‘Cat eye’ slit in spray bar – how to make?

[iconParticipant @icon]

I’m trying to replace a spray bar in an SC 108 glow pug engine. Most of the job is pretty staightforward, but then there’s a boat-shaped, or cat eye-shaped slit centred around a cross-drilled hole that I can’t easily replicate. I tryed whittling down a dremel cut-off disk, but that doesn’t produce the desired result. How are these slits produced? Is there a way of acheiving the same results at home?

[Speedy Builder5Participant @speedybuilder5]

First question, is it really needed ?

Before you machine the spraybar, you could cross drill a hole in a bit of bar, deform the bar to close the hole up and then machine the bar to the spraybar drawing – OR

Use a very sharp Swiss file and file across the bar. If the “eye” is longitudinal along the spraybar, make up a boring bar and use a piece of HSS or hardened silver steel with a sharp angled point on it, support the spraybar in the lathe crosswise to the boring bar etc etc.

[JasonBModerator @jasonb]

As Speedy says a sharply pointed tool held at right angles in a bar and then use that as a cutter. Not my best photos but similar type slots in a bearing

 

[iconParticipant @icon]

In retrospect, a picture might have been helpful – the narrowest diameter of the spraybar is 3mm, with a 1.2mm hole through. The ‘fuel control rod’ which is co-axial with the rotating throttle barrel pulls from left to right as the throttle opens, finishing about the centre (where the hole is) as full throttle. I don’t really understand the ‘theory of operation’ of this slit – the part to the right of the hole seems redundant as it’s never exposed.

 

I wonder whether I could just chain drill some holes of diminishing sizes and join them up?

[Michael GilliganParticipant @michaelgilligan61133]

You’re down to the size where a plunge-cut with a suitably shaped ‘Dental Burr’ might do the trick.

MichaelG.

.

Some variation on this theme:

https://eternaltools.com/products/diamond-lens-burrs

 

[FulmenParticipant @fulmen]

I would use something like a boring bar or internal thread cutter as a mill.

[Andrew TinsleyParticipant @andrewtinsley63637]

Goodness knows why the odd shaped slit. I have seen hundreds of spray bars and nothing like that. If the photo is of the original spray bar. then it looks to be an amateur mod, If it is your attempt to reproduce the original, then my apologies.

Either way, I would ignore it and simply drill a round hole. The same as every other manufacturer does. I have a couple of smaller SC 4 strokes and they have the simple round hole.

Andrew.

[Andrew Tinsley]

On Andrew Tinsley Said:

Goodness knows why the odd shaped slit. I have seen hundreds of spray bars and nothing like that. If the photo is of the original spray bar. then it looks to be an amateur mod, If it is your attempt to reproduce the original, then my apologies.

Either way, I would ignore it and simply drill a round hole. The same as every other manufacturer does. I have a couple of smaller SC 4 strokes and they have the simple round hole.

Andrew.

Nope, it really is the original spraybar! I can vouch for it as I’ve owned it from new. I misunderstood how it fitted in the carb, which was completely seized up after a decade of neglect – hence it’s missing the flange at the left hand end. Oops.

How would a single round cross drilled hole work with the sliding fuel control rod? It seems like that would just give an on-off control to the fuel, rather than steadily increasing the amount of fuel as the carb opened? I can see a row of holes perhaps working, each one allowing a bit more fuel in as it’s exposed?

 

[JasonBModerator @jasonb]

The method Speedy and I suggested will give a slot like that. The redundant right hand side is because that method produces the cats eye rather than a triangle stopping abruptly in the middle.

[FulmenParticipant @fulmen]

If it’s original I say recreate it unless you know for sure it can’t be important. But if that was the case we wouldn’t be having this discussion in the first place.

One of these would be perfect:  https://www.amazon.com/Long-Double-Ended-Boring-Holder/dp/B07XNYBM4J

 

 

[JasonBModerator @jasonb]

Just modeled it in cad and for the size of the part you are looking along the lines that Michael suggested but it needs to be quite a sharp Vee profile so probably best to turn one from silver steel and cut a few teeth in it.

 

This is 3mm dia with the cats eye cut

The rotated triangular profile that cut it

And in solid

Which can be developed into a tool about 6mm dia head and 20deg included angle, shown in section. Shank needs to be a bit smaller than I show or at least necked down just above the business end.

 

 

[Michael GilliganParticipant @michaelgilligan61133]

Nicely done, as usual, Jason

MichaelG.

[Colin CreedParticipant @colincreed93792]

@icon

Being as my original trade was a Jeweller/Goldsmith, might I offer a suggestion on “how” I’d try to achieve this “cats eye” groove, if in your situation.

JasonB’s CAD model predicts exactly what I was thinking when I looked at it from my “Jeweller’s” perspective.

 

Busch makes a “knife edge” cutter, model 234, the 14mm OD (Pg 26) would be my choice for handling any clearance issues.

https://www.busch.eu/en/

Jewellery Catalogue

[Michael GilliganParticipant @michaelgilligan61133]

Busch !

… that’s the name that escaped me !

MichaelG.

[JasonBModerator @jasonb]

It probably needs to be a lot smaller than 14mm dia, not for clearance but to get the width/length combination about right.

If the shank on the Busch ones is 1/8″ then you probably want the 070.

Cross section of holes produced by 7mm and 14mm cutters, the 7mm slot is 6mm long and about what it is in the photo, the 14mm slot is 10mm long.

I have coloured the inside of the tube so the hole is easier to see, changing the included angle will alter the width relative to the length.

[Author]

Posts