Britbike forum Forums Thread Like Summary - O-ring.

Hello. Has anybody tried ditching the O -rings when mounting carbs to the spacers on the cylinder head on a A65? .I am considering using 3bond1211 or Loctite 518 after warping my carb bodies. Cheers, Max.

FWIW
I tossed the O rings decades ago
The trick is to use a soft gasket that when compressed between the manifold & carb flange will partially fill the the slot where the O ring used to go thus make a good seal and without the O ring to be a fulcrum point, no chance of warping the glang thus distorting the carb body and making the slide stick.

The O ring is a good idea except when a shaved gorilla is let loose on the bike.
The original carb nuts were 1/2 thickness like a proper lock nut and a very light split ring went underneath
So it was a ase of tighten till the split washer just flattened and all is good
Being 1/2 thickness they are easy to fit as well.
However just about every bike I have worked on the original nuts & washers were gone in favour of a full sized nut & regular split washer which is way too tight if done up till flat.
As previously mentioned when properly tightened you can twist the carb a little .

With old carbs there is a good chance that the flange will be worn thin
This happens because the "ix" for a warped flange was to rub it on a surface plate of some sort till it was flat again.
This of course removes metal and makes the O ring groove shallower so the flange will be more prone to bending over the O ring which wil sticj out further so be even more of a fulcrum point that it was to start with .

Way way back John posted the details of the tool he uses to straiten the flange and instruction haw to use it.
Should be one in every BSA owners tool kit and many thanks to John for doing it .

Instead of saying that petrol starts to boil about 38 C, it would be better to say that about 1 or 2 percent of the petrol volume will vaporize at about that temperature with the rest remaining as liquid. ACEA has a specification for gasoline that requires that at 70 C no more than 20% to 45% by volume is allowed to evaporate in ambient areas above 15 C. At 100 C, ACEA's spec allows 50% to 65% to evaporate. At 180 C, 90% is allowed to evaporate.

Iso-octane, a component of petrol boils at 99.3 C. At 100 C its all gone.

I hate to say this, but it isnt the "O" ring that is causing your problem. Not that difficult to straighten if you make up a proper tool to do it.

Motalia sell the correct BSF/Cycle studs at various lengths.

6mm is about the thinnest you can get away with and not suffer fuel boiling in the carbs(least not in the uk).

You can get cycle thread nyloc nuts from Middleton stainless.

To reduce heat transfer to the carbs (causing me bother), I fitted 9mm Tufnol spacers, stainless studs, fibre washers and nyloc nuts. Any bigger the air cleaners would foul the side panels.

As mentioned, the BSC ones are hard to get so I made the studs all BSF which is the thread for the cyl. head.

The nyloc nuts can be fine tuned easier, once set never move.

To insulate the carburettor studs what is really needed are top hat shaped washers like what is used to insulate the points from the base plate. The mounting holes on a 930 Amal are 0.340" so it would need a stud stepped down to 1/4". 5/16" is overkill for holding a carburettor. Besides, with 0.028" difference be tween the hole and stud can leave a big match between the carburettor and port.

Been reading and can’t understand what the fuss is?
If tightening the stud nuts to the point off flange contact, that’s all wrong. Think of the O-Ring in terms of a fluid HydraStatic lock, as example: a brake system. If the O-RING captured in the groove has nowhere to escape, it acts as a solid and the flange bends around it, and distort the slide fit. ANSWER: don’t tighten the nuts any more than good O-Ring contact. Don’t know where I saw this mentioned years ago, but have been doing this for years. In fact, if proper contact/seal is made, the carb will still shift on the studs if twisted back and forth. Need I mention self locking nuts? I can even see a sliver of space between flange and head. Works fine, seals, and may even help with the mentioned heat transfer. My

It is worth thinking about

I tend to agree KC, i've been using o-rings (triumph ones) for years but
each to his own eh?
Dave's suggestion is a better engineering answer but i've never found it
necessary to go to that length.
I have made up spacers from 'type x' busbar support material in the past though
as some of the spacers around are like blotting paper (if anyone remembers what
that is!)