Author Topic: CB400F Crankcase Oil Leak  (Read 585 times)

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Offline jakec

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CB400F Crankcase Oil Leak
« on: September 04, 2018, 12:43:29 pm »
So I have this bike, I've been riding it for about 5-6 months now, daily. It has always had this oil leak when warm (stops and does not leak once it cools down). Immediately after I noticed the leak I went to investigate and found this stuff, what I assume is JB weld tacked onto the engine mounts. It's near where the oil leaks. I've drawn an arrow on the photo to show where the oil's path is each time I ride. After a ride I'll lean down and wipe up some oil from the underside of the frame there.



Anyways I knew I was going to have to fix this, but I wasn't sure how tough it would be. I figured I would go to a guy who can weld aluminum and he would be able to seal it up. However this weekend I was at a swap meet and this older guy came up to talk to me about the bike. He said he had the same bike in 77, and then he told me this story.

He said that when he owned the bike he tried to install crash bars. One of the bars utilized this mount, and in the process he somehow punched a hole in the case. The way he was able to fix it was to get new case halves and rebuild the entire engine into them. He said that they weren't able to weld the leak from the outside. So now he has me thinking. I have the same leak. I know that 750's get patched up under the sprocket cover all the time. But is this different somehow?
1975 CB400F
1972 CL175 K2

Offline Tim2005

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Re: CB400F Crankcase Oil Leak
« Reply #1 on: September 04, 2018, 01:24:02 pm »
Oh dear, yes it's not unknown - if you fit a too-long engine mount bolt it goes through the casing into an oilway. I've never heard of one being welded, it would be very difficult to do as the damage area is in a recessed hole.

Offline jakec

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Re: CB400F Crankcase Oil Leak
« Reply #2 on: September 04, 2018, 01:58:52 pm »
Is the "oilwawy" the cylindrical part of the casting that has the JB tacked on?

The guy I talked to said that they couldn't weld it from the outside, couldn't weld it from the inside either. That's how he ended up with new cases.He told me all this before I even told him I had the leak.
1975 CB400F
1972 CL175 K2

Offline Tim2005

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Re: CB400F Crankcase Oil Leak
« Reply #3 on: September 04, 2018, 02:45:22 pm »
Yes that's it. So you can see, if the bolt was a bit too long it would go into that and crack it as it was tightened up.

Offline jakec

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Re: CB400F Crankcase Oil Leak
« Reply #4 on: September 04, 2018, 02:52:15 pm »
I suppose if you're doing 30 ft/lbs. Wow. So the fact that the area beyond is an oil passage probably makes is pretty difficult to patch with a weld. But I wonder if it depends on the severity of the crack/leak. If they were able to go at it with JB weld perhaps it may be worth a shot taking it an experienced welder and seeing what they think of it.
1975 CB400F
1972 CL175 K2

Offline Tim2005

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Re: CB400F Crankcase Oil Leak
« Reply #5 on: September 04, 2018, 03:03:02 pm »
Well it might be worth a shot, but the problem is going to be getting the welding gear onto the cracked surface, the raised boss that the bolt goes through is in the way.  However, I would think that they could cut off that boss, do the weld repair, then weld the boss back on.

Offline jakec

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Re: CB400F Crankcase Oil Leak
« Reply #6 on: September 04, 2018, 03:14:22 pm »
Thanks for the advice. I would love to hear more from anyone else who may have had experience with this. I feel like if I buy a set of cases on Ebay I'll be taking another risk.
1975 CB400F
1972 CL175 K2

Offline przjohn

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Re: CB400F Crankcase Oil Leak
« Reply #7 on: September 04, 2018, 03:40:13 pm »
How bad is the leak? Is it leaking enough that you need to add oil on a regular basis or is it just spotting the floor in your garage? If it is just a spotter I would leave well enough alone. I have a 2008 KTM 990ADV that got caught up in some rocks and is a spotter. The crack is in a reinforced area and has had a JB Weld repair. It has since had 25,000 mostly off road miles put on it. I just put a pan under it, maybe leaks a cup of oil all Winter. I ain't tearing a motor apart for that, never mind the cost of the cases, etc. I know that having a bike leak oil is not the thing you want, well, unless you ride an English mount  ;D. Ever been to a vintage bike museum? These places are practically advertisements for Aluminum Roaster Pans. If it ain't bad just ride it.
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Offline robvangulik

