View Full Version : Long bolts drilled through.

05-26-2008, 09:49 PM
How in the world do they drill such small holes , about 1/16, length ways through a 7 or 8 inch bolt?????

05-26-2008, 10:00 PM
VERY carefully! High rpms and move the drill bit in and out advancing a little bit each stroke. Too much swarf or turnings in the lands will grab the bit and snap it!

Or maybe they are using EDM?


05-26-2008, 10:17 PM
It depends on what they are used for but a "hole popper edm" could do it very fast and accurately. They have a rotating electrode in a spindle assembly that moves up and down to drill the hole, and the hole is flushed with fluid of some sort.

05-26-2008, 10:22 PM
Mark61 and Jonathon,
These bolts are from about 1959, I am not sure EDM was even known of at that time. The only other option I could conceive of was drilling also, but that seems so unrealistic. I do know that they were a NAS bolt modified by the factory, today at $200 per pop I was looking for alternatives :D

05-26-2008, 10:45 PM
There are some odd single-flute drills used for deep holes. Pecking and high pressure coolant were probably used.

The holes are 1/16? What's the shank size?

If the shank is over, say, 3/8", the hole wouldn't be for lightening. It would probably be so the installer could drop a long micrometer probe down there to measure stretch.

05-26-2008, 10:48 PM
Thats why they are $200 each. would probably be more now.

05-26-2008, 10:49 PM

I'd guess that at that age "micro-drilling" would have been used.

Try some of these links as they are a good read:

05-26-2008, 10:50 PM
Carbide gun drills and 1,000psi coolant running 12,000rpm and a feed pressure rated in grams.

05-26-2008, 10:52 PM
Shank is 5/8, the hole is for lube with a grease fitting at each end.
The hole diameter is a guess as I have not removed any of the fittings, I was suspecting it to be the same size as a perpendicular hole in the center to lube the center bearing which is about 1/16.


05-26-2008, 11:02 PM
I really did not consider the $200 (current price) at all out of line, but still they add up to a lot of money, that is why the search for an alternative, which does not look promising, I would guess the NAS close tollerance bolt alone would cost $60 - $70 each

OT, thanks, I will check that out just for the info.

wierdscience, well, I dont have one of those handy :D


Jim Caudill
05-26-2008, 11:29 PM
try these folks:

Ken, what are you working on? Do the bolts really need to be in the 7" to 8" range? I have drilled and tapped grease holes(passages) for use in torque link applications, but not 7 or 8 inches.

05-26-2008, 11:37 PM
If it is a "NAS" job will it not require certification? If not a "NAS" job and you require NAS bolt specs, are you really limited to the existing hole size? Why drilled right through with a grease nipple on each end?

05-26-2008, 11:43 PM
A landing gear swivel bolt? My Archer had a long through-drilled bolt about 5/8" holding the main landing gear scissors assembly togther. Yep, about $200 each.

05-26-2008, 11:44 PM
Shank is 5/8, the hole is for lube with a grease fitting at each end.
The hole diameter is a guess as I have not removed any of the fittings, I was suspecting it to be the same size as a perpendicular hole in the center to lube the center bearing which is about 1/16.


Yuk,I hate those.I'm assuming they have a good reason for using that design.I always prefer the grease zerks in the bushing and not the bolt.Are the bolts timed so the cross hole ends up in the neutral axis I hope?

05-27-2008, 12:10 AM
Yes, to be FAA legal these these bolts do require FAA certs, but it is an old aircraft and some wiggle room is available, likely not these though as they are still available from the factory. The main cost is obviously the result of the modification.

Three bolts are required, they are the main rotor blade clevis bolts that attach through the rotor hub. Each clevis has three bearings, one at each end and a center bearing, these bearings require lube and with the design I see no way of doing that apart from the mentioned bolts.

Yes they are timed at 180 degrees permitting the rotation of the bearings at 600 hours and replacement at 1200.

If I could find an accurate and reliable way to put a hole through these I would do it in a heart beat, unfortunately, at this point is just does not seem possible.

Jim, that is a good link, and I will sure check them out.
Just measured it and overal length undJer head is 7 inches.


05-27-2008, 12:17 AM
Sounds like yours is very similiar.

