Thread tapping drill size help
Discussion
I need to tap some holes for some 7/16 Helicoil inserts. In the kit that I ordered, which arrived this morning, the drill bit is 29/64, but I was expecting a 25/64 drill. In tap drill charts, a 25/64 drill size is listed as 'tap drill' and a 29/64 drill is stated for a close fit 'clearance hole'. I have no idea what a 'clearance hole' is for, but I'm assuming I need a tap drill for my Helicoils?
Can someone please clear this up for me? Why have I been provided with a 29/64 clearance hole drill with a 7/16 tap for inserting 7/16 Helicoils?
Thanks.
Can someone please clear this up for me? Why have I been provided with a 29/64 clearance hole drill with a 7/16 tap for inserting 7/16 Helicoils?
Thanks.
ian_uk1975 said:
Of course, it makes sense to me now (don't know why it didn't before).
Bugger though... means I'll have to buy another cordless drill as my current one only has a 10mm chuck!
Thanks.
Be better drilled with a pillar drill in my view if the item is reasonably small, a cordless will turn much too fast for a drill that size, what material is it in?Bugger though... means I'll have to buy another cordless drill as my current one only has a 10mm chuck!
Thanks.
tr7v8 said:
ian_uk1975 said:
Of course, it makes sense to me now (don't know why it didn't before).
Bugger though... means I'll have to buy another cordless drill as my current one only has a 10mm chuck!
Thanks.
Be better drilled with a pillar drill in my view if the item is reasonably small, a cordless will turn much too fast for a drill that size, what material is it in?Bugger though... means I'll have to buy another cordless drill as my current one only has a 10mm chuck!
Thanks.
stevieturbo said:
will a cordless drill that only has a 10mm chuck even have the power to turn a bit that size ??
Providing it has a 13mm chuck, I'm going to ask to borrow my mate's DeWalt cordless... that should definitely have the grunt (I've borrowed it for oil pump priming purposes in the past, to good effect).Helicoiling, or in fact any drilling and tapping, by hand is something I tend to consider extremely dodgy if you require a finished thread that is mechanically sound and actually points in approximately the right direction. Fortunately having a Bridgeport mill it's not something I ever have to do by hand. It's a fairly regular op for me to have to reclaim stripped manifold threads in cylinder heads and on the mill it's a simple operation.
I assume you're reclaiming stripped 7/16" existing threads rather than creating helicoiled threads from scratch? By hand I suggest you double drill i.e. run a 7/16" through first to remove the old threads and then run the 29/64" through to enlarge to tapping size. You have a better chance of drills following the alignment of the existing hole if you go up in small increments. You'll also have less tendency to drill oversize. It's also handy to have a mate watching to make sure you have the drill truly vertical because it's very easy to be miles out when you're looking down from on top thinking it's vertical but are really about 20 degrees out.
How you then start the tapping operation correctly aligned by hand is another problem. That's much easier with a taper tap which has a long lead-in than a 2nd tap but the latter is all you'll get in a helicoil kit. Trying to start a 2nd tap accurately aligned really requires a jig or machine tool. Frankly every time I come across a thread someone has already tried to restore in a head or block by hand it's a complete cock up both in terms of position, alignment to the vertical and usually even to having any decent depth of new thread because they drilled oversize as well as on the piss. Most of the really tricky repairs I've had to do have been after someone has had a go first and ended up with an even bigger stripped thread than they started with. I had a Pinto head in last year which someone had tried to restore a stripped 8mm exhaust manifold thread in. They'd screwed that and had to enlarge out to 10mm, screwed that too and ended up with a partially stripped 10mm thread that had had almost no depth of thread to start with, was nearly 1mm out of position so the manifold didn't fit over it anyway and about 10 degrees on the piss. I had to mill that true to the vertical and back into the right position, tap the hole out to 12mmm, make a threaded plug out of an old bolt to fit and locktite into it and then finally drill and tap the desired 8mm hole back into that.
I've also seen plenty of DIY attempts at the CVH/Zetec ZVH conversion where you have to tap some new holes in the block for the waterpump and that's rarely a happy story.
If these are critical threads I'd get it done on a machine!
At the very least practice on a scrap component first to see if you can achieve a satisfactory result. Once you've screwed a helicoil operation you're left with very few cheap or easy options to get back to the desired starting size.
