Swapping float bowl screws on the 38's

weekendrider

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So you've gotten tired of the phillips head screws and decide to swap. You pack one to the hardware to play the match game. The selection is a little thin, they are either too short or too long but hey longer is better right? And the little bride has always assured you size doesn't matter so go BIG.
Well there is a gottcha on the 38's so before you go cranking them puppies home with the allen wrenchs LOOK. 3 of them don't matter screw away BUT when you get to the 4th, the one under the choke manifold you better have your eyecrometer working. When that screw hits the manifold it will peel those threads out of the throttle body in short order. And it happens I have repaired two sets of carbs (e-bay purchases) this has happened to. One pair with both holes stripped. The picture should let you see what ima sayin.
IMG_20170820_121150_135.jpg
 
Good heads up, WER.

Might as well add this "fix" in here. A suitable repair to stripped carb body/bowl threads is to use slightly enlarged #8-36 machine screws.

BS38 Carburetor Bowl Screws.
Stock screws are M4 x 0.7 - 12mm
An #8-36 screw is about 0.007" larger in diameter and has nearly the same thread pitch

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Close-up of the stock carb body threads:
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A #8-36 screw can be modified as an effective thread tap, or chaser, by dremel-ing a slot or notch on one side of the screw. Then slowly and carefully get the threads to align/match, and do the usual "tapping" routine, 1/2 turn in, then withdraw 1/4 turn, then another 1/2 turn in, and continue until fully rethreaded.
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#8-36 x 1/2" socket head screws are easily available, like this offering on eBay:

http://www.ebay.com/itm/110999227710

8-36-CarbScrews.jpg



Edit: WARNING. Be sure to measure the thread diameter of any/all #8-36 screws, to be sure that they're indeed 0.164" (4.17mm). Some scrupulous vendors have a history of conveniently slipping in some cheaper closely matched screws. Similar to the M5x0.8mm versus the #10-32.
 
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Yes, 12's are a little short, 14's a little long. I custom cut mine to about 13 from longer ones. Something that aggravates the problem is if you use the special "high collar" lock washers made for Allen bolts. They have a smaller O.D. so they fit under the Allen head better but to compensate for that, they are a little thicker than a normal lock washer. That cuts the reach down on the screw even more.
 
Adding a flat washer or two under the head works ok too.
When you buy the too long screws, buy a few nuts that fit. Thread the nuts on the bolts before cutting. When you thread the nut off it removes any burrs on the threads. I use a Dremel tool for this and slightly dome the screws before removing the nuts.
Unless you have a metric tap and die set, then use the proper die instead of the nut.
I have both an SAE and metric set of taps and dies. I often use them to clean up threads.
Leo
 
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