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Re: CB400F Crankcase Oil Leak
« Reply #8 on: September 04, 2018, 04:11:32 pm »
It is just trying to mark its spot, let it be. If it doesn't leak anymore you must top up the oillevel. ;D

Offline jakec

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Re: CB400F Crankcase Oil Leak
« Reply #9 on: September 04, 2018, 04:14:00 pm »
It's not that bad. But I don't know the history of how the repair was done, and I ride this bike quite a bit. I've probably put more miles on it in the last 6 months than it's seen in the last 20 years. For now it's rideable although it is annoying, spots my garage and also makes my stands wet with oil at all times. I don't know if the repair will hold up forever with daily riding.

On top of that, I have a long list of winter projects to do on the bike, that will put it out of commission for a while, so I figure maybe I should just see what I can do? At the very least I am rebuilding the carbs and doing a rocker cover gasket, maybe head. at that point, pulling the engine is only a little more work.
1975 CB400F
1972 CL175 K2

Offline Bodi

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Re: CB400F Crankcase Oil Leak
« Reply #10 on: September 04, 2018, 07:14:04 pm »
It will be quite tough to successfully weld oil soaked alloy but possibly it could be done from inside the case, closing whatever cracks are present? Or super clean the area inside the case and coat it with that good ol' JB Weld? It is best to seal leaks from the inside rather than patching the outside.
Pulling the engine and opening the cases is a bit of a chore but you get to see everything inside and may find some problems developing that, if left unresolved, may bite you in future. Depending on mileage - and abuse during those miles - a cam chain and primary chain could be a worthwhile investment.

Offline jakec

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Re: CB400F Crankcase Oil Leak
« Reply #11 on: September 04, 2018, 09:16:57 pm »
Thanks for the insight. I figured that even if it were welded in any case the cases would have to be apart.. just makes sense. I would lean toward that option rather than buying case halves from Ebay that could have their own problems... Buying things on Ebay is how I got into this mess  ::)

The mileage now is 12,000. I think I will ride this bike for a long time. Perhaps I should consider a donor bike/engine in the case that I want to do this work myself? That way I can stay on the road and/or have a source for parts. I could also just swap in a motor for probably less than it would cost to do the fix...
1975 CB400F
1972 CL175 K2

Offline awrawr

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Re: CB400F Crankcase Oil Leak
« Reply #12 on: September 08, 2018, 06:38:51 am »
Mine has a weld here. Same cause. Hasn't leaked for nearly 40 years now. I guess that counts as a successful repair ?

Offline jakec

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Re: CB400F Crankcase Oil Leak
« Reply #13 on: September 08, 2018, 08:56:33 am »
Would you be willing to take some photos of it? I know it's a lot to ask, but maybe you could pull the bolt out and take a photo? I would be very grateful.

Was the repair done when you purchased the bike?
1975 CB400F
1972 CL175 K2

Offline awrawr

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Re: CB400F Crankcase Oil Leak
« Reply #14 on: September 09, 2018, 12:26:13 am »
I noticed the leak shortly after I bought the bike. I went back to the shop and they gave the option of a refund or a repair. I went for the repair. I am about to remove the engine from the frame, so pics are no problem. Watch this space.......

Offline calj737

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Re: CB400F Crankcase Oil Leak
« Reply #15 on: September 09, 2018, 04:02:24 am »
It can be welded, but not easily. Body is correct, welding 40 year old cast aluminum is hard enough, but oil-soaked cast aluminum is the Devil’s Balls.

To weld it properly, split the cases, parts wash the cases at a machine shop. Then clean them some more with Acetone, inside and out. The surface area needs to be scrubbed with stainless brushes, and I use 4047 filler wire for cast ally and peen the weld as I go. Else, you will get tons of porosity that will produce seepage.

Someone proposed cutting off the bosses and edging those back on. I wouldn’t unless absolutely mandatory. Even then, I’d have donor bosses available. The bosses are thick, the cases not so much. You’d need to grind bevels into the boss mating surface to get sufficient strength and penetration and use multiple passes, peening as you go.

Welding aluminum is so difficult. Welding cast is tricky. Welding 40 year old cast is a whole other bailiwick. And the oil and grime saturation, and a competent welder who will insure it’s right will charge you by the hour, and expect it to be 3-4 hours if you clean it first.

