Engine stud hole retap question/confusion

[Marshall V. Daut]

I am working on an engine that has numerous problems. One is the presence of helicoil repairs, one of which came loose from its bore. The hole is too big for a new helicoil, so I just ordered the oversized threaded stud from a vendor, as well as the special tap. I have seen these oversized studs in another engine I repaired and they seem a little bigger than the helicoil hole. So I think the larger stud may just work.
But - I just received a call from the vendor stating they no longer have the tap in stock and don't know when new ones will be available. In their catalog, it specifies that the bit is 27/64" for oversized studs, yet that is the same size that one is supposed to use to drill for the standard 7/16" stud threads. How can the larger bit for the oversized stud be the same size? The threaded portion into the block of the oversized stud is noticeably bigger than the standard thread. It's tricky enough re-threading the standard size hole with a 7/16" tap without fear of snapping it. I should think using a 27/64" bit for the larger oversized stud would be too small for the larger tap. A tap would most likely break trying to re-thread the newly-enlarged hole that fits a standard stud. The tap size called for with the oversize stud in the catalog is 1/2"-12, not a common size to find. Which is wrong - the specified drill bit size or the tap? Or me?
If the tap size is in fact 1/2"-12, does anyone have this bit that I can borrow, paying for postage both ways? Or is it something a specialty tool store might have? It sure isn't at my local hardware store - I looked.
Why isn't anything simple???
Marshall

[John]

I doubt that the tap size is 1/2"-12 but most likely is 1/2"-13 UNC which is standard coarse thread. This thread series would take a 27/64" drill size. The 7/16"-14 UNC thread series takes a 'U' drill bit which is slightly larger than 23/64" according to my thread chart.

[Bob C]

I looked in Bratton's catalog and it shows 1/2"-13 for the oversize stud, 27/64"
drill is correct for that. You use a U size drill bit for the 7/16"-14 tap for the stock
stud. I would use a starting tap in the hole first if you are afraid of breaking the tap.


Bob

[Russ/40]

I have the 1/2 ×13 tap if you need to borrow it. It is available from multiple on line sources if you are still trying to buy one.

[Terry Burtz, Calif]

Tapping a hole to 1/2-13 UNC when it has already been tapped for a 7/16-14 Helicoil is a very bad idea. The hole is already too large and it is possible that the stud will pull out of the block when torqued.
Consider the use of a Twinsert which maintains the same 14 thread pitch.

https://www.stanleyengineeredfasteni...c943-rev4.ashx

[Big hammer]

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Marshall are you sure the hole is to big for a new helicoil ? I've had helicoil's back out and with a new coil on the proper tool they would go back in, you can us thread locker to keep them in. A new coil is like a tension spring and when installing it stretches and the tension keeps it in place.

[Bruce of MN]

If it somehow turns out that it is 1/2"-12, that is 1/2" BSW https://britishfasteners.com/threads/bsw.html and can be purchased here: https://www.tracytools.com/taps-and-...2-x-12-tpi-bsw

[BILL WILLIAMSON]

To lessen the chance of breaking a TAP, oil it well & after each 1/4 turn of the tap, back up a little, you'll FEEL the cuttings sorta' "snap" loose.
Bill Careful

[MALAK]

I like the Time-Sert inserts. Makes for a clean, strong repair.
https://www.threadkits.com/time-sert/

I highly recommend one of these to keep the drill perpendicular to the block:
https://www.shop.biggatortools.com/S...TD2000DGNP.htm

and this to keep the tap perpendicular:
https://www.threadkits.com/time-sert...l-tools/37164/

That is unless you have a mill or some other way of keeping things perpendicular to the block surface.

[GPierce]

Here is what we used to repair Diesel engine blocks and heads threaded holes.
http://www.repairengineering.com/keensert.html

[Marshall V. Daut]

Thanks, everyone, for these great ideas and suggestions. Some are beyond my tool/machining capability, but most look lie they will solve the problem with hand tools if the oversized stud doesn't work.
As shipping both ways to borrow a tap would cost almost as much as just buying a new one, I 'll go ahead and order one. That way, I still have something in my hand after the expenses.
Once the repair has been made, I'll come back with the story. It may be a while, though.
Marshall

[BILL WILLIAMSON]

Originally Posted by Marshall V. Daut

Thanks, everyone, for these great ideas and suggestions. Some are beyond my tool/machining capability, but most look lie they will solve the problem with hand tools if the oversized stud doesn't work.
As shipping both ways to borrow a tap would cost almost as much as just buying a new one, I 'll go ahead and order one. That way, I still have something in my hand after the expenses.
Once the repair has been made, I'll come back with the story. It may be a while, though.
Marshall

Marshall, my Friend,
The ONLY things the Dog & I do QUICK, is SNEEZE & FART--LOL
Bill Gross