Trying To Save A Carb

[moonpup]

At some point in it's life, one of my carbs had the choke plunger cap (part# 1647-5000) cross threaded into the carb body. The threads on the big end of the plunger cap are ok, but the ones in the carb body where you screw the cap in are a little buggered such that the plunger cap will only screw in at an angle. No bueno! 

I'd like to find out what size tap to use to chase the threads in the carb body. I've been all over this little podunk town checking any place I can think of (machine shops, hardware stores. nut & bolt supply etc.) and they all get this look like I’ve just asked them an algebra question.  No help at all.

Does anybody here already know the answer or maybe have a way to get the measurements for the big end of the plunger cap threads?  Sure would be appreciated!

[coxy]

You may have to drill it out and bung in a helicoil but I will have a look and see what size thread by the weekend

[moonpup]

I've considered that option, but I'd like to avoid that if it's at all possible. Take your time and let me know what you find out when you get a chance. Thanks...

[coxy]

Now Don't QUOTE me on this its late my eyes are blury and I think this one is tricky there is not a lot of room in there and a tap will have to not have a taper but the die fit to the plunger holder/adjuster appears to be m12x1.5 though both threads are so short this may not be the case .I think that someone who specialises in threads may want to jump in .or take it to a thread repairer .I have to do something similar with stripped  oil pump threads

[moonpup]

Whatever it is, I think it's going to be an oddball size based on what I've checked so far.

I have a metric tap & die set and when I checked that, I found that the 10mm die was too small and the 12mm was too big, regardless of thread size/pitch.

Also, I went to Lowes today and checked things out on one of those universal plastic cards used to determine sizes for nut/bolt/threads. Got the same results putting the cap in the 10 & 12mm holes, but it seemed to fit somewhat good in the 7/16 hole. The thread count also appeared to be 24. Of course they had nothing that size in the store, so I couldn't try a fit in person.

I'm going to try calling Mikuni or Sudco next and see if someone there can help.....

UPDATE- Sudco is clueless  

UPDATE #2- Mikuni equally clueless  

[BRT-GTR]

Hi Mike,

            I don't specialise in threads and I am due to have a cataract done next month but I've just measured the screw-in part (1647-5000) from a 67 GTR, Vin 6750.
       I get 11mm OD with 1.0mm pitch.  You wont find one of those in a cheap tap and die set or your local hardware store, it's a bastard thread size (official term, honest  ) but they are not as unusual as I would have thought. Just Googled them and plenty came up here in the UK.

You could try Amazon or here-             http://www.tracytools.com/taps-and-dies/metric-taps-dies/11-x-1-metric        -sure they will be available locally in the States.

  Could be tricky to get the thread to start correctly, because of the step in the carb, you will have to start with a plug tap and may have to grind the end off that to clean the threads to the bottom.

  Good Luck and no guarantee on the thread size, they might have changed it later,  but nothing to lose.    Brian.

[slawsonb]

I love it when a manufacturer doesn't know anything about their product. Sheesh! Yes, it old, but it says Mikuni on it.
...bert

[moonpup]

Mochas gracias mi amigo from over yonder!

That sounds about right, but before I go rush off to order a tap, I think I'll give this thread a little more time and see if anybody can jump in and confirm your measurements.

Please don't take offense to me waiting, I think I have only one shot at this and would like to know that all my ducks are in a row before I move forward with this "repair".   

[disc_valve]

Hi, Moonpup,

Yes, the thread size of M12 * 1.0 sounds right for a 350 choke plunger, but beware. Mikuni changed this thread on the carbs fitted on later model 350s - presumably around the same time that they changed the float pivot holes to give a light interference fit for the pin and stop the holes wearing out through vibration.

I did measure both sizes of thread a while ago, but I can't get my hands on the figures right now - I didn't update the file on my PC memory stick, but I've got a couple of old carbs out in the back of the shed. I'll go out with a measuring stick and get back to you.

Graham

[OldSwartout]

I grabbed an old 350 carb body I had in a box - a 10 x 1.0  bolt slides right in (too small) and a 12mm is too large to even start, so the 11mm sounds correct (at least for that carb body I have).  It is 1.0mm pitch.

[moonpup]

The carb is off a 1967 GTR, so I'm pretty sure it's one before any thread changes as mentioned by Graham. Also Graham, you said that "the thread size of M12 * 1.0 sounds right for a 350 choke plunger". Was the "M12" a typing error or are you saying that's the size on the later 350's.

