 though because of the frame being in the way.  I didn't get any oil on my finger on #2 and 3 either.

I did see a wet bolt head (6mm), bolt on the end of #4 cylinder that you can see through the fins (on the side of the cylinder), see pic.

Not sure how oil would end up there without the puck leaking, and it is not on #4.

Not sure where to go with this yet.

[Stev-o]

I did see a wet bolt head (6mm), bolt on the end of #4 cylinder that you can see through the fins (on the side of the cylinder), see pic.

I see the oil and also the remnants of something, if I didnt know better, looks like coolant.
Is it possible water got in there from either washing or rain?

[denward17]

I did see a wet bolt head (6mm), bolt on the end of #4 cylinder that you can see through the fins (on the side of the cylinder), see pic.

I see the oil and also the remnants of something, if I didnt know better, looks like coolant.
Is it possible water got in there from either washing or rain?

No rain or wash, I don't use it enough to wash it.

[BenelliSEI]

Den…. On a few heads, I have seen the threaded holes for the cam box hold down bolts that are drilled and tapped right through to the first “air gap” in the head. Occasionally, oil can creep down those bolt threads and drip. Take a bright light and carefully inspect in that first open slot….. Because the engine is canted forward, it will end up dripping out front by the exhaust ports.

[denward17]

Thanks Guys,

I think I will leave it be until it gets worse.  I was able to tighten down the valve cover a little bit on all screws.

I will just watch it a while and if it gets worse will pull engine and deal with it.

When I put it back together, I did put sealant on the cam tower studs (use of mirror indicated they were not leaking), but not sure if I did the cam tower tower bolts.

[BenelliSEI]

I had a good look at the K0 I am working on today. It only weeps the tiniest amount, and I think it is one of those fasteners. I feel your pain!

[seanbarney41]

My F0 does weird leaking stuff from time to time.  It's like a sweat that comes and goes.  One time when I had a bunch of stuff in the middle of the bike torn down, I noticed that the breather/cpllector was plumbed wrong.  So I rerouted the three hoses from the valve cover, to the airbox and the drain so they matched what was shown in the service manual....and on the first ride the thing literally puked it's guts out.  I thought I was going to have to rebuild the thing after all these years.  I put all the hoses back where I originally found them which according to the manual is incorrect, and the massive leaks stopped, just the slight sweating that comes and goes that it has always had since I owned it...almost 20,000 miles I have put on it's 23,000 when I got it.  Basically, make sure your crankcase vent is working properly.

[HondaMan]

This all sounds like the 2 O-rings through the head gasket for the 2 oil passages at the back of the cylinders. The 'clue' to me is: it appears between cylinders 1-2 and 3-4, not in the middle of the cylinders.

These are the 2 O-rings that are 2.4mm thick in OEM and gasket kits: they are not thick enough for modern head gaskets made since 1996 when asbestos was outlawed. The new head gaskets are 0.2mm thicker, so those 2 O-rings must also be 0.2mm thicker, or they will leak. You can also mill back the cylinder deck by 0.010" (technically, 0.008", but 0.010" comes out more consistently in practice) when it is apart if you really want to use the normal 2.4mm thick O-rings: I usually do both.

The leak track follows the lesser-pressured zones of the (torqued-down) head gasket between the 2 cylinders on either side, making the oil appear in between the 1-2 and 3-4 pipes at the front. It takes about 1500-2000 miles to leak after a rebuild, typically.

[denward17]

This all sounds like the 2 O-rings through the head gasket for the 2 oil passages at the back of the cylinders. The 'clue' to me is: it appears between cylinders 1-2 and 3-4, not in the middle of the cylinders.

These are the 2 O-rings that are 2.4mm thick in OEM and gasket kits: they are not thick enough for modern head gaskets made since 1996 when asbestos was outlawed. The new head gaskets are 0.2mm thicker, so those 2 O-rings must also be 0.2mm thicker, or they will leak. You can also mill back the cylinder deck by 0.010" (technically, 0.008", but 0.010" comes out more consistently in practice) when it is apart if you really want to use the normal 2.4mm thick O-rings: I usually do both.

The leak track follows the lesser-pressured zones of the (torqued-down) head gasket between the 2 cylinders on either side, making the oil appear in between the 1-2 and 3-4 pipes at the front. It takes about 1500-2000 miles to leak after a rebuild, typically.

Thanks Mark,  If I recall, I did get the o-rings from you when it was put together about 2 years ago.  I'm working on re-painting the tank/side covers, when that's finished I will ride it a bit and see if I can tell where the oil is coming from, may have to tear it apart again.

[denward17]

Getting back to looking at this.

After my tank/side covers refinishing work, I changed the oil/oil filter and went on a 25 mile ride. 
The only oil seepage I saw was around the 6mm bolt at the end of the head.

Also had a little too much oil in it, so retrieved some out of the oil tank, to put me up about 3/4 way of the hash marks.

With use of a small 1" mirror in #4 spark plug access region, I could see underneath the puck on the end was dry, as well as the front cam tower stud hole, and the end valve cover hole.

I did use thread sealant on all cam tower studs, but not the cam tower bolts.

I'm not sure of the rear cam tower bolt, as I could not see if/where it could have access to open air.

I marked up a picture to identify the only places I think the oil leak could be coming from to wet the top of the 6mm bolt on the end of the head.

I'm waiting on a new battery so I can start it again in the garage for about 20 mins to see if it is still leaking in this area.

What's your thoughts?

[HondaMan]

A long shot, but maybe this: loosen and slide off the exhaust pipe's header clamp for the #4 cylinder, see it things are 'wet' in there?

I've had a K7 and a K8 both did this, but it was an expensive fix. The exhaust valve guide was very worn on the #4 cylinder. The #1 was, too - but, it didn't weep a drop(!). I got the bikes at a good price & rebuilt their top ends: they are still running around here almost 15 years later.

[denward17]

A long shot, but maybe this: loosen and slide off the exhaust pipe's header clamp for the #4 cylinder, see it things are 'wet' in there?

I've had a K7 and a K8 both did this, but it was an expensive fix. The exhaust valve guide was very worn on the #4 cylinder. The #1 was, too - but, it didn't weep a drop(!). I got the bikes at a good price & rebuilt their top ends: they are still running around here almost 15 years later.

Something to look at indeed, but I did have a head job during resurrection about 2 years ago.  All valve guides, valves were replaced.

Edit to add:
All valve cover screws on the front were open to atmosphere on the bottom, not sure they could leak there, but went ahead and took all of them out and put thread sealant on them.

Also took off the header clamp on #4  (didn't remove pipe), and there was a wetness of oil on the bottom of the clamp.  Hopefully not valve related, just seepage from above that.

Will investigate furhter next week when I get new battery.

[bryanj]

F2 was renowned for chocole exhaust guides

[newday777]

F2 was renowned for chocole exhaust guides

chocole exhaust guides??

[denward17]

F2 was renowned for chocole exhaust guides

chocole exhaust guides??

I think Bryan is referring to worn exhaust guides......easy to check I guess, I think I will remove the exhaust pipe on #4 and have a look see.

[bryanj]

Was suposed to say chocolate but my dyslexic keyboard got in way, plus i am day 4 into a supposedly 3 day "bug"

[Stev-o]

Was suposed to say chocolate but my dyslexic keyboard got in way, plus i am day 4 into a supposedly 3 day "bug"

I was going to ask you to "speak English"!  Ha...