AMAL JETS/SMALL DIAMETER HOLES

This thread could also be placed under motorcycles but I thought I might get more replies from engineers under this heading

Amal carburetters used to claim in their advertising that their jets were reamed to size for accuracy. However in the smaller carbs the jets are tiny, with very precise apertures around 19 thous. Does anyone know how Amal could have achieved this 60 or more years ago? The jets are made from brass.

What on earth would a 19thous reamer look like and where would one be obtained?

Watchmakers broaches or something similar ?

For those of us who make small injectors for miniature locos this is not a problem.

Cheers,

Julian

drill service will do that size for you as a slot drill for £19 it seems..

**LINK**

Depends how much its worth to you I guess.

"For those of us who make small injectors for miniature locos this is not a problem"Thats a lot of help Julian...

For those of us with electron drilling machines it is not a problem either!

Mark

Hi Mark,

You can make a reamer out of silver steel or HSS to any shape of size to ream very small holes. It need only be of 'D' bit half section type for brass.

The problem usually in injectors of small size for miniature locos is drilling the pilot hole accurately and true.

1. centering for the drill which is smaller than any centre drill will produce. I use some very small dental burrs or some very small clock makers spade drills or an old fashioned graver.

2. getting the lathe fettled up and altered to produce an accurate hole of very small size to some depth for the pilot hole. This requires accurate tailstock adjustment, a high speed for drilling, and a sensitive lever feed tailstock attachment.

Cheers,

julian

Edited By julian atkins on 15/01/2017 22:04:42

Thanks Julian.

I don't have any need of small holes currently but you never know what is around the corner.

I did need some aperture plates making for a confocal microscope some time back but that required extremely tight concentricity and size (and also a tapered cylindrical profile) which was done using photo etching on nickle foil.

Mark

As Roy said; Watchmaker's broaches are readily available in that sort of size ... 'though they do produce a slightly tapered hole.

Julian, of course, knows all about making injectors.

That only leaves me to mention that good quality sewing needles can provide a very useful source of material for a small 'toolmaker's reamer' [a diamond hone or an Arkansas slip will soon produce the flat] ... which I have found very suitable for enlarging Amal jets. 

The needle-making industry was alive and well long before the carburettor was invented.

MichaelG.

Edited By Michael Gilligan on 15/01/2017 22:44:00

Charles

I was told you opened up an Amal jet by taking a needle, or round broach, that was significantly smaller than the hole, and repeatedly passed it through the hole. I don't know when you decided to stop. The last thing you do is to use a drill.

JA

Not familiar with watchmakers' broaches but am familiar with old fashioned toolmakers' reamers. Just an accurately turned-to-size piece of drill rod, with one end ground with a flat about 20 degrees included angle across it. Harden and temper to straw colour if cutting steel, harden and temper not required for working brass or die cast alloys. Worked by hand or with slow speed these will cleanly open any hole to size and are easily made.

Used them for years for work on oddball carburetors and also on mould and die precision work.

(have not used them in injectors though - too much top secret info there)

I've always just drilled the jets on Amals when enlarging them to suit megaphone mufflers etc on classic Brit bikes. You can buy sets of jet drills made for the purpose, they come in a little holder like a pen, with a set of drills in the handle and a tiny chuck on the other.

Perhaps if you were jetting a TZ750 to run around Daytona without seizing you might ream the jet holes. But for the average bloody old Brit bike banging around the streets the drills do the job. Any Amal carb is going to be so worn anyway that it is never going to be a precision mixture metering device, so reamed holes is like waxing a dirt floor.

I've used the jet drills for drilling holes in virgin material in the lathe. Put the jet drill in its holder and put the other end of the holder in the tailstock chuck. Run lathe flat out. Feed very cautiously and take your time.

Jet drills can be purchased from bike parts dealers and sometimes from welding shops, used for cleaning jets in acetylene torches etc.

Edited By Hopper on 16/01/2017 05:07:52

Hopper, I also use drills. Amal jets go up in 10s for sizes Mikunis go up in 5s. I am now tasked with replicating Mikuni sizes in Amal jets in the quest to get good carburation using this dreadful excuse for petrol we now have with ethanol added.

.

Jeff,

For info. they are pentagonal in section, and slightly tapered.

The negative rake angle works beautifully on brass.

MichaelG.

.

https://www.cousinsuk.com/category/broaches-cutting

Edited By Michael Gilligan on 16/01/2017 08:40:38

They were never precision mixture metering devices when new. I've seen pictures of Triumph twins with SU carbs, seems a much better bet to me

You can buy new Amal Monoblocks and Concentrics. They make a considerable improvement in the running of a bike from a worn out carb. However the improvement is nothing compared to using a Mikuni.

The Mikunis are far better designed and made. For a start proper aluminium is used instead of metal rejected by Dinky.

JA

Just a thought, but aren't carburetor jets tapered anyway ? ( I could be wrong )

Roy

Amal now is not what Amal once was back in the day. Far, far superior today than back then. Chalk and cheese. Different alloy used, up to date machinery, better fits and tolerances, high quality components internally. Same co. owns SU too.

I presume because there is no current British motorcycle manufacturer left that uses Amal on a production machine, they don't have to be built DOWN to a price like back in the old days...

"Jeff,

For info. they are pentagonal in section, and slightly tapered.

The negative rake angle works beautifully on brass.

MichaelG."

Thanks for the info on the watchmaker broaches. JD

.

Thanks to everyone who replied to this thread, it has been a major help in understanding the whole business of making small diameter holes.

The background to my request is that I am the Vintage Motorcycle Clubs(UK)marque specialist for New Imperial motorcycles (although I live in Australia) and I am trying to sort out why some of our members are having difficulties with the special, small Amal carbs fitted to pre-war lightweight New Imps.

These carbs use main jets of the order of 24 thous with only 1 thous steps between different jet sizes. These jets are no longer available, even from the current makers of Amal carbs because they are a completely different style to the jets fitted to the usual size of Amal carb.

These replies have reminded me that jet reamers were once to be found in any veteran motorcyclists toolkit but dropped out of use circa 1910, presumably when jet manufacturing techniques improved

Whilst I more-or-less agree with Hopper about the need for precision, it seems that these tiny carburetters are very sensitive to jet size variation

Julian: what size drill would you use to make the pilot hole, in brass, prior to feeding in the D-bit?

Thanks everyone, Chas

You can remove the ethanol by getting a 25 liter can with a tap at the bottom ( Camping water carrier). put 20 litres of petrol in it then add 2 litres of water. Give it a good shake up and leave to settle overnight, then drai n the water off the bottom and you will have ethanol free petrol. Ethanol being an alcohol mixes with water.