715 Holley question

[6s1640]

Hi Texas Swede,

I called Daytona Carburetor about this metering  block "Timed Spark plug".  Their best offer was the plumbing hardware store for a 1/8  NPT plug.  I did some searching and found a close replacement on Amazon.  The quantity five with a little shipping gets the price down per part pretty good, $2.49 each.  There are also singles that can be purchased.  Search with "1/8 NPT slotted plug" and several choices will pop up.

I also found a few on a popular auction site, but the pricing and shipping was higher than the five on Amazon.

With a little patients and a grinding wheel, the domed top could be replicated on these for a close facsimile.

Thanks for the tip.

Cory

[texas swede]

The head on these are flat and the original rounded.
When getting my carb back from Joe Bunetic it had a flat
screw (had no screw when I sent it to him) and after a phone call he sent me an original.
The same happened on my friend Roland's (6S923) carb and Joe sent a correct screw
that somebody had made for him. I couldn't see any difference between the original and
the one he had made up.
Texas Swede

[6s1640]

The head on these are flat and the original rounded.
When getting my carb back from Joe Bunetic it had a flat
screw (had no screw when I sent it to him) and after a phone call he sent me an original.
The same happened on my friend Roland's (6S923) carb and Joe sent a correct screw
that somebody had made for him. I couldn't see any difference between the original and
the one he had made up.
Texas Swede

Hi Texas Swede,

I agree with you on the plumbing plug.   I am a pretty good customer of Joe's too.   I expect he has these made or has a good source.  Maybe he will sell me one.  Also I might try to modify one of these flat plugs for grins.  I like the challenge.

Take care

Cory

[Dan Case]

I got several carburetors, a nos plug, and measuring tools out, then worked under my lighted microscope, and made some drawings today. Any machine shop should be able to make all you want.

[6s1640]

Hi Dan,

Very nice.  Do you have a recommended tolerance on each of the dimensions?   Standard three place tolerance would be +/- .010.    Do your recommend tighter?

The tolerance on the 0.375 and 0.500 radius and the remaining material under the slot 0.250 could be +/- .030.  Do think the total height 0.385 should be hard and not reference?  I realize you are trying to avoid double dimensioning with the definition of the 1/8 NPT definition.

Thanks for making.

Cory

[Dan Case]

Hi Dan,
Very nice.  Do you have a recommended tolerance on each of the dimensions?   Standard three place tolerance would be +/- .010.    Do your recommend tighter?

The tolerance on the 0.375 and 0.500 radius and the remaining material under the slot 0.250 could be +/- .030.  Do think the total height 0.385 should be hard and not reference?  I realize you are trying to avoid double dimensioning with the definition of the 1/8 NPT definition.

Thanks for making.
Cory

You are welcome.

Tolerances General:  I hesitate to assign tolerances based on a small collection of used parts and one new old stock (nos) one.  I would really like to have at least five (5) new old stock parts each made in different time frame to see what kind of variation the maker(s) had.  This is not a functionally critical part like an intake valve so most dimensions probably didn't mean a lot to the designer.  I didn't do an exhaustive survey yesterday nor did it get my Machinery's Handbook out but this does not seem to be a standard "size" commercial plug, at least not currently. The designer also probably was calling out fractions and not decimal dimensions. Some issues:
1)   Just in the parts I have access to there is more than a little variation is surface finish (tool mark artifacts) and crispness of cuts (hanging chip(s) and or burr(s) left behind). Surface finish and cut defects affect cut accuracy, easy of measurements (reproducibility), and repeatability between persons doing measurement.  Measurement tools used makes a difference; micrometers, calipers, analog, digital, a high resolution optical comparator, or co-ordinate measuring machine (which would be tough on this little part). I don't have access to a surface texture measurement machine anymore but from experience I would guess that non-thread machined surfaces on the parts I have range between maybe 64 and 125 rms over a 0.03 inch stroke. One used part had a roughly 0.003 inch tall rotary cutter burr left on both of the slot edges of the crown.

