Oil leak question, cooler area and underneath CM400A 1981

Windmill John

Windmill John

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Hi All,
I'm getting ahead of myself at this point, but… As per the picture, oil cooler and underneath sump is covered in oil. Haven’t yet looked in detail, just preparing.
Are there default seals and areas I should be looking at? I saw a couple of seals in the parts diagram on the oil cooler for example.
Thanks.
IMG_2026-04-16-144122.jpeg
 
I would verify that the 5 (M6mm X 60mm) oil cooler mounting bolts facing the front wheel are tight to the Factory Service Manual (FSM) listed tightening torque figure, especially the 2 bolts at right side which close on the gasket there between oil cooler and engine. Also verify tightness of the 2 bolts (M6x28) at left side of oil cooler that close on the gasket at left end of oil cooler.
THEN spray wash off as much of the leaked oil as you can, blow it dry with an air compressor and blow nozzle, or hair dryer, or shop vacuum with hose in blower hole on vac, THEN run engine with fan aimed at engine while looking for leak source.

More likely that leak is from the valve cover gasket at engine's top or one of the 2 rubber-grommetted valve cover mounting bolts (shouldered), or maybe even the cylinder head gasket

And when you verify the valve-tappet clearances and have the valve cover removed for that, check the tightness of the 8 cylinder head bolts in 2 to 3 steps following the tightening sequence shown with a photo in the FSM. For final of those 3 steps, set your torque wrench to 22-24 Ft-Lbs
 
Thanks for the info Boosted. There is leak from the valve cover as well, but this is separate to the lower ones.
I think I will take off the cooler to replace the seals. There are no caveats you’re aware of to removing the oil cooler?
 
Seals are 100% unavailable. dont remove this unless youre dead certain it is the source of the leak. they dont leak unless disturbed.
 
packing oil cooler gasket l. there are two. they are not available.

trust me on this. they are not available. nobody makes them. they are not in an aftermarket kit. its the rarest part on the bike next to the change switch.
 
clean the area well, replace valve cover gasket and bolt grommets then see what you got. Most oil leaks are there and the dowel pin grommets that are between the head and cylinder.

If you think the oil cooler is leaking and you've confirmed its not the above mentioned areas then the starter motor o-ring is more likely.

do not remove that oil cooler except as a last resort. because of the gasket being long gone it might be the only one id say a thin layer of hondabond (NOTE: NOT RTV!) would be appropriate. dont do any such thing before consulting in this thread.
 
Interesting that CMNSL show those parts as available, but if you cut and paste the part number in David Silvers site it shows the part as NLA. Usually, when DS don't have it, they say available by special order, which I always assumed that they meant that they'd source it from CMNSL.
 
Not sure what youre seeing because #4 and #5 are out of stock. It says add to watch list. Cannot physically add to cart.

Believe me, its unavailable. I have two of each. One I sourced from France from a place that would not sell out of France and I had a friend who lives there buy it for me.
 
If you need to remove the starter motor which I did on the 1984 CB450SC I bought two years ago. I could not get it out of the motor frame without removing the oil cooler. It was real close to coming out, yet the cooler needed to come off. I was fortunate my bike was a cream puff with very low kilometers, so I was able to smear a thin coat of Permatex Gasket maker on those two O rings and reuse them.
As Frank noted they are as rare as hen's teeth to find anywhere these days.
 
You can remove the motor with oil cooler installed. Requires patience, but I have pulled it off.

EDIT: Suspending the frame with winches or similar and balancing act with a motorcycle jack to remove the engine can get it out in one piece. Including the cylinder and head attached.
 
Thank you all, I might look at this first then. As I’ll be draining the oil too, then if recommended, I might pull the right side off to check cam chain tensioner too.
 
Not sure what you mean. do you mean the balancer chain? If yes, this requires new gaskets and an o ring.

Before you do things you might want to ask us as its very easy to fall into these traps and cause unnecessary expense.
 
Maraakate said:
Not sure what you mean. do you mean the balancer chain? If yes, this requires new gaskets and an o ring.

Before you do things you might want to ask us as its very easy to fall into these traps and cause unnecessary expense.
Click to expand...
Thanks Maraakate, I’d like to ensure I don’t make things worse.
Yes, the balancer chain. I’ve bookmarked your post regarding this.
 
