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Thread: How do I measure and drill small holes?

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    Jim_Lou's Avatar
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    Question How do I measure and drill small holes?

    In a few days I'm going to start modifying a carburetor, including changing to smaller jets. The current jet is between a #39 drill and a #38, probably a .100". I have some smaller jets that are unmarked as to size but are surely too small. Jets are usually sized in increments of .001" so there might not be a number drill the size I need. So how do I measure a hole that small, and how do I make it a predictable amount larger?

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    jackal's Avatar
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    Jim,

    I worked in a carburetor repair shop for about 10 years. We had reamers that we used to open up the sizes in jets.
    If you use a drill the swirl from the flutes and cutting will affect the fuel flow.

    We tested everything on "wet-flow" benches.

    For gauging there were several sets of round plug gauges. Starrett I believe????

    Some people couldn't figure out how a jet ( say .055 size could be drilled out to .063, and flow less fuel).

    On Holley jets, there is a funnel shaped chamfer on the entry side of the jet. This is critical of the flow.

    Also, I have seen guys store their extra jets on a piece of mig welding wire.

    After they rumble around in the toolbox for a month or so, the mig wire eats small imperfections into the bore of the brass jets.

    The jets don't flow like they were supposed to anymore.

    I have measured new jets from Holley before and find the same # jet with 3 different sizes.


    After around #55 or so, the hole size is not the jet number size.

    A #60 jet may measure .060, but a #78 jet may measure .090.

    Try using reamers for opening up the flow and deburr the holes the best you can.

    Hope this helps,

    JAckal

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    opscimc is offline Stainless
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    Quote Originally Posted by Jim_Lou View Post
    In a few days I'm going to start modifying a carburetor, including changing to smaller jets. ... So how do I measure a hole that small, and how do I make it a predictable amount larger?
    Measuring the dia. of a hole is one thing, measuring its flow is another. You cannot drill (or ream) a jet and have it flow the correct amount. The flow through a jet depends on several factors (gravity, pressure head across the jet, viscosity of the fuel, etc.), but it also depends linearly on the"contraction coefficient" of the orifice. This is the problem with drilling.

    It makes sense that the shape of a jet affects flow, for the same reason a streamlined car has less drag than one that isn't. Unfortunately, the contraction coefficient varies between 0.62 for a sharp edge on the exit orifice, to 0.97 for a rounded edge. What this large range means in practice is that two jets carefully reamed one after the other almost certainly will vary in flow by at least 10%. Because of this, the main jets sold to you by the manufacturers are all individually installed on a flow guage, measured, and tweaked into compliance by burnishing part of the exit orifice. (this isn't necessarily done by an aftermarket manufacturer, causing no end of headaches for tuners).

    Again, even though the bores of two 0.1"-dia jets can be identical to better than 0.0001", they can flow more than 10% different than each other. This makes boring your own futile, unless you have a way of measuring the flow. A standard SuperFlow-type flow bench can be used for this (it doesn't have to be a wet bench to give valid numbers).

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    opscimc is offline Stainless
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    Quote Originally Posted by Jim_Lou View Post
    The current jet is between a #39 drill and a #38, probably a .100". ... So how do I measure a hole that small ...?
    p.s. I hate it when I ask a question (e.g. "how to I measure a hole that small?") and someone -- me, in this case -- answers as different question. So, one answer to this question is: Diatest split anvil bore gage. Several other companies make similar split anvil gages, but if you search eBay for 'Diatest' you will find the kind I'm talking about. However, if you buy a set, it is essential you get one with the necessary ring gauges to set the diameters of the balls. You can set the diameters with a micrometer but, at the level of a ten-thou., it's tough to do and be confident you've done it as accurately as possible.

    My Diatest set covers diameters from 0.057" to 0.165", with precision of a ten-thou. A Mahr Intramess set takes over where that leaves off, continuing up to 0.28". For a specialized carb. application, i.e. measuring Amal needle jets (diameters between 0.105" and 0.109"), I have a Starrett bore micrometer that covers the smaller range 0.100"-0.120" to a ten-thou. using a split cylinder, rather than a split ball. Where the Diatest and Starrett overlap, and where I have two ring gauges to set/check them, they agree to better than the smallest division on each gage (i.e. better than 0.0001"), so these gages accurately answer your question "how do I measure a hole that small?"

