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Author Topic: Making venturis without a lathe  (Read 1736 times)

Offline Steve Glass

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Making venturis without a lathe
« on: January 18, 2023, 11:31:14 AM »
http://controlline.org.uk/phpBB2/viewtopic.php?t=16596&highlight=lathe+venturi

Here's a link to a thread on the Barton website on how to make venturis with only basic workshop tools.
I've been asked to expand on the subject here on stunthanger, so here goes........ Please ask questions.

I made a range of different sized venturis to fit the OS 46LA.  The 46LA was on a Teosawki that was overpowered with the smallest venturi that I had at the time, so I thought I might try this idea to make several smaller venturis to help throttle it back.

I used two pieces of aluminium tube that were a sliding fit with each other and glued with JB weld.
For the OS LA and FP engines you need a length 10mm o.d. aluminium tube with 2mm wall thickness and a length of 12mm o.d. aluminium tube with a 1mm wall thickness.

Steve

 

Offline Steve Glass

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Re: Making venturis without a lathe
« Reply #1 on: January 18, 2023, 11:31:38 AM »
You will need a fixture to hold the tube when drilling with a drill-press.  Any hardwood offcut will do 3/4 to 1" thick.  Cut a length off the offcut about 1" wide and 6" long and bevel one edge at 30degrees. Cut this length in half in half and glue to give a 60 degree V groove.

Take the 10mm tube and dress one end to be square. I use a disc sander. Clean away all swarf and offer the tube up to the engine intake and mark the NVA hole with a sharp pencil.

Centre punch each side and drill a 2mm pilot hole, just each side wall only, not all the through. take a 3.5mm drill-bit and again drill both sides of the tube and finish with a pass all the way through.

Test for a good fit with the nva hole by passing the 3.5mm drill-bit through the engine/tube.  If unhappy start over.

Steve

« Last Edit: January 18, 2023, 12:22:16 PM by Steve Glass »

Offline Steve Glass

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Re: Making venturis without a lathe
« Reply #2 on: January 18, 2023, 11:31:59 AM »
10mm of tube protrudes into the intake, depends on the thickness of the 'O' ring.

Make the 10mm tube 25mm long and the 12mm tube 15mm long, leaving 10mm to fit inside the engine.  Glue with JB Weld.

Steve
« Last Edit: January 18, 2023, 12:32:39 PM by Steve Glass »

Offline Steve Glass

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Re: Making venturis without a lathe
« Reply #3 on: January 18, 2023, 11:32:22 AM »
The first photo shows squaring-up the end of the tube against a disk sander.

Shaping the venturi...... try to visualise what you are trying to achieve here. To get the 'venturi effect', that is suction, you must have the narrowest part of the bore where the NVA passes through.  There should be a straight taper from the intake to where the venturi narrows and another straight taper from the NVA to the crankshaft.

The second photo shows a drill-bit held in a vice and the venturi held in the chuck of a pistol drill.  This removes a lot of material very quickly.  Final shaping is with small round files.  Again, the the venturi can be spun by the drill and the file held in a gloved hand.

A vernier calliper is worthwhile investment here, if you haven't got one, to measure the size of the bore.

« Last Edit: January 18, 2023, 03:08:12 PM by Steve Glass »

Offline Steve Glass

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Re: Making venturis without a lathe
« Reply #4 on: January 18, 2023, 12:47:42 PM »
I use this air filter.  JP accessories supply to UK model shops.  Part number 558088 12mm filter.
« Last Edit: January 18, 2023, 02:55:41 PM by Steve Glass »

Offline Steve Glass

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Re: Making venturis without a lathe
« Reply #5 on: January 18, 2023, 02:34:58 PM »
saved
« Last Edit: January 18, 2023, 03:31:38 PM by Steve Glass »

Offline 944_Jim

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Re: Making venturis without a lathe
« Reply #6 on: January 18, 2023, 04:03:27 PM »
Mr. Glass,
Outstanding, and just in time for me.
Saved x2!

Thanks so much for the quick tutorial.

Online Brett Buck

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Re: Making venturis without a lathe
« Reply #7 on: January 18, 2023, 04:53:48 PM »
The first photo shows squaring-up the end of the tube against a disk sander.

Shaping the venturi...... try to visualise what you are trying to achieve here. To get the 'venturi effect', that is suction, you must have the narrowest part of the bore where the NVA passes through.  There should be a straight taper from the intake to where the venturi narrows and another straight taper from the NVA to the crankshaft.

The second photo shows a drill-bit held in a vice and the venturi held in the chuck of a pistol drill.  This removes a lot of material very quickly.  Final shaping is with small round files.  Again, the the venturi can be spun by the drill and the file held in a gloved hand.

A vernier calliper is worthwhile investment here, if you haven't got one, to measure the size of the bore.

   Not that I want to encourage anyone too much, but, an adjustable reamer will allow you to get the right size within a few thousandths. Drill a hole for clearance, then ream it to the final diameter by adjusting the reamer, do a little, measure, adjust larger, measure, until you get just what you need. A few thousandths is a noticeable change, so you need to both be able to ream it to the right size ,and measure it to a few thousandths, to have any hope of reliable results.

    Also, a split-ball type small hole gauge is necessary to get reliable measurements. Adjust the ball so it just goes through, then measure the ball with a micrometer. Both micrometers and the hole gauges are usually available for modeler-type money used on eBay. Calipers will get you pretty close but will tend to measure slightly under, because of the width the the blade at the measurement end.


         Brett

Offline Steve Helmick

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Re: Making venturis without a lathe
« Reply #8 on: January 25, 2023, 12:33:08 PM »
Being a retired machinist and familiar with adjustable reamers, I was a bit surprised by Brett's suggestion, because I'd never seen any small enough for our use. So, I did a search (using DuckDuckGo search engine instead of Google), and found out he's right, they are available.

https://www.mcmaster.com/adjustable-reamers/straight-blade-adjustable-size-reamers-8/

The smallest size is what you need to order. And, it's only $116.90, a plus?  LL~ Steve
"The United States has become a place where professional athletes and entertainers are mistaken for people of importance." - Robert Heinlein

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Online Brett Buck

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Re: Making venturis without a lathe
« Reply #9 on: January 25, 2023, 06:10:15 PM »
Being a retired machinist and familiar with adjustable reamers, I was a bit surprised by Brett's suggestion, because I'd never seen any small enough for our use. So, I did a search (using DuckDuckGo search engine instead of Google), and found out he's right, they are available.

https://www.mcmaster.com/adjustable-reamers/straight-blade-adjustable-size-reamers-8/

The smallest size is what you need to order. And, it's only $116.90, a plus?  LL~ Steve

  I have one from India, via eBay, it was about $12. Fully functional, although a bit cruder than you want.

       Brett

Offline Steve Glass

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Re: Making venturis without a lathe
« Reply #10 on: November 02, 2023, 07:54:38 AM »
An update.

To make it easier. I've substituted the outer aluminium tube and O ring with simply a length of silicone tube.

The inner aluminium tube was shaped as in the instructions above.  The aluminium tube to fit the old Enya is 11mm o.d. 7mm i.d., sourced from China via Ebay.

Steve

Offline Paul Smith

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Re: Making venturis without a lathe
« Reply #11 on: November 02, 2023, 12:34:56 PM »
Nylon spacers from the parts drawers at Ace or Acco Hardware are good place to start for a simple venturi.

These were for compliance with the FAI 4mm rule.
Paul Smith


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