I assume you're reclaiming stripped 7/16" existing threads rather than creating helicoiled threads from scratch? By hand I suggest you double drill i.e. run a 7/16" through first to remove the old threads and then run the 29/64" through to enlarge to tapping size. You have a better chance of drills following the alignment of the existing hole if you go up in small increments. You'll also have less tendency to drill oversize. It's also handy to have a mate watching to make sure you have the drill truly vertical because it's very easy to be miles out when you're looking down from on top thinking it's vertical but are really about 20 degrees out.
How you then start the tapping operation correctly aligned by hand is another problem. That's much easier with a taper tap which has a long lead-in than a 2nd tap but the latter is all you'll get in a helicoil kit. Trying to start a 2nd tap accurately aligned really requires a jig or machine tool. Frankly every time I come across a thread someone has already tried to restore in a head or block by hand it's a complete cock up both in terms of position, alignment to the vertical and usually even to having any decent depth of new thread because they drilled oversize as well as on the piss. Most of the really tricky repairs I've had to do have been after someone has had a go first and ended up with an even bigger stripped thread than they started with. I had a Pinto head in last year which someone had tried to restore a stripped 8mm exhaust manifold thread in. They'd screwed that and had to enlarge out to 10mm, screwed that too and ended up with a partially stripped 10mm thread that had had almost no depth of thread to start with, was nearly 1mm out of position so the manifold didn't fit over it anyway and about 10 degrees on the piss. I had to mill that true to the vertical and back into the right position, tap the hole out to 12mmm, make a threaded plug out of an old bolt to fit and locktite into it and then finally drill and tap the desired 8mm hole back into that.
I've also seen plenty of DIY attempts at the CVH/Zetec ZVH conversion where you have to tap some new holes in the block for the waterpump and that's rarely a happy story.
If these are critical threads I'd get it done on a machine!
At the very least practice on a scrap component first to see if you can achieve a satisfactory result. Once you've screwed a helicoil operation you're left with very few cheap or easy options to get back to the desired starting size.
Edited by Pumaracing on Wednesday 13th May 04:43
Pumaracing said:
Helicoiling, or in fact any drilling and tapping, by hand is something I tend to consider extremely dodgy if you require a finished thread that is mechanically sound and actually points in approximately the right direction. Fortunately having a Bridgeport mill it's not something I ever have to do by hand. It's a fairly regular op for me to have to reclaim stripped manifold threads in cylinder heads and on the mill it's a simple operation.
I assume you're reclaiming stripped 7/16" existing threads rather than creating helicoiled threads from scratch? By hand I suggest you double drill i.e. run a 7/16" through first to remove the old threads and then run the 29/64" through to enlarge to tapping size. You have a better chance of drills following the alignment of the existing hole if you go up in small increments. You'll also have less tendency to drill oversize. It's also handy to have a mate watching to make sure you have the drill truly vertical because it's very easy to be miles out when you're looking down from on top thinking it's vertical but are really about 20 degrees out.
How you then start the tapping operation correctly aligned by hand is another problem. That's much easier with a taper tap which has a long lead-in than a 2nd tap but the latter is all you'll get in a helicoil kit. Trying to start a 2nd tap accurately aligned really requires a jig or machine tool. Frankly every time I come across a thread someone has already tried to restore in a head or block by hand it's a complete cock up both in terms of position, alignment to the vertical and usually even to having any decent depth of new thread because they drilled oversize as well as on the piss. Most of the really tricky repairs I've had to do have been after someone has had a go first and ended up with an even bigger stripped thread than they started with. I had a Pinto head in last year which someone had tried to restore a stripped 8mm exhaust manifold thread in. They'd screwed that and had to enlarge out to 10mm, screwed that too and ended up with a partially stripped 10mm thread that had had almost no depth of thread to start with, was nearly 1mm out of position so the manifold didn't fit over it anyway and about 10 degrees on the piss. I had to mill that true to the vertical and back into the right position, tap the hole out to 12mmm, make a threaded plug out of an old bolt to fit and locktite into it and then finally drill and tap the desired 8mm hole back into that.
I've also seen plenty of DIY attempts at the CVH/Zetec ZVH conversion where you have to tap some new holes in the block for the waterpump and that's rarely a happy story.
If these are critical threads I'd get it done on a machine!
At the very least practice on a scrap component first to see if you can achieve a satisfactory result. Once you've screwed a helicoil operation you're left with very cheap or easy few options to get back to the desired starting size.
