SOHC.co.uk Forums > CB350/400
400 four main jet o ring
ozzybud:
I was only trying to be helpful.
The original poster was asking for the correct size and I posted the picture of the bag with the correct size
Peace ✌️
scifi:
The only place in the UK for Ethanol Free is at airfields. My local garage only sells E10, and does not even have E5 these days.
Thank God the Budget kept our 40 year tax and MoT exemptions. I would hate to pay for all that on my fleet of vintage bikes.
Seabeowner:
Just reading this whole thread. The Fowlers listing is in 1/10s of a mm! And is 4.9 x 2.5m so still far too big:
https://www.ebay.co.uk/p/1815886647
I have found that nitrile O rings on the main jets actually just expand a bit with E5 and seal better. Have seen no deterioration apart from that over the years.
Just to add to the confusion I thought the Honda size was 3.5 X 1.2mm but never found them listed as a separate part for the 500 as they came with the main jet:
https://www.davidsilverspares.co.uk/CB550K0-1974-USA/part_5119
But I have used 3.5 x 1.2mm, 4.0 x 1.2mm (which I think HM recommended some time ago)
I use E5 80% of the time and have have thrown E10 is sometimes when topping up and found it OK. If storing for a period only E5.
But don't break up the happy home. I find it's always best to put your argument once and disagree, but don't prolong it and hijack the thread....which I have done.
exvalvesetdabbler:
These are easy to find. And what I have settled on. Thanks
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Oddjob:
--- Quote from: Seabeowner on December 08, 2025, 07:29:09 PM ---Just reading this whole thread. The Fowlers listing is in 1/10s of a mm! And is 4.9 x 2.5m so still far too big:
https://www.ebay.co.uk/p/1815886647
I have found that nitrile O rings on the main jets actually just expand a bit with E5 and seal better. Have seen no deterioration apart from that over the years.
Just to add to the confusion I thought the Honda size was 3.5 X 1.2mm but never found them listed as a separate part for the 500 as they came with the main jet:
https://www.davidsilverspares.co.uk/CB550K0-1974-USA/part_5119
But I have used 3.5 x 1.2mm, 4.0 x 1.2mm (which I think HM recommended some time ago)
I use E5 80% of the time and have have thrown E10 is sometimes when topping up and found it OK. If storing for a period only E5.
But don't break up the happy home. I find it's always best to put your argument once and disagree, but don't prolong it and hijack the thread....which I have done.
--- End quote ---
If you look at a very early parts book for the 500k0 you’ll find them not only listed as a separate part but with a size shown. Bryan allowed Ash to copy his original parts book and it’s available to download on the site if you want to look and confirm what I’ve said. It’s why my first post on this thread was to advise what Honda says is the size. You’d expect them to get it right. Knowing how Honda think and do things it doesn’t surprise me they stopped listing them as available separately as it’s more profitable to get you to buy a main jet as well just to get the oring.
I do seem to recall Julie saying they were testing nitrile orings in ethanol petrol by leaving them in a jar and seeing how they performed and the answer was not that well iirc. But I’ll leave her to confirm if that’s true or not.
Those should be ok size wise dabbler. To get the exact size from Polymax means ordering 500 a time, not a great idea if you just want 4.
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