Radiator Neck Inner-Diameter?

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    Gents & Ladies,

    I recently bought a dog-bone radiator cap on eBay and believed it to be the correct size (outer-diameter threads) for my Series 80.

    I was wrong, as it is slightly larger in outer-diameter than the inner-diameter of the radiator neck on my Series 80 ( +/- 0/02″).

    I don’t want to rework it.

    I believe that the Large Pierce-Arrow cars have a slightly larger inner-diameter in their radiator neck than on the Series 80.

    I am looking for information on the inner-diameter for those cars, as I am now the owner of a “useless-to-me” dog-bone radiator cap.

    The piece is in good condition, near NOS, with one or two minor nicks.

    It is a solid 9/10, and note that it is chrome.

    I will appreciate any info on the matter.

    I am also looking to sell it for the price that I paid, which was quite reasonable.







    The inside diameter (top of threads) of the radiator neck on my Series 36 is 1.97″.  Cap is 2.03″ diameter across the top of the threads. I didn’t check the pitch, but can if you need it.



    Peter & Dave,  Based on Dave’s measurements, the nominal cap size is 2 inches; the next sizes are out of range at 2-1/4 and 1-15/16 inches.    Peter, can you please measure (micrometer preferred) the outside diameter; top-to-top of the threads, in 4 locations around your new “dogbone” and check thread pitch with a thread gauge?   Dave, could you also measure the threads pitch on your cap?  Within the standardized UN  thread series, the 2 inch size has several threads counts, each with different inside diameters and fits, which determines how tight the threads will be.

    If both Peter’s new cap and Dave’s existing radiator cap have the same external thread diameter and pitch, then it may be possible that Peter’s new cap can be used on his car without trouble. Based on Dave’s neck inside measurement of 1.97 inches, I expect either 6 or 8 threads per inch, but please check and let me know (by email?).     End of lecture.


    Herbert and David,

    Measurements of the “new” dog-bone radiator cap OD is as follows.

    2.095″ – 2.096 and 2.092″. (only three measurements).

    The OD of an existing and usable cap threads is 2.013″.

    The ID of my radiator neck is 1.977.

    The pitch on both caps is 14.



    Peter and Herb,

    My cap is 16 TPI.


    < sigh > Welcome to the wonderful world of Pierce Arrow, where you can get ANYTHING customized.

    I can think of two explanations why the threads of these to caps are different: If Pierce made the parts in their factory as odd sizes, they made new taps to fit each radiator cap dimension. The more likely explanation is that the radiator and cap were purchased as sets, and Pierce changed vendors at some point between the production dates of these two cars.

    Peter,  you have a choice; since your dogbone threads match the radiator, it be made usable if you reduce the thread crest (OD of the threads) by about 3/32 of an inch, to match the outside thread of your existing cap. Then make sure the bottom of the threads of the new cap also fits the neck and use it …..  or keep using the cap you have now and let the dogbone find another home, as you mentioned in your first email.   Thanks for the interesting problem, it was fun to chew on a while.



    Nearly 3 years ago you and I were emailing back and forth about a Pierce Arrow Dogbone I bought on ebay. If you would like all of the information that Bob measured I can send to you.

    Here is what my friend Bob came up with back then. I can email you everything I have if you would like. It was a lot of work to rework the dogbone. Bob went way out of his way to make it work for me. The Delrin Cap I bought off this site had the incorrect pitch for my Series 80. It was slightly off and Bob reworked that one so it would fit my Pierce. Below are the measurements that Bob came up with.

    The Dog Bone that I bought on ebay is the 14 thread. It measure 2.095. I was originally thinking to grind the dog bone down and change
    the pitch to 16 so I could use it on the Pierce. Bob did wind up cutting the threads off and putting a sleve on that worked great. I believe I have the correct measurements if you would like to have them please email me.



