The Stovebolt Site Forums The Shop Area The Tool Chest Re-tapping stripped bolt hole.

Guys, I have a stripped out bolt hole on my zinc alloy carb. The bolt appears to me to be a 7/32" x 28tpi. What would be the proper next size larger to tap? (I have available what appears to me to be a 1/4" x 20tpi machine bolt.)

I'd be leery of changing thread pitch - might consider a Helicoil if you can find the right size

Bill

If you choose to tap to a larger thread size, try a 6MM X 0.5. 7/32" is 0.218", and 6MM is 0.240". The 0.5 metric thread pitch is close to the same thread spacing as a 28 TPI thread in fractional measurements. a 1/4-20 would be too large, and too coarse a thread.

If you choose to re-tap, I'd suggest opening up all the other holes in that area of the carb to the same thread and replacing all the screws. That way there won't be any confusion caused by one oddball screw.
Jerry

If it is possible to put a thru bolt with a nut, that's another way. You can run a longer screw with same threads up thru bottom and put a nut maybe. May be too Mickey Mouse for you. Many carb parts available on Ebay. Maybe consider a donor carb section. Is this a B1? A new bowl is on Ebay now for $20. It may actually be new.

I agree that the metric would be the better fit.
However, instead of changing all the other screws, use a different head style to remind you (or someone else) where it goes. Either a cheese head or a button head socket.

M6X.75 or M6X1 are closer to 28p in actual measurements. The taps are $6 at home depot if you don't have one and a M6X1X20 Socket head cap screw is $1.10 for two. 7/32 screw does not exist, you probably have a 12-28(.216). Hens teeth. You better do some math to see if M6 taps a 12 OK. May have only .010 on the radius to play with. Check it out. Depends on how bad the "strip" is. No drilling, just start tapping. Agree with Sears about doing only one.

http://www.grainger.com/ec/pdf/Loctite-Form-A-Thread-Data-Sheet.pdf

dg

Denny, looks like a neat product. Especially for someone trying to do a points on restoration. Have you tried this stuff?

Not that mark, it's the same thing as JB Weld, just a filled
epoxy. You could use any of the filled epoxy's with a release
agent and get the same results.
Not gonna get the same torque values as you would with virgin
metal or a thread insert but for a lightly loaded fastener
that should work just fine. I assume the OP was dealing
with a Rochester 'B', the die casting won't take all that
much torque to begin with, that's why it's stripped out
now.
I did work with epoxy's at my last job, even the filled, i.e.,
metal or graphite resins don't take the place of virgin cast
metal. No matter what you fill it with, it's still limited by
the resin that binds it all together and epoxy resin isn't
all that strong. The less binder the stronger the composite,
as in the carbon fiber structures.
The metal fillers are good in compression.
dg

Gentlemen, Firstly, I simply have to apologize- I neglected to mark my thread for watching and I have missed out on all of the great advice here. Sometimes I forget that on this forum, one needs to mark a thread for watching. I did mark a related thread and it somehow got into my mind that that is 'the thread' about my tapping task. Only now was it brought to my attention that I have this thread going. Again, please excuse me.

So, not having had the benefit of the advice from you guys, I went forward and tapped the existing holes for 1/4" x 20 tpi. Luckily, it actually went ok. I drilled the clearance hole out in the air horn, and then I went directly to tapping the float bowl. There was no need to use a tap drill -the hole was well stripped and seemed perfectly ready to take the 1/4"-20tpi tap. I put in the new bolts and it seems really good. However, reading now your responses, I suppose I was more lucky that smart. Now I just have to re-install the carb and see the result. Thank you, gentlemen!

7/32 screw does not exist, you probably have a 12-28(.216). Hens teeth.

Bartamos, Thank you. I went back and checked the old screws with my rather inexpensive mic and I come up with 0.219 -which is probably the 12-28 you mention. I understand that it is a metric, which I did not expect to find on a 1965 Rochester B carb. I am not familiar with 'hens teeth'.

To be clear. I was saying you probably have a Unified Standard No. 12 thread size. Not 7/32. Not metric. Finding a 12-28UNF screw is tough locally at hardware stores. They are as rare as Hens Teeth. The B1 would not be metric.

Bartamos, Ah, thank you.

12-28 fasteners were fairly common in the past for electric/electronic things, pretty much dropped now, but some fastener specialty places still stock'em in limited selection of lengths/head style

Bill

McMaster Carr has them

Denny Graham, I was unaware that there are epoxy fixes for stripped threads. Thank you for that link. I think I will keep a product like the Loctite on hand. While it does seem a little tedious, entailing more than half an hour to do, it might come in handy when a bolt hole is difficult to get to for using a drill, tap, etc. Also, I was unable to find any torque values for the Roch B in the books. But all in all, we are talking about fastening an airhorn to a floatball in zinc, so I did it snug, pretty snug, but I did not put my shoulder behind it. I did two of the four bolts, and I will do the other two at the next cleaning of the carb. I actually like the look of the socket cap bolts that I put in. Regrettably, this has not solved the issue that I am trying to repair which we address in my Bogging/Stall thread.

Patching on A Rochester can be an exercise in frustration. Find a properly-calibrated Carter YF and don't look back. The oldtime advertising slogan is pretty true- - - - -"The ABC's of carburetion are 'Always Buy Carter'- - - -!"

Jerry

Hotrod Lincoln, Is there a Carter YF that you think would be particularly suitable for my stock I6-250 w/2spd PowerGlide and Delco P&C dizzy? So far, I figured I would take the Autoline c935 from AutoZone, which is exactly the Rochester B that I have now.

Search Ebay for "Carter YF Chevy 250". There are at least six of them listed right now, several different model numbers specifically for 2nd. generation Chevy I-6 engines. Carb King is probably much more qualified to recommend a specific model than I am.
Jerry