Fitting round float bowl o rings

[exvalvesetdabbler]

I was wondering if anyone has some good tips for fitting round float bowl o rings.  I kind of remember that they used to be shaped to fit.  The last remaining OEM kit that I bought, probably 15 years ago has a round one as do the current offerings.

If you glue them down at the corners, the ultimate seal is only as good as the glue is to resisting fuel.

Also thinking if a length of o ring chord might be just as easy/difficult.

Regards
Dave

[Nurse Julie]

The lightest smear of red rubber grease gives enough stiction to hold them in place or a small dab if super glue at each bend also works.

[Oddjob]

I doubt cord would work as they’d be a gap where the 2 ends meet. It may just leak a little as a result.

As for the round orings supplied these days, if it’s a genuine kit from Honda you’ll find that it fits with no glue. There are pinch points around the float bowls where it grips the ring, the problem is when you buy kits not made by Honda. Most haven’t got them right as regards the thickness, then the pinch points struggle as there just isn’t enough for them to grip securely. So you need glue or grease to bond them to the bowl until you can fit the screws. I’d imagine they’ll be fine afterwards as they’ll adapt to the shape over time. I did have the size of the float bowls where rings at one time, they fitted with no glue required. That was for the 500/550 bowls but they may be the same as the 400. The thickness should be either way. I’ll see if I can find it.

[exvalvesetdabbler]

I have one genuine honda o ring from a kit so I'll just measure that.  Getting it laid out on the table  in a perfect circle to measure it is the challenge. Can't decide if it's 63,4 or 5 ID.  Others have reported using 63x2. The OEM one I have here is more like 1.8 cs.  Haven't noticed any pinch points when cleaning years of gasket goo out with acetone.

Regards
Dave

[Oddjob]

Just found it. The 500/550 uses a 67x 1.9 oring. However that’s not readily available so I used a 67 x 2.0mm and that fitted with no glue etc required. You could always try that size and see if it fits ok.

There is a formula for working out the id of an oring, you cut it, measure it’s total length then divide by a set amount and that gives you the id you need. It’s around on the web somewhere.

[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]

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

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. 

[exvalvesetdabbler]

The lightest smear of red rubber grease gives enough stiction to hold them in place or a small dab if super glue at each bend also works.

Thanks Julie.  I had some stiff silicone 0 ring lube and that held them down in the grooves long enough.

Dave