SOHC.co.uk Forums > CB350/400
400 - “elasticity†in headnuts
K2-K6:
--- Quote from: Erwin83 on July 07, 2022, 09:33:46 AM ---So, couple months (and just a few kilometers, really) later. I'm back at square one.
Again, Cilinder 2 is smokey. I started riding, nice and easy. No smoke. Once everything was nice and up to temp I gave the ol'girl the beans and cilinder 2 (I have a non-balanced 4-4 exhaust, so easy to diagnose) started smoking again. Looks like head gasket again.
I'm going to retorque the head again, first without changing the gasket. Remember last time I found 'loose' head nuts.
Could there be anything wrong with my Studs, or Head?
Is there any harm in using an extra ring under the head nuts, just to make sure the nuts are not bottoming out on the studs (because of the aforementioned elasticity)?
--- End quote ---
I can't immediately remember but someone on forum had one of these head with crack in casting allowing oil to leak into exhaust port to smoke on #3 I think ?
It was long convoluted chase through all the other possibilities, but ended definitely with this outcome.
Erwin83:
Well I'm hopefull it's not that. Afterall, replacing the headgasket and (re)torquing the head nuts stops the smoking completely.
It appears to come back only after riding hard (full throttle up to red-line).
I think my issue is with the studs/nuts, as I have this 'elasticity' on torqueing, and headnuts that appear to come loose after a short while of riding (couple 100 km).
royhall:
--- Quote from: K2-K6 on July 07, 2022, 10:12:44 AM ---
--- Quote from: Erwin83 on July 07, 2022, 09:33:46 AM ---So, couple months (and just a few kilometers, really) later. I'm back at square one.
Again, Cilinder 2 is smokey. I started riding, nice and easy. No smoke. Once everything was nice and up to temp I gave the ol'girl the beans and cilinder 2 (I have a non-balanced 4-4 exhaust, so easy to diagnose) started smoking again. Looks like head gasket again.
I'm going to retorque the head again, first without changing the gasket. Remember last time I found 'loose' head nuts.
Could there be anything wrong with my Studs, or Head?
Is there any harm in using an extra ring under the head nuts, just to make sure the nuts are not bottoming out on the studs (because of the aforementioned elasticity)?
--- End quote ---
I can't immediately remember but someone on forum had one of these head with crack in casting allowing oil to leak into exhaust port to smoke on #3 I think ?
It was long convoluted chase through all the other possibilities, but ended definitely with this outcome.
--- End quote ---
That was me on the 350F. It was a real head scratcher problem. I ended up rebuilding the top half three times. In desperation I took the head to an NDT place who gave it to a chap called Eddie Current. He was good, he found a crack next to a valve guide that was invisible cold but opened up when hot. I replaced the head with a really good low mileage one off eBay and problem solved. It's never smoked since.
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350Simon:
When I rebuilt my 125s I torqued the head down properly (circa 16ft lb iirc) and I distinctly remember looking at the manual multiple times thinking is that it?! Is that the correct figure, have I misread something? That torque seemed way to low to me after having worked on car engines with aluminium heads countless times when your generally over 50ft lb on head studs.
However the 125s studs are on the small side so I thought OK that must be correct and duly proceeded to finish off the build. After about 400 miles of riding I noticed oil seeping from the cylinder base gasket. So I duly re torqued the studs and noticed exactly the same as what you did, the stud would turn with the nut for a couple of degrees, then go tight allowing the nut to turn on its own. I put it down to everything just settling in.
Only for it to happen again at about 1000 miles. This time I thought, this is really strange I've never known this to happen before so I approached one of the engine builders at work and asked him for advice. He said the torque spec does seem very low but if that's what honda specified then it must be correct.
We work on vintage/pre war cars so are used to dealing with worn out and re used parts. He suggested that perhaps the threads in the crankcase were stretching / already stretched and the studs and nuts likewise. Therefore although torqued to the correct setting they were able to fret loose with engine vibration due to the wear on each component. I had after all re used my studs and nuts which were original.
So I asked what he would do and he said come here my boy....... and pulled out a 12mm spanner from his drawer. Go home and tighten those nuts as hard as you can with the ring end of this spanner. I suggested that I might run the risk of over tightening them if I did that, to which he replied you can't, this 12mm spanner is this length for precisely that reason.
So I did what he said and a few thousand miles later hey presto all was well. I'm not suggesting that you do this to your engine, merely sharing my similar experience!
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Erwin83:
Took the valve cover off last night and found that the 2 nuts on the intake side of cil 2 (the smoking cilinder) were indeed a bit loose again.
I've tightened them up, but one would not stop with the 'elastic' feel once torqued. So I added another ring and that helped (for the feeling of stretch at least). Maybe this nut is binding on the threads or binding in the dome of the nut?
No idea, but I'll put everything back together this weekend and see what happens.
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