Deep interrupted small bore

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Coupe31

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I am working on an old Model A carburetor. It is a Zenith updraft, typical of the ones used in the 20’s and 30’s. These carburetors tend to wear at the throttle shaft causing a vacuum leak. This carburetor has had this problem in the past and was repaired by installing throttle shaft bushings. The bushing have failed leaving oval shaped holes in the casting. I would like to bore the casting to .375 dia., install oversized bushings and then drill and ream for the final shaft size.

The problem is the carburetor is about 2.5 inches wide, which means that I will have an interrupted bore of almost 2.5 inches and it ends in a blind hole. I have not been able to find a small boring bar that is long enough.

Any suggestions??
 
It is odd that a throttle shaft ends in a blind hole. Can you replace the bushings with a pre calculated amount of crush? That way avoiding reaming the final bore. Normal repair is to use an oversized shaft and ream to fit. Boring a hole the size of a throttle shaft 2.5 inches is a real challenge.
 
Nutfarmer said:
It is odd that a throttle shaft ends in a blind hole. Can you replace the bushings with a pre calculated amount of crush? That way avoiding reaming the final bore. Normal repair is to use an oversized shaft and ream to fit. Boring a hole the size of a throttle shaft 2.5 inches is a real challenge.
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I have repaired several by reaming the hole and using an oversized shaft and it works great but the hole left by the former busing is too large. Both holes need to be in line and that's why I wand to bore both holes together for the oversized bushing. I will drill and ream the new bushing in place. A rebuilt Model A carburetor is between 350 and 500 dollars and then you are not sure what you are getting. I can drill out the blind end and plug after the machining is complete. Many have a thru hole and plug from the factory.
 
Start with a long end mill. The end mill is side cutting so it will ignore the oval hole and get you a straight starting hole. Once you have a clean hole you can drill and ream to final dimensions. That last little bit of drilling will not wander with the rest of the hole supporting the drill bit. Make sure the reamer has enough flute length to bridge the venturi.
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Or this if you really want to mill to final depth but the hole will end up a bit over .375 by the time you clean it up.
www.mcmaster.com

McMaster-Carr

McMaster-Carr is the complete source for your plant with over 595,000 products. 98% of products ordered ship from stock and deliver same or next day.
www.mcmaster.com www.mcmaster.com
 
I would bore the open side round first. Then make a crude cutter out of drill rod so that it has the same od as the newly bored hole and cuts only at the end, like a reversed piloted reamer. You can even make a step in the cutter od at the point where your blind bore ends, to act as a positive stop.

Now that you have both holes round and in line, you can ream to size.

Another way is to drill through the blind side then use that as a guide for a piloted reamer fed from the open side.
 
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