Where to buy a metric bolt

[doxford jim]

Drill and tap to the next larger common size - imperial or metric, if at all possible. Drill and use a convenient size helicoil if it can be done. That way you will be able to get the right bolt in future.

I know it isn't the preferred way to fix things, but sometimes you gotta do what you gotta do.

 

[oldtimer 66]

Bolts are generally rolled threads and if you chase even the same thread with a die it cuts deep into the bottom of the V of the thread. Nothing wrong with that but it does take some extra effort to re-cut the same rolled threads of a bolt with a die. Just wanted to make every one aware of that. AKA cut threads versus rolled threads. Plus you have the hardness of the bolts, Grade 2,5,8 etc. to consider when chasing new and or re-cutting the threads. Later

 

[tcartwri]

7/8-14 is 0.088" up in size. You shouldn't even need to run a drill into it. Tap is going to be expensive though..

 

[LD1]

.088" is alot when talking drills and taps. No way you are gonna run a 7/8-14 tap in a hole threaded for M20x2.0.

7/8-14 requires a 13/16 bit (.812")

The M20x2.0 would have been drilled with a 18mm bit. (~.708")

Just aint gonna work. The root of the threads in the existing M20 hole are still smaller than needed for the 7/8-14. IE: of you drilled it out for 7/8-14, you would not only remove the threads of the M20 hole, but also about 0.040" of virgin material.

So...yea, you would have to drill if you want a UNF 7/8 bolt

 

[tcartwri]

.088" is alot when talking drills and taps. No way you are gonna run a 7/8-14 tap in a hole threaded for M20x2.0.

7/8-14 requires a 13/16 bit (.812")

The M20x2.0 would have been drilled with a 18mm bit. (~.708")

Just aint gonna work. The root of the threads in the existing M20 hole are still smaller than needed for the 7/8-14. IE: of you drilled it out for 7/8-14, you would not only remove the threads of the M20 hole, but also about 0.040" of virgin material.

So...yea, you would have to drill if you want a UNF 7/8 bolt

Finished diam of M20 hole is 0.787"....

In reflection, there may not he enough material for a 7/8 thread.

 

[LD1]

.787" is only where the threads have been cut.

The "peaks" of the threads in the hole will still be .708" diameter.

Give a 7/8-14 requires a .812 drill, that will remove ALL of the M20 threads AND a little virgin material. How can you possibly say there may not be enough material?

 

[tcartwri]

I was just thinking it could get pretty sloppy, particularly in cast iron.

 

[LD1]

This may help. These are the threads to spec, superimposed on top of each other.

M20x2.0 in red
7/8-14 in white



The threads look flat on the tips, because they are. Threads with a drill and tap are NOT 100%. Those dimensions are assuming using the reccomended 18mm bit for the M20, and 13/16 bit for the 7/8 UNF. And assuming cut to the maximum depth spec of the particular thread

 

[tcartwri]

I'm just thinking that if you use the existing hole to pilot the drill and it wanders even a little things could get complicated in a hurry. It has to be square and dead center.

 

[LD1]

I'm just thinking that if you use the existing hole to pilot the drill and it wanders even a little things could get complicated in a hurry. It has to be square and dead center.

Agreed. But there is no way you are gonna get a 7/8-14 tap down that hole without drilling (or breaking the tap)