Fuel pilot Screw

Does anyone know the thread pitch of the VM26 fuel pilot screws. I’m guessing they are 4mm. I’m limited in my thread pitch gauges and don’t have one to match. Carb bodies were vapor blasted and one thread is tight. I’m assuming there is media caught in the threads. Or any tricks to chase the threads.

 

Whip out one of the float bowl screws and compare them. Float bowls screws are 4 x 0.7 thread. I would imagine they are the same. 

5mm x .5

That is a good suggestion.  I do not know the pitch on the pilot screws. Look you have been inside enough carbs (seen your work) that I am not sure I can tell you anything you do not already know. Though it is never a good idea to take parts from one carb and use them in another you could try one and see if it is the threads on the screw or in the carb. I am not sure how to help you with finding the pitch.  I am not sure what it is like where you live but here (Seattle) there are brick and motor stores that can assist with this.

wrote:

5mm x .5

Thanks. I was playing with some taps I have matching up the pitch. The best match I had was using a 4mm x .5 tap to match for the threads. Then the caliper confirmed it is a 5mm. Feeling good now I see Ness independently coming up with the same. I questioned myself as I stopped at a local well known bolt and hardware supplier and they told me it was a priority size and pitch developed by the carb manufacturer. First time in 30 years they failed to help. Might have been beer night as they locked the door behind me as I walked in and was the last customer of the day. Yes they let me out———LOL. 

Thanks for the help and kind words. Next move is to get a 5mm x .5 nut and see if it threads on the pilot screw. If a go then a bottom tap will be ordered. 

wrote:

wrote:

5mm x .5

Thanks. I was playing with some taps I have matching up the pitch. The best match I had was using a 4mm x .5 tap to match for the threads. Then the caliper confirmed it is a 5mm. Feeling good now I see Ness independently coming up with the same. I questioned myself as I stopped at a local well known bolt and hardware supplier and they told me it was a priority size and pitch developed by the carb manufacturer. First time in 30 years they failed to help. Might have been beer night as they locked the door behind me as I walked in and was the last customer of the day. Yes they let me out———LOL. 

Thanks for the help and kind words. Next move is to get a 5mm x .5 nut and see if it threads on the pilot screw. If a go then a bottom tap will be ordered. 

www.amazon.com/gp/product/B07BF96X3M

Be very careful with a tap like this.  The threads are so fine that it's easy to start it incorrectly and take out all the threads in the hole.  Start the tap by hand and make sure it starts properly in the old threads.  A regular tip is better than a bottoming tap because it has a more gentle lead-in, which helps make sure the tap starts properly.

I use grease on the threads to catch the chips.  Regrease as needed as you go.  The idea is to pull out the swarf, don't allow it to collect in the hole.  When finished, spray carb cleaner into the port in the carb throat, which will exit the pilot screw hole, thus blowing the chips backwards.

Thanks Ed, lucky the “Swarf” seams to be deeper in the bore. The pilot screw threads in most of the way till it gets tight. That is done without the oring, and spring. This should help with getting the tap started. I was thinking a bot tap because the bad threads are near the bottom and I was concerned the tip of the tap might hets the tapered seat before it cleared the threads. I’ll buy both. Thank You

Just make sure the pilot screw port is open. As you know some of these screws have been broken in the past and left points behind.

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Just make sure the pilot screw port is open. As you know some of these screws have been broken in the past and left points behind.

Yes, Ed save my butt in that regard. I twisted off two tips because I couldn’t feel the bottom because the threads were boogered. He talked off the ledge with a repair procedure to remove the tips. 

   

I've fixed buggered threads before on CV carbs, but the idea is the same as VM's.  This photo is of an older set, but I did a set just this last weekend too.  I'm (sadly) getting pretty good at it.

20170527_134643 by nessism , on Flickr

 

Researched types of taps and picked a “plug tap” it is a tap between a bottom tap and a taper. Just an FYI.

 

Seeing how this post was about these carbs I figured I’d post the 95% finished product here. Needs top and bowl gaskets, bench sync, fuel levels set clear tube and one last go through to make sure everything is tight. 

Thank you to Nessism for talking me off the ledge over broken fuel pilot tips, and helping with the fuel transfer tubes. 

A thank you also to Zed1015 for rebuilding the choke plunger seals and supplying the air corrector Jets. 

These are going to Wheelhop for thanks for his Most Excellent Adventure and tossing my parts in with his. Thank you Jim!.