Honda-SOHC
SOHC.co.uk Forums => CB350/400 => Topic started by: Murf on February 24, 2025, 11:02:15 AM
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Hi all I am normally on the 550 section but I am just rebuilding the top end of a 400/4 for a friend.
The barrel has been rebored to 0.5mm oversize and I am using DS pistons. I tend to work in Thousands of an inch so all following measurments are it thou. I have given 0.001†bore clearance for pistons and I am just setting piston ring gaps. As the rings came I have approx 0.007 gap for top and second ring, the spec in the manual is 0.006 to 0.014 so they are in spec but at the tight end, I would tend to open the gaps a bit more maybe to 0.008 or 0.009. Maybe I am being a bit cautious my thoughts are to stop any chance of the rings binding if the engine gets really hot. I would be interested to know what people who have experience building these engines have used with good results. If you work in metric that is ok I can convert any measurement. Thanks
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I have rebuilt about six 400/4 motors and have never had to gap the rings even following a rebore and using pattern pistons and rings (unlike British bikes where I have always had to gap the rings). The rings you have sound as though they are within spec and I'd leave them as they are. What make are the pistons and rings, genuine Honda (rare) or pattern, IMD or Cruzinimage?
More important though is who did the rebore, are they familiar with Honda piston to barrel clearance, these are given in the manual and these measurements are critical.
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It’s always worth checking. Folk make mistakes, the trick is not being the victim. If the gaps are within specification I’d leave well alone.
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In 50 years never had to gap Honda rings
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I've measured the gap but never had to gap them with a file for instance.
7 thou sounds just fine, sometimes you get them very low and some were bored by Honda to a spec you'd think they'd seize but never known one do so.
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On that topic can anybody recommend a decent, but cheap manual ring grinder. I’ll only need it for one two cylinder engine build. The market place is flooded with them and I was wondering if anybody has had previous experience.