Author Topic: Spark Plugs 4.5 mm Thread  (Read 5710 times)

Offline Graham Meek

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Spark Plugs 4.5 mm Thread
« on: August 07, 2014, 06:23:31 PM »
Following on from the "Spark Plugs 8-36 thread" post I thought the readers might like to see my 4.5 mm spark plugs that I have made for the model of a Fiat 702 tractor that I am building. The construction follows closely the method I used for my larger 1/4-32 UNEF plugs that I made some time ago. (These were made for my Seagull engine)

My best regards
Gray,

Offline gbritnell

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Re: Spark Plugs 4.5 mm Thread
« Reply #1 on: August 07, 2014, 06:57:15 PM »
Nice work Gray. How does the brass center electrode hold up?
gbritnell
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Online Roger B

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Re: Spark Plugs 4.5 mm Thread
« Reply #2 on: August 07, 2014, 07:15:27 PM »
Very nice  :ThumbsUp:  :ThumbsUp:

How's the rest of the tractor progressing?

I am still working on my long term plan for a real diesel Field Marshal.
Best regards

Roger

Offline Jasonb

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Re: Spark Plugs 4.5 mm Thread
« Reply #3 on: August 07, 2014, 07:54:01 PM »
Looking good Gray, I see you have gone for a tapered seat, should need a bit les sforce to get a good seal than trying to compress a washer with such a fine thread.

How did the trial one go in the adaptor?

J

Offline Graham Meek

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Re: Spark Plugs 4.5 mm Thread
« Reply #4 on: August 07, 2014, 08:19:20 PM »
Nice work Gray. How does the brass center electrode hold up?
gbritnell

A pair of larger plugs as shown in the photograph have completed nearly 100 hours running in my Seagull engine. Taking one out yesterday to try the mini plug as I call them I thought I would just measure the gap to see how much it has increased. The gap was barely a 0.001" larger than my original setting.

Hi Roger,

Work on the tractor has been slow, my wrist was giving me a great deal of trouble but luckily that all seems behind me now and I am back on the tractor in earnest. I recently made a pair of crossed helical gears to drive the water pump and dummy magneto off the camshaft. As is usual with me I decided to try a different approach, (see photographs below)

Hi Jason,

The adoption of the tapered seat was more by necessity than design. The length of the plug is governed on the tractor engine by the depth of the Valve Bonnets, (see photograph below). On the original tractor the plugs are partially buried inside the hexagon pocket used to tighten the bonnet. A copper washer here would raise the plug body and expose more plug than is needed. The tapered seat seemed the easiest option, but as you say they require hardly any torque to make a good seal.

Having obtained a bottle of Colman's fuel on Tuesday evening I decided to give the mini plugs an endurance test. The Seagull engine was started this morning and has run non-stop today for 8 hours. Given that these small plugs are working on a standard 12v coil I think I can safely say they are going to be OK.

My best regards
Gray,

Offline Admiral_dk

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Re: Spark Plugs 4.5 mm Thread
« Reply #5 on: August 07, 2014, 11:04:10 PM »
I'm very impressed that those sparkplugs works so well while being that small - electricity and insulation does not scale well ....  :zap:

Very nice tractor engine - are you doing a build post I haven't seen ?

Offline steamer

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Re: Spark Plugs 4.5 mm Thread
« Reply #6 on: August 08, 2014, 05:30:57 AM »
I would love to see a dissertation on the machining of those cross helical gears.....Hmmmmmm.....Pretty Please!


Dave
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Offline Thor

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Re: Spark Plugs 4.5 mm Thread
« Reply #7 on: August 08, 2014, 05:51:28 AM »
Hi Gray,
as usual - very nice work. Glad to hear that your wrist is better.
As Dave says, I too would like to know more about how you make those helical gears.

Thor

Offline Steam Haulage

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Re: Spark Plugs 4.5 mm Thread
« Reply #8 on: August 08, 2014, 07:30:40 AM »
And me! Please.

Jerry  :happyreader:
Dogs look up to you, cats look down on you, pigs treat you as equal.

Offline Jasonb

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Re: Spark Plugs 4.5 mm Thread
« Reply #9 on: August 08, 2014, 07:48:29 AM »
Now you can't go posting a picture of those adaptor bushes without letting on how you made the shallow hexagonal recess to screw them in with. I'd hazard a guess they were turned from socket head screws but could be wrong.

J

Offline Graham Meek

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Re: Spark Plugs 4.5 mm Thread
« Reply #10 on: August 08, 2014, 09:43:11 AM »
I'm very impressed that those sparkplugs works so well while being that small - electricity and insulation does not scale well ....  :zap:

Very nice tractor engine - are you doing a build post I haven't seen ?