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    jim rozen is offline Diamond
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    The other non-answer that nobody's yet supplied is, it won't work well.

    A better plan is to simply purchase the size jets you need for this, if they are still
    available from the factory. Jets like this correctly sized are very inexpensive.

    If it's from a carb still supported by the manufacturer.
    smalltime likes this.

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    bryan_machine is offline Titanium
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    Another part of the general answer responsive to the question is that you can get gage pins down to about 0.006 (I suppose smaller somewhere) and up to whatever size you want. You can get them in pairs, where on is "-" (0.000,2 under, a "go" gage) and the other "+" (0.000,2 over, a "no-go" gage.) So if the go-gage fits and the no-go doesn't, that gives you a nominal size +- 2 tenths. There are surely fancier pin gages closer tolerance than that.

    But that doesn't tell you if the hole is round, or smooth, just what diameter of cylinder will or will not pass through it.

    I've seen drills listed down to 0.004".

    None of this will deal with how surface finish affects flow (see above) nor with chamfers or burs on the entry or exit edges (see above.)

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    Wow! It looks like I'm in deep doo doo here. I knew instinctively that there is more to the flow of a jet than just the hole size, but hadn't figured on all the other variables. As to buying jets form the manufacturer, Carter in this case, I think they're out of business, and probably would be building something besides carbs if they weren't. I found a supplier of jets, but the biggest they list is .094, which I suspect will be too lean, knowing how much difference a single step can be. A supposed expert told me that one size smaller would probably be good for going from 300 feet of elevation to 10,000 feet, two sizes at the most.

    I have an exhaust analyzer coming. My hope is that I can test a given jet, and take the next step from my supply, or if the largest is too lean, open it up a little. A bigger hole with the other variables constant MUST flow more fuel. I hope.

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    In my twenties I worked on Porsches and became the guy in the shop that did all the carb work. I had several trays with 40 or 50 compartments for almost all the jets we needed and tiny plug gages to measure them with. I had a bunch needle like reamers with very slight tapers to open up a jet if necessary. Those were some fun days, figuring out which carb out of six needed and intermediate jet cleaned or rejetting after an engine build. I loved it.

    I think you can still buy the jet reamers and gages in sets.

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    I made Critical Flow Orifices for air in the long distant past. We used carbide form tooling in 303 SS. Then we calibrated them on a flow bench. If we made 20 we could sell 6 pair matched to 2%. It was air I know but there was 2 to 5 % variation. I presume liquids would be similar...

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    Quote Originally Posted by jkruger View Post
    I think you can still buy the jet reamers and gages in sets.
    The relevant equation for flow rate through a jet has squares, square roots, and subscripts in it so I won't type it, but it depends on the square of the diameter. That you can accurately change and measure. Unfortunately, it depends linearly on the "contraction coefficient," and the large variation it has depends on the precise shape of the outlet orifice, making this a very, very difficult (or, hopeless) problem to do at the necessary level of matching flow rates to better than 5%. Yes, lots of tuners used to use drills and reamers, and they thought they were doing it correctly, but they weren't. You certainly can use reamers to increase the flow rate, but only by measuring the rate will you know what it is you've produced.

    A while back I wanted to make a cheap, but precise, portable gage for measuring diameters of Amal needles. I bought a range of high quality reamers and experimented making holes in Al blocks. I also had my technician do this. Although the holes looked pretty good, if I do say so myself, unfortunately I had the Diatest and Starrett gages to measure them with. I should have left well enough alone, but instead I measured them.

    I have a holder for my Diatest gages that holds the gage steady and lifts a platform straight up to insert the gage in the hole. Since they are ball gages, only the widest circumference makes contact, so this serves as a roughness meter for the inner surface of the holes at a length scale of ~0.1mm. We reamed them dry, and with cutting fluids of various kinds, and all I can say is none of them were as good as the bores in commercial jets (although, we were using Al, not brass). Anyway, maybe my technician and I just don't have the talent to do this, but making a precision, smooth bore of dia. ~0.1" with no taper isn't trivial (and that's quite aside from the problem of the shape of the exit from the orifice).

