Honda-SOHC
SOHC.co.uk Forums => CB350/400 => Topic started by: Bradders on November 05, 2024, 10:49:44 AM
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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.
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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.
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Thanks Ted. Does that mean if they were replaced it could well still be a problem?
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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.
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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!
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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.
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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.
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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.
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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.
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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.
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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 ;D ).
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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.
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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.
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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?
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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.
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To replace the head gasket risks disturbing the bottom block seal gasket & four base rings. Not sure if members here would take a risk and just replace the head gasket only.
Care has to be taken when removing the camshaft as you don't want a bolt or the chain to drop down into the engine.
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That sounds quite a way beyond my mechanical skills. Thanks for the info Ted.
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The thing about mechanical skills is they improve if you challenge yourself to try to fix something, when it works you gain confidence to progress to the next level, we were all novices in the past.
Although I was/am semi-skilled as a teenager back in the 1960's working mainly on BL A series engines so I did have some basic mechanical knowledge. I knew the difference between a normal left hand thread and a right hand thread. When I worked on the only Alfa Romeo I have ever owned it took me a while to remove the left side road wheels - yep like some old Jaguars the wheel nuts are a left hand thread on one side, right hand thread on the other side. By the time I was 40 I had stopped servicing my own car as I could afford to take it to a garage. For the next 30 years my A/F spanners remained in a tool box going rusty.
In 2020 at the age of 72 I bought my first full Metric Tool Kit so I could try to restore a 400/4 , I had never fully dismantled a motor cycle crank case in my life. I learnt the hard way (hence my Ground Hog days), spurned on by the help on this site I found confidence in leanring new skills and re-discovering long lost ones.
I'm not saying this to blow my own trumpet or big myself up. I was where you are in 2020 we can all learn new skills if we take a risk - even more so if we don't have to have the bike running by the next day to get to work.
I draw the line with learning to play any kind of musical instrument - my CD player does that for me. As for learning to dance well I did manage the Waltz until the teacher stopped calling out 1-2-3, 1-2-3, 1-2-3 - it's a bloke thing I suspect.
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I changed the head gasket on the 400/4 that I am looking after in around four hours of spanner time, although this was divided into two sessions as the new gasket, when fitted, was left to settle overnight and then re- torqued in the morning before fitting the camshaft etc. The cylinder base gasket and everything below head level was left untouched and has not shown any issues since. When I lifted the head, I did, however ask the bike's owner to firmly hold the cylinder barrels down, to avoid moving the base gasket.
I often work on the "old" bikes and did the same work on my Benelli Sei a good few years ago, however my only real skills were learned in the 1970's and 80's when I was a "chalk and talk" technical trainer for Jaguar Cars.
In all honesty, providing that you can follow the manual and possess a few workshop tools including a good low - range torque wrench, I don't see the job as something that the average DIY mechanic would have trouble handling.
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The thing about mechanical skills is they improve if you challenge yourself to try to fix something, when it works you gain confidence to progress to the next level, we were all novices in the past.
....and when something goes BANG confidence wanes! ;D Actually a lack of facilities meant I have to farm out the big jobs to the professional's, but I've got to know my bike really well this past six years doing what I can do.
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The thing about mechanical skills is they improve if you challenge yourself to try to fix something, when it works you gain confidence to progress to the next level, we were all novices in the past.
....and when something goes BANG confidence wanes! ;D Actually a lack of facilities meant I have to farm out the big jobs to the professional's, but I've got to know my bike really well this past six years doing what I can do.
True we learn more from our mistakes than our easy success's, during my 400 rebuild I did have my Groundhog Days - I call it the learning dip as opposed to the learning curve.