Also, thanks Karl for taking the time to check one of your carbs. Sounds like 11mm x 1.0mm is the way to go.

[disc_valve]

Hi, Moonpup,

Whoops! It is an M10 thread - a slight case of "finger trouble" (too many keys to choose from, and I hit the wrong one!)

I've been and measured the choke caps on both types of GTR carb. The early one (like yours) is M10 * 1.0. The later carb was an even weirder size - 10.8mm * 1.0. I'm pretty sure it's not 11mm as I tried screwing the choke cap into the end of a new spare Speedo cable, and it didn't want to go. (The speedo cable bottom fitting is definitely 11 * 1.0). Whether 10.8mm equates to some strange BA type thread I'm not sure, but I've never come across that size anywhere else.

The M10 * 1.0 is the same thread size used on the smaller Bridgestones (hence the "1647-5000" BS90  part number) and these choke caps are made of brass. The later 10.8mm choke caps seem to be zinc-plated (steel?), and are distinguishable by a groove machined around the cap just above the hexagon part. From memory, my GTR parts book only shows the "-5000" part number for it, which will be the one for the early type carb.

Best of luck with your salvage scheme!

Graham

[moonpup]

Here's a pic of the cap from the carb. Based on your description, you're saying this IS the later thread version and therefore should be 10.8mm x 1.0mm...... right?

I'm soooo confused now!  

UPDATE: Just got back from the garage and checked out the speedo cable fit. The fitting on speedo end was too big, but the fitting that screws onto the wheel hub side fit fine. Didn't feel loose and tightened up snuggly. (at least for me it did)

[slawsonb]

My head is throbbing away trying to figure out why Mikuni would use such an off the wall size...(thump, thump, thump)...
...bert

[BRT-GTR]

    You've got me confused now.   Not sure which cap you've got.                 (oh, can see pic now)

     I've just measured a later steel choke cap with a micrometer, it comes in at 10.9mm which would be correct for an 11mm tap size. (You are measring on the thread peaks which are slightly rounded off). The choke cap happily screws into the bottom fitting on 3 used speedo  and one tacho cable. As a cross check I screwed a Tacho gear bush, 2813-9000, into the carb thread, it fits a treat, did it for me.  .     I would be 99.9999999% happy to run a 11x1 tap (if I had one)into the carb.  

     Graham, will your new speedo cable screw onto a front wheel fitting or the tacho drive, should fit either and thanks once again for the cable thread info.  
     I agree with you Bert, it's an oddball  size but they did use it also on the cables at one end.          I have found variations in the 4mm threads used on the bike but lets leave that for another time.   My ed urts.         Brian.

[slawsonb]

If its an 11, that's less oddball (off the wall) than a 10.8. My head feels somewhat better now. Probably need a toddy to really get back to normal.
...bert

[moonpup]

More pics of the cap, this time as it sits cross-threaded in the carb.

Also decided to just go ahead and try the 11mm x 1.0mm size tap and either it works or it doesn't. I'll report back with the results. This is the one I ordered on Amazon......

http://www.amazon.com/1-0-High-Speed-Steel-Bottoming/dp/B0006G4MVG

[BRT-GTR]

   Mike ,
             Some people shouldn't be allowed near a wheelbarrow with a spanner never mind a BS carb.   That tap will be fine, good choice, HSS taps tend to made to a better standard than Carbon Steel taps.
   
  As a matter of interest, the top thread on the speedo/tacho cables seems to be 12mm x 1.0mm.             
  I'm a big fan of the metric thread system, never thought I'd say it, one set of spanners fits the lot.    Have struggled in the past with imperial BSW, BSF, BA, BSP, AF, UNF, UNC etc to mention but a few. 
  Brian.

     

[disc_valve]

HBi. Folks,

I've already had a re-think last night, and I now tend to agree that the later choke cap thread is actually 11mm , not 10.8. The choke cap I measured was probably a bit worn down on the thread peaks which fooled me on the diameter. I did try a speedo cable fitting from an old BS90 brake plate, and that screws into the carb choke thread very nicely. We'll settle for 11.0 * 1.0.

I will post fresh thread (pun intended) about Bridgestones and Mikuni carburettor thread sizes , which members may find useful for reference.

Graham
.

[moonpup]

Thanks to everyone's help, I've managed to save this carb. 

It's official now.... 11mm x 1mm is the correct size to use. My plunger cap now screws in at the correct angle and fits snuggly. 

Thanks again everyone!