Thanks for all the advice.I assume you're reclaiming stripped 7/16" existing threads rather than creating helicoiled threads from scratch? By hand I suggest you double drill i.e. run a 7/16" through first to remove the old threads and then run the 29/64" through to enlarge to tapping size. You have a better chance of drills following the alignment of the existing hole if you go up in small increments. You'll also have less tendency to drill oversize. It's also handy to have a mate watching to make sure you have the drill truly vertical because it's very easy to be miles out when you're looking down from on top thinking it's vertical but are really about 20 degrees out.
How you then start the tapping operation correctly aligned by hand is another problem. That's much easier with a taper tap which has a long lead-in than a 2nd tap but the latter is all you'll get in a helicoil kit. Trying to start a 2nd tap accurately aligned really requires a jig or machine tool. Frankly every time I come across a thread someone has already tried to restore in a head or block by hand it's a complete cock up both in terms of position, alignment to the vertical and usually even to having any decent depth of new thread because they drilled oversize as well as on the piss. Most of the really tricky repairs I've had to do have been after someone has had a go first and ended up with an even bigger stripped thread than they started with. I had a Pinto head in last year which someone had tried to restore a stripped 8mm exhaust manifold thread in. They'd screwed that and had to enlarge out to 10mm, screwed that too and ended up with a partially stripped 10mm thread that had had almost no depth of thread to start with, was nearly 1mm out of position so the manifold didn't fit over it anyway and about 10 degrees on the piss. I had to mill that true to the vertical and back into the right position, tap the hole out to 12mmm, make a threaded plug out of an old bolt to fit and locktite into it and then finally drill and tap the desired 8mm hole back into that.
I've also seen plenty of DIY attempts at the CVH/Zetec ZVH conversion where you have to tap some new holes in the block for the waterpump and that's rarely a happy story.
If these are critical threads I'd get it done on a machine!
At the very least practice on a scrap component first to see if you can achieve a satisfactory result. Once you've screwed a helicoil operation you're left with very cheap or easy few options to get back to the desired starting size.
To answer your queries, yes, I am reclaiming stripped 7/16 threads rather than creating new threads from scratch. To aid in keeping the drill and tap perpendicular, I'll definitely be asking someone to look along the horizontal plane as I take care of the vertical plane. The depth of the holes I'll be drilling/tapping are only ~1" and the Helicoils will be inserted just below the surface, so at least that allows a slightly greater margin for error. Your suggestion of using a taper tap seems like a good idea, so I'll definitely obtain one.
An alternative to a machine for drilling and tapping true is to make a drill and tap guide. Get a block of scrap metal about an inch thick and using a drill press drill vertical pilot holes through it for the drill sizes and tap o/d you need to use. If a friend holds this lightly in position so it can still move to let the tool find the centre of the hole you're working on it'll ensure you do everything vertical to the job.
At a pinch you might even manage with a block of hardwood for limited use.
Of course it begs the question if you can find a drill press to make the drill guide then why not just do the helicoiling on it instead?
At a pinch you might even manage with a block of hardwood for limited use.
Of course it begs the question if you can find a drill press to make the drill guide then why not just do the helicoiling on it instead?
A few years ago, for reasons unknown other than I probably have far too much spare time on my hands, I did a little exercise to work out the geometry of helicoil taps. It should be apparent that if the result is to work then the i/d of the thread the helicoil tap creates must be slightly larger than the o/d of the bolt it's designed for otherwise the bolt would foul the crests of the threads in the parent material after the insert has been wound into place.
This generates a minimum size for the helicoil tap which for the most common 60 degree thread pitch (metric, UNC and UNF threads) is approximately 20% larger in diameter than the bolt i.e. for an 8mm bolt the helicoil tap o/d is about 8 x 1.2 = 9.6mm (actually 9.67mm).
It also determines the thickness (horizontal thickness not vertical thickness) of the wire needed for the inserts which turns out to be a tad larger than the thread pitch. So for a 6mm x 1mm pitch metric thread the helicoil wire is 1.06mm thick. For an 8mm x 1.25mm thread the wire is 1.26mm thick. For a 10mm x 1.5mm pitch thread the wire is 1.57mm thick.
This in turn determines the size of the drill needed to prepare the hole for tapping which is clearly always going to be a bit larger than the nominal bolt size so it removes all the old threads. It turns out that the drill size is about 1.04 x the bolt size so for an 8mm bolt the drill is 8.3mm to 8.4mm.
So although I don't keep imperial helicoil sets I can predict the size of the various components in it. For a 7/16" thread, which I'll assume is a UNC given it's probably in a block or head, we get the following. The bolt size is 7/16 x 25.4 = 11.11mm so the tap size is going to be about 11.11 x 1.2 = 13.33mm.
