SOHC.co.uk Forums > CB350/400

Fitting round float bowl o rings

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exvalvesetdabbler:
Thanks Ken, 67 Id would be way to big.

I don't want to cut up a new OEM O ring. I'll try measuring in a few places and take an educated guess/average.

I was taking notes from here
https://www.vintagehondatwins.com/forums/threads/a-1975-cb400f-high-mileage-complete-engine-rebuild.216/page-5
scroll down to 25 September.


Regards
Dave

Oddjob:
Not sure if Jensen has got it right tbh. Some of the parts used are the same between the 400 and the 500/550. Like the push in main jets for instance. The sizes of the original orings was published by Honda in the very first parts book for the 500k0 but was omitted for the later books. The float valve is listed as being 5.7 x 1.3, Jensen says 6.00 x 1.3, odd the thickness is identical isn’t it. Main jet is 4.2 x 1.1 versus 4.5 x 1.0.

Personally I’d believe the published figures before someone else’s measuring. The fuel line fittings are different between the 400 and 500 so no use comparing those and the float chamber drain bolt has the thickness figure missing in the parts book but the id is almost the same, 4.5 versus 5.0.

I must say the float bowl looks identical at a glance and oddly he has the same thickness of 1.9 as the parts book.

I do have a spare 67x 2.0 if you’d like to try it. Just PM me your address.

exvalvesetdabbler:
I get what you're saying regarding opinions on sizing.  Every other o ring I have measured the grooves and housing and then plugged sizes readily available into an o ring calculator to check the % void fill and % compression is within specs.  Strange thing is the float valve oring as honda documented is borderline on ,% compression, not sure if keihin relied on a degree of swelling to do the job long term.

Regards
Dave

Mikep328:
When I first overhauled my 400F carb setup I bought the Orings from Nurse Julie.  A tiny smear of grease held them in position for assembly with no problem at all.  I have disassembled/reassembled them twice since then using all the same O-rings on reassembly and never any leaks from the carbs.

One thing worth being aware of is that it is easy to warp carb bowls by over tightening the screws.  IF the mating surface is warped, leaking is common.

K2-K6:

--- Quote from: exvalvesetdabbler on April 25, 2026, 08:47:47 PM ---I get what you're saying regarding opinions on sizing.  Every other o ring I have measured the grooves and housing and then plugged sizes readily available into an o ring calculator to check the % void fill and % compression is within specs.  Strange thing is the float valve oring as honda documented is borderline on ,% compression, not sure if keihin relied on a degree of swelling to do the job long term.

Regards
Dave

--- End quote ---

I don't feel that the carb bowls in general have a surplus of structural integrity, that's if they're expected to clamp a thicker O-ring section.

The tabs often orientated to "outlying" are a little too vulnerable for that. Design seems to be that a generally distributed light clamping is sufficient. 

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