hallo David nice bike haven't been on one of those since i was a nipper if
your crank-case is good i.e. not warped not really necessary , no science in
the return side if sinking one side of the crankcase a nice press fit to a
depth of 2 thirds of the thickness of the o ring the other third is the
compression factor the o ring will mould itself into the recess experiment
with 2 pieces of perspex fit o ring and clamp perspex together that way you can
see what the ring is doing ... and you can adjust to depth on the final
drillings of the job ... most transfer ports that i have come across , and not
many have a small tube and located into the crank case locating pins that's if
there is enough meat around the particular area if you want to bore both sides
of the case that's a different story not advisable you could loose the o ring
on assembly ( here again experiment with the piece of perspex and you will
realize what i mean the way i would go is to bore both sides to a depth which
is not detrimental to the casting in both sides of the casing and fit a small
length of brass in the hole which has been bored out to the inside measurements
of the transfer port to locate exact boring of the second case , the use of
engineering blue around the area enough to transfer a good locator mark on the
other casing that has to be bored then bring both sides of the case together
and tighten the bolts with a bit of luck this will give you the central locating
position for the second hole to be bored basically that is going to the
extremes but do experiment with a piece of perspex to see what is happening ,,,
your ,, RE is it a square four ... the one i had was the single cylinder grass
track bike lot of fond memories returning here David
Charles w.a.
----- Original Message -----
From: david_steam
To: halfinchlivesteamforum@...
Sent: Friday, May 29, 2009 5:24 AM
Subject: [Half inch live steam forum] Re: O ring query





Ok I will come clean, its for a Royal Enfield 250 cc engine. The two crankcase
halves have a 3/16 passage for (I think a low pressure return) are just butted
together with a bit of sealing gunge and no gasget. So what type of material and
what size and depth seating do I need?
David

--- In halfinchlivesteamforum@..., "Simon Wass" <simon.wass@...>
wrote:
>
> Your experience of using o rings is totally opposite to that I experience. I
> only use red or white silicone rings and they have quite a good stretch and
> compression range. If a piston is a bit tight I simply skim the groove
> deeper and lo and behold the fit is freer, its the same for piston rods
> although I measure the od of the ring whilst it is on the rod and bore to
> suit.
> O rings are amazing if used correctly to suit our applications, remember
> they are designed for super high pressures in hydraulics etc and the fits
> need to be tighter, our fits need to be far less so need common sense
> applied, which is where most who fail are failing.
>
> ----- Original Message -----
> From: "John" <catboat15@...>
> To: <halfinchlivesteamforum@...>
> Sent: Wednesday, May 27, 2009 6:12 AM
> Subject: [Half inch live steam forum] Re: O ring query
>
>
> > Also remember the "rubber" or elastic material of an O ring is not
> > compressable like convential packing. So the cut out for the ring must be
> > wide enough to allow the ring to flex and expand out to the side as it is
> > squeesed between piston and cylinder wall or shaft and gland.
> >
> > To me "O" rings were the greatest invention since mixing gin with bitters.
>





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