I would just get it running first before you tackle that. Balancer chain is nice to do, but the bike will not disintegrate if you ride it for a summer without tensioning it.
 
Actually, when I heard the engine running, it was quiet. At this stage do you feel, from your experience, to leave checking the balancer chain? I don’t normally ignore things, but maybe out off for a bit….
 
Balancer chain counterbalances vibrations. I've never heard one with very bad slack, but have seen a few old posts mentioning it. Regardless, just focus on the leaks and getting it running before you put money and time into that.
 
Thanks again. Having tremendous fun trying to find tyres; more accurately, the rear. Being 81, it’s tubeless, one of the reasons for choosing.
I know tyres might not be a priority compared to leaks, but if I want to try it on the road… They are old and hard.
There are other jobs, but I’ll stick with tyres and leaks, plus fork seal leaks….
I’ll try and leave you in peace for a mo until something rears its head!
Oh, need to clean carbs due to the misfires… hoping it’s carbs anyway… Don’t say anything! 😉
 
Out of curiosity I looked up 4 and 5 on E-bay and 4, 15911417000 shows several listings with this being the best price and free delivery.

www.ebay.com

GENUINE HONDA GASKET PACKING OIL COOLANT C CAP GASKET SEAL OEM NEW 15911417000 | eBay

GENUINE HONDA PART # 15911417000.
www.ebay.com

Number 5 15912417000 only shows 1 listing with only one left but at a good price IMO.

www.ebay.com

NOS GENUINE HONDA PACKING, OIL COOLE CB400 CM400A 1978-1981 OEM # 15912-417-000 | eBay

Find many great new & used options and get the best deals for NOS GENUINE HONDA PACKING, OIL COOLE CB400 CM400A 1978-1981 OEM # 15912-417-000 at the best online prices at eBay! Free shipping for many products!
www.ebay.com
 
Windmill John said:
As per the picture, oil cooler and underneath sump is covered in oil.
Click to expand...
Also, the "sump" could be oily if the oil filter case's (housing, sump) gasket (#11) or the O-ring for the oil filter bolt (#10 are either missing or hardened and or shrunk from age and heat-cycles.
These 2 parts (gasket & O-ring) are included new with some brands of oil filters

1776376932104.png
 
Boosted said:
Also, the "sump" could be oily if the oil filter case's (housing, sump) gasket (#11) or the O-ring for the oil filter bolt (#10 are either missing or hardened and or shrunk from age and heat-cycles.
These 2 parts (gasket & O-ring) are included new with some brands of oil filters

View attachment 56923
Click to expand...
its literally $2 more for an OEM filter and includes all you need. why buy any other brand with such a small price difference?
 
RobMan said:
Out of curiosity I looked up 4 and 5 on E-bay and 4, 15911417000 shows several listings with this being the best price and free delivery.

www.ebay.com

GENUINE HONDA GASKET PACKING OIL COOLANT C CAP GASKET SEAL OEM NEW 15911417000 | eBay

GENUINE HONDA PART # 15911417000.
www.ebay.com

Number 5 15912417000 only shows 1 listing with only one left but at a good price IMO.

www.ebay.com

NOS GENUINE HONDA PACKING, OIL COOLE CB400 CM400A 1978-1981 OEM # 15912-417-000 | eBay

Find many great new & used options and get the best deals for NOS GENUINE HONDA PACKING, OIL COOLE CB400 CM400A 1978-1981 OEM # 15912-417-000 at the best online prices at eBay! Free shipping for many products!
www.ebay.com
Click to expand...
Second one gone already which is unfortunately the most needed one.
 
Just posted a picture after first ride today.. brilliant. Anyway, got home and noticed some oil underneath the engine when I covered him.
Obviously we’ve touched on this, but just want a current view, as I’ve degreased the engine, new drain washer, new oil filter o rings.
You can see the oil filter and drain are clean.
The two red marks look like drips, but nothing on floor yet.
The two green areas you can see have oil.
Anymore narrowing down the area?
It‘s not from oil filter; reasonably sure.
It‘s not from the valve cover; new seal and nothing running down.
Someone had mentioned the starter motor, but can’t see anything obvious.
Thanks.
IMG_2026-06-29-124128.png
 
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