2)   Plating details. My sketches are intended to be before plating. (It can take many hours to completely study, make conclusions, and define every detail and that assumes there is access to determine what the original plating was and how thick it is.  We have been saying bright zinc for years but under a microscope this nos one looks like it might be plated in cadmium.  Whatever the sacrificial coating is it is very thin. It might be in the 0.0002 to 0.0008 inch range. It does not hide tool marks.)

3)   Defects. All the used parts on hand have some degree of what could be called wear and tear. That made using good lighted magnification to see exactly what I was measuring very important; an artifact of manufacture or an artifact of an installation or extraction tool.  The nos part I have had a chip from the slot cutting operation jammed into the groove bottom. The chip was fully plated and there was no shadow in the groove so the chip found its way there after plating.  I didn't notice it until I got the radius gauges out and put the part under the binocular microscope do test fits. To measure slot details I had to remove the chip. If I had just been out on the bench top I don't think any gauge would have "fit" very well.

0.385 Reference Dimension:  I chose not to make it a hard number.  Of all the dimensions that would be, my interpretation, the hardest for a machinist or numerically controlled machine to control. Across the parts selection I have that slotted crown surface has the most variation including most variability in surface finishes from pretty smooth out near the perimeter to pretty rough near the center.  Some parts have tiny semicircle almost flats near center of the slot as an artifact of machining. That also doesn't address the situation that it is not an even nice neat number. Most dimensions for countless parts before the finite element analysis revolution were based on being easy and typical. That was true of tolerances also. If I did a tolerance loop analysis and came up with ±0.012 I would put ±0.010 on the drawing. That did two things, 1) it was simple and 2) if gave a little wiggle room for different people measuring in different places under different ambient conditions, and with different tools.  Specially if the machinist got +0.010 average and I got +0.011 average I knew the part was still okay and would in writing allow the deviation. Where the dimension worked out I can't conclude whether the designer specified 3/8" or 25/64". It could have been either but 3/8 would have been simple. As a non-critical dimension it really doesn't matter I believe. I also measured a brass part that preceeded the plated steel part. The corresponding dimension came out to 0.363. I don't have a for sure clue if the designer meant 23/64 or 3/8. It very well could have been 3/8 for both of them as a ±0.02 inch tolerance would have covered both parts.  (Then you step into do you make replacement parts like the factories did or do you clean up dimensions and tolerances.  My best examples are Shelby American drawings, mostly by Moore and Remington, for parts and assemblies Shelby American had to provide for new street and race Cobras. In many cases you cannot take a "factory" drawing and exactly replicate what they made / had made and actually used.  Sometimes it is lack of details and sometimes the fabrication methods didn't lend themselves to making exactly what was drawn for some reason or another.) Anyway, how many would notice a part once installed if the dimension turned out to be anywhere between 0.36 or 0.39? 0.385 just happens to be what my new old stock part measures.  If I was having some made I would probably go for a 0.385 MAX just to be simple.

[s2ms]

Fantastic, thank you Dan!

[Dan Case]

Fantastic, thank you Dan!

You are welcome. These little plugs have come up in threads on multiple sites.  The design was shared in Ford and Holley brand carburetors. The super hard to find ones in excellent condition are the brass ones used in select Ford and Holley brand carburetors for specific applications before the steel part was introduced. Brass ones are almost universally damaged. These pipe thread parts are not to be confused with the straight thread parts used in 1964½-1967 Ford 4100A carburetors on HP289 engines.

I put more detail in the straight thread 1964½-1967 plug for Ford 4100A carburetors.



Owners, fans, restorers, and authors get the two different designs mixed up frequently.  It does not help that the internal threads in very many carburetors have been damaged in all kinds of ways and currently nothing threads in easily. The pipe threaded part and straight threaded parts are not interchangeable.  It is important to not use a pipe tap to try and chase the threads in a 1964½-1967 HP289 Ford 4100A carburetor. It is also important not to run a 3/8-24 tap into the ports of C3OF-AB and C3OF-AJ carburetors. The pipe threaded part, brass early then plated steel, shows up in selected Holley carburetors and 1963½-1964½ Ford 4100A models for HP289 engines. The straight threaded part shows up in 1964½-1967 Ford 4100A carburetors for HP289 engines.