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    jim rozen is offline Diamond
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    Has anyone tried extrude-honing jets to size them?

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    JohnEvans is offline Hot Rolled
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    Jim_Lou : Give Jon at Carb shop of MO. a call at 573-392-7378 Mon-wed. central time. He is a Carter expert and is more than willing to give advise.

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    Quote Originally Posted by JohnEvans View Post
    Jim_Lou : Give Jon at Carb shop of MO. a call at 573-392-7378 Mon-wed. central time. He is a Carter expert and is more than willing to give advise.
    I traded e-mails with him early in this project when I was thinking about switching to a different carb, but he pretty much blew me off. Maybe if I get him on the phone he'll be more helpful. I'm ordering a gauge that reads air-fuel ratio directly in real time, and have decided not to make any changes until I have that working. That will take a whole bunch of the guesswork out of the project.

    Thanks for all the advice everyone!

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    Just curious what is the application that you are working on ? The reason I ask is you mention Carter as the carb that you are working on and I see a Jeep in your avatar if you are working on an electronic BBD on a 4.2 and looking for tuning for altitude there is an altitude jumper in the wiring harness that you ground to be able to run above 5000ft. without having to change the carb. Billl

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    No BBD, this is a '78 that came with a YF 1-barrel. I was going to switch to a Motorcraft 2150, thinking I could mill the top off the manifold and weld on a proper two-barrel plate. It turned out to be a lot more work than I initially thought and the shop got busy, so now I am just going to try re-jetting the YF. It's rich because the EGR has been removed, I guess.

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    opscimc is offline Stainless
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    Quote Originally Posted by Jim_Lou View Post
    I'm ordering a gauge that reads air-fuel ratio directly in real time, and have decided not to make any changes until I have that working. That will take a whole bunch of the guesswork out of the project.
    I have an Innovate air/fuel gauge with tach for which I've made a bracket to attach to the handlebars of motorcycles. You're right -- having such a real-time gauge changes carburetion from a guessing game, to a science.

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    EGR elimination wouldn't make it run rich. Float level would .Going to a later 2bbl intake would make it easier to convert to a 2150. When I get to work tomorrow I will look up and see if I can find what the High altitude jet for that application should be. Bill

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    Thanks! The float is set at the factory recommendation, whatever that is, the stock jet is stamped 120 401 and measures about .100", and the metering rod is in its lowest setting, but it's definitely rich; the exhaust system from the ports to the tailpipe is sooty black, and it's been running that way for years. It never bothered me much because I drive it very little, but in Colorado I expect it will hardly run.

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    jocat54 is offline Plastic
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    Jim, I know this might not be the answer for you but has worked for me several ties in the past (last on Kawasaki Mule).
    I use the a tip cleaner for a oxy/act torch, not fast but they work okay. Have no idea of what sizes they are, never measured them. I wanted a bigger orifice so I just found one that would go thru the jet and used the next bigger size to ream it out.

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    9100's Avatar
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    Re comments about measuring holes with a Diatest, slide one through a normal drilled hole and watch it jiggle. A drill naturally walks around rather than going straight through. The spiral it cuts is regular, almost like a shallow thread. You can see it when drilling clear Plexiglas. A really good tool and die maker I worked with had the view that a drill was only a way to get excess metal out of the way so you could make a good hole. He had a set of end mills ground to the sizes he normally used and would drill holes undersized, then bore them.

    Re discharge coefficients, I make propane - oxygen torches for silver brazing that ignite the fuel stream inside the nozzle and send it out through an orifice designed to have a bad coefficient. The flow necks down so much that the torch stays cold. You can hold it in your hand and I have even had moisture condense on it from expansion of the oxygen cooling it. By getting the fuel lit while everything is moving slowly, there seems to be almost no limitation on the maximum flow rate. They don't blow themselves out, but the noise is unbearable when you get up to high flow rates. I have considered marketing them but sure as hell some idiot would run one up to where he blew out his hearing and sued me.

    Diatest had a display at IMTS. They make a lot more than the little ball end gauges.

    Bill

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