The thread pitch is 14 tpi or 1.81mm so the wire thickness is going to be about 1.9mm.
Alternatively if you add the nominal bolt size to the thread pitch and multiply by 1.045 you again get approximately the tap size. 11.11 + 1.81 = 12.92 x 1.045 = 13.5mm. So the tap is going to be somewhere between 13.3 and 13.5mm o/d.
The clearance drill is going to be about 1.04 x 11.11mm = 11.55mm = 0.455" which is almost exactly 29/64".
It's all relatively simple when you've worked it out. Like I say I clearly have far too much spare time on my hands
Obviously this also means that helicoil taps are never a standard size given that they must have the thread pitch of the original bolt but be about 20% larger in diameter.
If the OP has a digital vernier he can maybe tell me how close my estimates are.
This generates a minimum size for the helicoil tap which for the most common 60 degree thread pitch (metric, UNC and UNF threads) is approximately 20% larger in diameter than the bolt i.e. for an 8mm bolt the helicoil tap o/d is about 8 x 1.2 = 9.6mm (actually 9.67mm).
It also determines the thickness (horizontal thickness not vertical thickness) of the wire needed for the inserts which turns out to be a tad larger than the thread pitch. So for a 6mm x 1mm pitch metric thread the helicoil wire is 1.06mm thick. For an 8mm x 1.25mm thread the wire is 1.26mm thick. For a 10mm x 1.5mm pitch thread the wire is 1.57mm thick.
This in turn determines the size of the drill needed to prepare the hole for tapping which is clearly always going to be a bit larger than the nominal bolt size so it removes all the old threads. It turns out that the drill size is about 1.04 x the bolt size so for an 8mm bolt the drill is 8.3mm to 8.4mm.
So although I don't keep imperial helicoil sets I can predict the size of the various components in it. For a 7/16" thread, which I'll assume is a UNC given it's probably in a block or head, we get the following. The bolt size is 7/16 x 25.4 = 11.11mm so the tap size is going to be about 11.11 x 1.2 = 13.33mm.
The thread pitch is 14 tpi or 1.81mm so the wire thickness is going to be about 1.9mm.
Alternatively if you add the nominal bolt size to the thread pitch and multiply by 1.045 you again get approximately the tap size. 11.11 + 1.81 = 12.92 x 1.045 = 13.5mm. So the tap is going to be somewhere between 13.3 and 13.5mm o/d.
The clearance drill is going to be about 1.04 x 11.11mm = 11.55mm = 0.455" which is almost exactly 29/64".
It's all relatively simple when you've worked it out. Like I say I clearly have far too much spare time on my hands
Obviously this also means that helicoil taps are never a standard size given that they must have the thread pitch of the original bolt but be about 20% larger in diameter.
If the OP has a digital vernier he can maybe tell me how close my estimates are.
stevieturbo said:
It maybe isnt to easy to bring the workpiece to the bench drill etc....
Sometimes you have no choice. I've drilled and tapped a Helicoil insert on my VW sump in-situ with a cordless drill. Used a Taper tap to start and finish the whole thread operation of the oversized thread and then flushed any remaining swarf out with the previously drained oil. Helicoil process was 100% after. Don't you just love, overtightened, butchered sump plugs. So much for FSH 
Only other option was to remove the engine
It had only started out as a routine oil/filters service on a newly purchased car Edited by FWDRacer on Wednesday 13th May 10:00
Sometimes you are safer with no service history...it means nothing has been butchered by idiots at main dealers.
Must say....I did drill a RV8 block out once to accept 1/2" studs for the heads. I bought a cheapy drill guide...although to be fair it was never bolted down during use.
I will say it was interesting getting the cylinder heads on lol I did have to make use of a "special" tool to get them on.
WHen I eventually had to strip it again....they did come off quite easily though. I guess the pulled straight lol.
It is obviously very important any drilling/tapping operations are carried out perfectly straight and vertical.
Must say....I did drill a RV8 block out once to accept 1/2" studs for the heads. I bought a cheapy drill guide...although to be fair it was never bolted down during use.
I will say it was interesting getting the cylinder heads on lol I did have to make use of a "special" tool to get them on.
WHen I eventually had to strip it again....they did come off quite easily though. I guess the pulled straight lol.
It is obviously very important any drilling/tapping operations are carried out perfectly straight and vertical.
Gassing Station | Engines & Drivetrain | Top of Page | What's New | My Stuff