Now you may get lucky and there’s a defined crack inside the case, and it can be ground out and grooved, filled, then filed. That might take an hour. But expect the worst, and pray for the best. With cast ally, the heat from welding causes the air pockets in the surrounding area to expand. You end up with “blisters” and those are weakened. So it takes some care not to cause more damage than repair.

I’m not trying to say it’s impractical or impossible. Just explaining how it should be done correctly so you know ahead of time what to expect. If you’re splitting the cases over the winter anyway, get it done. If not, use some donor cases then keep track of the bearing shells.
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Offline awrawr

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Re: CB400F Crankcase Oil Leak
« Reply #16 on: September 09, 2018, 10:50:44 am »
I will be dismantling to bare crankcase soon, so better pics in October I hope. The nut on this mount has been shaped with a grinder to fit the recess. (Apologies for the tight crop - only way I could get below the size limit on my phone).

Offline jakec

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Re: CB400F Crankcase Oil Leak
« Reply #17 on: September 10, 2018, 09:47:06 am »
Interesting, thanks for the photo. It looks similar to mine. Has your repair still held up or were you saying it is now starting to fail? All of this information is great, and it makes me consider whether the fix is worth it. I just really hate having an oil leak.
1975 CB400F
1972 CL175 K2

Offline awrawr

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Re: CB400F Crankcase Oil Leak
« Reply #18 on: September 10, 2018, 10:27:43 am »
The repair has lasted since it wad done and is still ok with no sign of any leakage.

Offline jakec

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Re: CB400F Crankcase Oil Leak
« Reply #19 on: September 20, 2018, 08:37:14 pm »
I currently have a plan.. I think that either way (repair or replace) the cases have to come apart. So I can tear the engine down and show the cases to a welder. And if he says no go, then I buy another set and rebuild with those.

As for timeline, I need another bike if I'm going to take my bike out of commission for a few months. And I predict that I'll need a few months, since it seems that I'm always spending a week or more on each stuck screw, or waiting for tools I need, etc. So, I'm thinking maybe I just keep riding the bike as is until the right replacement comes along, then i can take my time and do a good restoration of this engine, and maybe other parts of the bike. (I've been reading about big bores  ;) )

Here's a good candidate...  https://denver.craigslist.org/mcy/d/1975-honda-cb400f-ex-race-bike/6701123479.html

Anyways, if I were to buy a set of cases off of ebay, I don't really know what signs of wear to look for. Since I have never seen the inside of the cases, I don't know what's "normal" wear, and what to stay away from. I have viewed a couple of sets online. They range from $80-200.

https://www.ebay.com/itm/1975-HONDA-CB400F-SUPER-SPORT-CRANK-CASE-EMPTY-MOTOR/330708816198?hash=item4cffc81146:g:L5wAAMXQvJVRQXfL&vxp=mtr
1975 CB400F
1972 CL175 K2

Offline jakec

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Re: CB400F Crankcase Oil Leak
« Reply #20 on: September 24, 2018, 01:36:05 pm »
Watching this series of great disassembly/rebuild videos and I notice that this guy's crankcase has the same repair as mine.

1975 CB400F
1972 CL175 K2

Offline awrawr

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Re: CB400F Crankcase Oil Leak
« Reply #21 on: September 29, 2018, 02:14:39 pm »
Better pic of my repair
 

Offline jakec

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Re: CB400F Crankcase Oil Leak
« Reply #22 on: September 29, 2018, 06:56:28 pm »
Thank you. That is helpful. I found a friendly guy in NY who is willing to sell me some nice cases, so unless that fall through, I will probably be doing that. I think the highly skilled welding required would quickly get expensive.
1975 CB400F
1972 CL175 K2

Offline Tim2005

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Re: CB400F Crankcase Oil Leak
« Reply #23 on: September 30, 2018, 05:29:27 am »
If you change cases you may well need new main bearing shells as their sizing varies with each set of casings (it would be good practice to change them in any event) - are all sizes of 400f main shells actually available nowadays?

Offline jakec

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Re: CB400F Crankcase Oil Leak
« Reply #24 on: October 02, 2018, 09:24:38 pm »
Unfortunately I don't know what you mean, perhaps you could point them out?

The guy selling me cases has a complete engine and he was going to strip the cases off for me, he can send me any other parts I request.


1975 CB400F
1972 CL175 K2

 

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