    Below is what Bob sent me December 25, 2018

    I should clear up a couple of details.  The photos I sent you labeled Delrin, dog bone, stainless are of the three caps you own.  If you look closely you will see a series of grid lines on the green colored back ground.  These images are taken from an optical comparator display.  The comparator uses a light and lens system to enlarge the item and project a shadow on to a screen.  In this case the part was enlarged 20 times.  Reading the grid against the shadow is how I took my measurements of the thread depth and angle.  I used my phone camera and simply took a photo of the screen for each part.


    My conclusion was the Delrin part was not cut deep enough leaving the large, oversize flat at the top (crest) of each thread.  The thread angle and spacing were very good.  The stainless part showed an irregular thread profile with poor angular dimensions.


    The info you received from Greg Long is very helpful. He measured several caps and found a range of 2.030” to 2.045”  This gives an average of 2.038”


    George Teebay measured his cap and found it to be 2.032”


    Paul Jacobs did not offer a measurement but stated his Delrin caps were made to fit his radiator neck.  His (now yours) Delrin cap measures 2.040” as shown on my drawing.


    One last bit of information I came up with is based on the dimension I measured off your radiator neck.  This measurement inside the neck is called the minor diameter of the thread.  I came up with 1.971”  Doing some math, trig and geometry you can come up with a calculated major diameter (this is what you measure on the cap)  I calculated a 2.047” diameter.  This is larger than all of the caps but is still hitting pretty close.  Going off the minor diameter and working backward is not the best way to go for several reasons.


    In measuring your radiator neck, I did NOT take a series of measurements.  Greg mentions his neck is 1.995” and brings up the question of roundness.  I did not check roundness on your neck and should have done so.


    Conclusion and Confusion:  Somewhat surprised to see your neck at 1.971 and Greg’s at 1.995.  This difference of .024” is a lot.  If Greg’s is more typical (large) it would explain why Paul’s Delrin caps have NOT given him any reason to question size.  Getting a  measurement from Paul on the size of the neck he uses to qualify his parts would be interesting.  We need to check your neck for roundness.


    We know Paul’s Delrin cap does NOT fit your neck.  Your neck is too small (or out of round) OR the Delrin part is too big.  Betting your neck in NOT out of round, we should be able to machine down the thread on Paul’s Delrin part to get it to work.


    Going forward—picking a dimension close to Greg’s average of 2.038” should work for your small neck and anything larger.


    Paul’s comments on the “last photos—-” showing a damaged thread—-must be the photo of your stainless cap.  He must have missed the caption titles and assumed all three photos were of his Delrin?


    Greg is correct—the threads on the Delrin part are NOT deep enough (especially to fit a small diameter neck like yours)


    Greg’s suggestion on making stainless caps with flat tops, etc. is interesting.  I would like to see the photo of the cap profile he mentions.


    The gas cap photo you show is interesting as it looks just like the Delrin cap.  You mentioned your gas cap is not threaded and will not help us check any dimensions for your radiator cap.


    This email is detailed but does summarize what we now know.  Will check In with you when I get back in town.

    Bob C.



    I have a substantial collection of dog-bone caps which now hold various sized Moto-Meters, so buying the most recent one was done on the chance this one would fit.

    It didn’t!

    I am currently not in the mood to spend another bunch of $$ to not only get this one to fit, but to re-plate it in nickel.

    I will probably be offering it for sale on the PAS website as soon as I take a few photos.

    Thanks for all of the input.





    I completely understand you position. I was completely blessed by Bob when he made mine fit. Just a slight difference in the pitch can even make the cap not fit. It may fit one Pierce of the same model and not another one of the same.

    I wanted to share a little of what I went through to get mine to where it fits today.

    Thank you for helping me on my other projects. Was hoping I could help you with yours.





    I also appreciate your input.

    I decided to use it as a mascot display piece, rather than as a functional Moto-Meter cap for my car.

    I have plenty of those.

    Thanks again.


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