Oil leak from head.

[Bradders]

Morning all. I have had an oil leak on my 400/4 for some time which I’ve been trying to ignore but seems to be getting slowly worse. It seems to be coming from the head gasket area, more from the front and blowing backwards whilst riding. Interestingly all the gaskets were changed in September 2019 ( David Silvers kit ) by my old school mechanic, so am quite surprised they have gone already.
Is there anything I can do to fix this or is it head off and renew the gasket? I’m not sure my limited mechanic skills are up to that.

[McCabe-Thiele (Ted)]

When they banned Asbestos the thickness of the head gasket changed, they did not however change the sizes of the seal rings that don't compress enough.

[Bradders]

Thanks Ted. Does that mean if they were replaced it could well still be a problem?

[McCabe-Thiele (Ted)]

I believe some members have fitted bigger seaks, that might lead to oil starvation to the camshaft so it's a trade off from what I've learnt. A new seal ring will hold the oil back initially then over time, heat cycles you can have a leak

Nurse Julie will know more as will other members, a lot depends on the head gasket thickness I guess.

[flatfour]

I lifted the cylinder head on a 400/4 belonging to a friend a few months ago for this same problem. A genuine gasket was bought from DS, together with the two short rubber tubes that seal the oil galleries on either side of the head. I understand that the 350 & 400/4's use these short rubber tubes instead of "O" rings, although as always, I might be wrong?

It has, incidentally stopped the leak!

[McCabe-Thiele (Ted)]

Correct me if I'm wrong but a DS genuine head gasket might not fix the issue as it will not have the original asbestos base that is thinner, it's probably illegal to sell an asbestos based gasket unless DS have compensated with larger seal rings or somehow made the gasket the same thickness as the original.

PS I keep on calling them O rings they are more like sleeves shaped.

[Bradders]

Thanks for your replies guys. For the time being I’m thinking I’m going to take the exhaust off and get all the old oil off and think about what to do next.

[Athame57]

About a year ago my leaky head was cured with DSS parts as part of a wider rebuild, no problems so far is all I can say.

[Athame57]

As a matter of interest, do you have a very early CB400F? I gather Honda made some subtle amendments to later models to redress an issue with leaky heads.

[Bradders]

As a matter of interest, do you have a very early CB400F? I gather Honda made some subtle amendments to later models to redress an issue with leaky heads.

Hi Athame57. My bike is an early one , I think 122. It had a top end rebuild in 2019 with a DSS gasket set and had been fine until about 6 months ago when it started leaking.

[royhall]

My 350F started leaking at the left front. I took the rocker cover off and re-torqued the bolts to a higher torque than specified in the book, about 15% over if I recall. Surprisingly, considering it had only done 600 miles from rebuild, some of the bolts had come very loose. So far after the re-torque it has not leaked (touching wood urgently  ).

[Athame57]

Hi Athame57. My bike is an early one , I think 122. It had a top end rebuild in 2019 with a DSS gasket set and had been fine until about 6 months ago when it started leaking.

Later models had the length of the cylinder head studs revised, as to whether you can retrofit I don't know, but there will be someone here who can answer that.

[Bradders]

Thanks for the replies guys.
Is it a difficult job to tackle?  My normal motorcycle repair garage didn’t want the job.  I guess they want jobs that they do quick and get on with the next one.

[McCabe-Thiele (Ted)]

Changing the head studs isn't an easy task, full of things that can go wrong, hopefully it's a later model 400, is there an engine number for the change of stud length?

[Bradders]

Sorry Ted, I meant the gasket change. I’m not the most proficient mechanic so not sure I would tackle changing the gasket myself but my repair garage didn’t want to change the head gasket either.