Carburettor screw size

[G3L1946]

Hi,

Just exchanging the bowl on my carburettor for the correctly angled one (15 degrees on, 3 degrees removed ) . Both bowls are for the pre monobloc 76 series and accept same floats etc. However, there’s always a snag! The bolt hole in the fixing stub ( see photo ) are slightly different in size. Therefore, the bowl I’m wanting to use will not accommodate the larger sized bolt taken from the original bowl.
What is the purpose of the hole which is, basically, sealed off by the bolt? Anyone know what the thread size might be ( it’s a 1J bowl ). Any reason why I can’t re thread it to take a bolt size I have?
Thanks in advance,

P.S. Since found out this is called the ‘plug screw’ . It’s described as either 1/4 or 3/16. I would presume I need a 3/16 as smaller. But what is the thread?

Steve

[Duncan]

The bolt is there as a by product of manufacture as a hole needed to be drilled to provide a passage for petrol, all the bolt does is blank the original drilling, I believe there was only one size but am happy to be corrected, have a look on the AMAL web site to see if there is further detail.

Edit: just looked at https://amalcarb.co.uk/shop they appear to have redesigned away their web site and removed the exploded views, Surrey Cycles have an exploded view https://surreycycles.com/carburettors/amal-276/ I believe the part number you need is 13-153. Some older Amal castings can become brittle so I would be cautious about any drilling and tapping.

[G3L1946]

Hi Duncan,

Looked on AMAL site but they don’t appear to stock this individual item. According to Hitchcocks motorcycles site they are either 3/16 ( early bowl ) or 1/4 ( later bowl ). So, the thread size would be handy, BA maybe? Ordering one from Hitchcocks proves expensive, with p/p , for just 1 bolt
Steve

[jackstringer]

Why can't you just use the bolt that is for the one you want to fit?

Maybe give Amal a call and ask thier tech types. I am sure they will help. I would also suggest getting the little books they have for the carbs, they are free and very handy way to find details for a carb.

I suspect you can tap it for any bolt you have that is the right length.

[G3L1946]

Hi, I can’t use the one I have ( from the previous, later bowl ) as its diameter is too great. It won’t fit! Apparently, the earlier bowls, like the J/1 had a different sized bolt. They are shown as 3/16 (earlier) and 1/4 ( later ).
I’ll give them a ring tomorrow to seek their input but I’ve found their postage costs are almost prohibitive, £6.54 for one small item.
Thanks for your advice,
Steve

[jackstringer]

I doubt its BA. More likely Unified. Try and work out the threads per inch and it will be easier to find. For a 1/4 it's either a 20 tpi or 28 tpi. Whitworth would also be a 20tpi. To work out the difference between wit and unc you would would need a pitch gauge as they allow you to see the angles of the thread so see the difference.

P&P for small bits is always a pain. There are physical shops around that will sell you the bolts you need but I do find I spend more than the p&p because I see sooo many shiny things I need to stock up on.

[G3L1946]

Hi J

I can spend money on shinny things too…….just haven’t got the funds to cover it! I’ve got a pitch gauge but the bolt is so short and my eyes so crap I doubt I could measure it correctly.
I’ll give Burlen a ring tomorrow, speak to technical and see if they can help.
Thanks mate,

Steve