Hi Admiral,

The scale factor and electrical theory was one of my main concerns when making these plugs, but I decided to give it a go anyway, I had everything to gain and nothing to loose, luckily for me they worked.

I did start a build post on the ME Forum, but as I no longer post there for reasons which I will not go into, the build there has stopped. If you Google Fiat 702 - Graham Meek, it should get you there.

Hi Dave, Thor & Jerry,

When I started out designing the Tractor these gears were going to be "bought in". Starting to make enquires about the price plus the minimum order surcharge I thought I must have a go at making these if only to remain solvent.  While I have seen various methods of generating the gears via curved rolled up helix, both in the Model Engineer, Vol 100 and on this Forum by Chuck, I always had the inkling to try the method shown in the photograph. While it is not a true Helical gear and it will probably make the purists cringe they do work surprisingly well.

I would like to say that the idea worked straight from the can, but I made one big mistake, and that was using information from the internet in the form of a chart for selecting the correct cutter for Helical gears. I spent a whole day fruitlessly trying different radii to swing the cutter to get the gears to mesh at the correct centres. In frustration that evening I referred to my Machinery's Handbook, (what I should have gone first), low and behold I was using the wrong cutter profile. The following day with the right cutter the first two gears off the set-up were much nearer my goal, but the form on the end of the gear looked nothing like anything I had seen in the books. I pressed on and eventually I found that the "correct" or "best radius" to use was half the "Helix Lead". Gears machined at this setting came out spot on as regards the centre to centre distance, but the profile was still off. As I had to thin the gears down to form bosses either side I thought I would wait and see what they looked like then. Sure enough as the gear is thinned the profile becomes more like a true crossed helical. 

 Hi Jason,

Putting the hexagon in was one of the biggest hurdles to this part of the build. The depth of the hexagon is only 1 mm and the A/F is 5.5 mm (approximately 7/32"). I did think about using stainless steel capscrews but availability and cost ruled this out, plus it would mean some very interesting set-ups to maintain the depth of the hexagon. In the end I decided to Broach them by making a HSS broach using my scale Quorn cutter grinder. The 5.5 mm A/F can just be ground out of an old 1/4" shanked milling cutter. The pockets were machined in the head of the Bonnet and a simple alignment fixture to hold the bonnet and broach was turned up, basically two concentric reamed holes. One hole to hold the bush carrying  the bonnet and the other to take the 1/4" shank. Resting everything on my bench block the broach was given a hefty clout with my copper mallet and then I held my breath as I dis-assembled the parts. The result was as shown in the photograph apart from having to remount the bonnet in another adaptor in the lathe and clean up the bottom of the recess to remove the curls of swarf produced by the broaching, barely 0.03 mm was removed during this clean up operation. A bronze A/F driver was milled up to insert the bonnets, my reasoning being that I would rather have to make a new driver than a new bonnet. The hexagon on the driver is just short of the 1 mm depth of the hexagon and the shoulder produced keeps the driver square with the bonnet. Downward pressure on the boss keeps everything in contact and a short lever supplies the torque.

My best regards
Gray,


Offline Jasonb

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Re: Spark Plugs 4.5 mm Thread
« Reply #11 on: August 08, 2014, 10:34:43 AM »
Thanks Gray, sounds like a similar method to what I tried recently when the subject came up on ME - hollowed out the end of an allen key and drove it through a hole of the AF size.


Offline Graham Meek

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Re: Spark Plugs 4.5 mm Thread
« Reply #12 on: August 08, 2014, 08:03:46 PM »
Hi Jason,

I have used this method in the past but found the Allen Keys were tight to get into the hole. It is okay if you are using the hexagon as a drive and you are making the mating part though. In the past I have  placed a piece of 0.0015" shim on one flat in the hole and pass the Allen Key broach through again to add a little clearance. Making the broach on a cutter grinder I usually make the A/F size plus 0.05 mm as not all Allen Keys are truly symmetrical. Below is the smallest hexagon I have broached I found it easier to adjust this screw with an Allen Key rather than trying to get the blade of a screwdriver in a slot, plus the short leg of the key acts as an indicator as regards how much the screw has been turned.

My best regards
Gray,

Offline Steam Haulage

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Re: Spark Plugs 4.5 mm Thread
« Reply #13 on: August 09, 2014, 08:59:16 AM »
Ingenious!

Often just knowing it can be done can lead to finding out how to do it. :happyreader:

Jerry
Dogs look up to you, cats look down on you, pigs treat you as equal.

Offline Allen Smithee

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Re: Spark Plugs 4.5 mm Thread
« Reply #14 on: August 09, 2014, 02:12:18 PM »
Digressing back to the spark plugs - what are you making the insulators out of? Am I right is assuming it's a plastic and you're retaining it by rolling or crimping the steel outer body onto the machined shoulder of the insulator?

AS
Quidquid latine dictum sit altum sonatur