05-27-2008, 01:22 AM
Hi Ken,

I have made some bolts similar to what you want, but not aircraft, just a grade 8. It is a pivot pin on a v-ripper. The bolts that I made these from are 8 1/2" long x 3/4" diameter drilled through 1/16. To drill these holes I drill from both ends. First I put into accurate chuck and center drill. Once center is picked up use a pin vise to hold drill and peck by hand. Use a light touch holding the pin vice, if it starts to pack up let it spin in your hand and stop lathe. This works surprisingly well for deep small holes, use lots of cutting oil. Use a good aircraft drill bit, they make them 12" long but I have only used a 6". Unless the bolts you need to drill are really hard I think that you have a doable project.


05-27-2008, 03:21 AM
That is very encouraging and has renewed my interest in looking further into this.


05-27-2008, 07:11 AM
I'd have thought that all NAS/FAA "required" items would need to be 100% "traceable" and "logged" from point of origin to point of use - including the qualification and signature of each person involved in the process. How would you meet these requirements is you bought a "traceable" non-drilled bolt and drilled it yourself?

Can a qualified air-craft Engineer certify your work to achieve compliance? And if he can but won't, where does that leave you?

05-27-2008, 10:32 AM
I am reasonably certain that only the factory can certify parts, so neither myself, AI nor a qualified air-craft Engineer could make self modified bolts be FAA approved.

I believe these could be self modified if say the factory was out of business and/or they were no longer available..

If it seems that I don't care about that little FAA approved stamp on these particular bolts, that is because, well, I guess it's because I don't really care.

It could well be that the cost of the bolts, gun drill and magnafluxing just will not make it worth the time it will take.

Jim Caudill
05-27-2008, 10:55 AM
Anything that has to do with either the main rotor or the tail rotor of a helicopter, strikes me as a critical application. I would be concerned that the "cross drilled" hole to connect to the longitudinal passage, might create a "stress riser" or do something else that might compromise the bolt's strength or fatigue resistance. I have made aircraft components and hardware for other antique aircraft (including a Pitcairn autogyro), but this would cause me some concern. If it were necessary to fabricate the bolts, I would want to find someone who knew and understood these issues to guide me. It may be that after drilling the holes, you would need to chamfer the opening and possibly lap the bore or follow some other procedure (maybe a shot peen or heat treatment) in order to preserve the integrity of the bolt.

If you can come up with a NAS #, we could try and locate the bolt or get them made and certified. The bolts I made for the torque link application were for an old Bellanca tailwheel (not quite as critical as a main rotor)

05-29-2008, 11:30 PM
There is not much on a helicopter that is not critical Jim :) but you can rest assured that safety is utmost on my mind, who would I be kidding here.
I have owned this thing for 36 years and there is not a thing on it that I have not personally overhauled. Some of the design, like the tail rotor is just dang near scary, but it has never failed, the main rotor system is very heavy duty for a two place copter. They also have a very reputable safety record with only a few re-occurring AD's, most copters have books of them.

I realize that just because I do not see a need for concern does in no way mean there is none, but I just don't see it here on a .040 hole. (that would account for why these were tough to grease) They are easily removed so a more frequent inspection could be done.

It likely will be that I just bite the bullet any way for sake of the time it would involve to make them.

Have not had a chance today to look up the bolt number but should tomorrow.

Are you thinking that these could be certified apart from the factory ?



Jim Caudill
05-30-2008, 01:35 AM
quote: Are you thinking that these could be certified apart from the factory ?

Sure, if it has an "AN", "MS", or "NAS" number, all you have to do is buy that part number. Once you have the number, you can look at options. Someone may have a few sitting on a shelf, or there are suppliers that specialize in buying up surplus stock. If it has a number, it can be pursued.

The factory probably doesn't make the bolts, they order them from some supplier in an agreed upon quantity. I doubt if the factory can "approve" or certify a bolt to NAS specs. They probably rely on certs from the hardware manufacturer. If it bears a particular approval number that is unique to your helicopter company, they may have some further testing that they have to subject the bolt to prior to putting their part number on it.

I have an AI friend that holds an STC to manufacture turbocharger wastegate actuator filter kits. He buys all approved components, and then must assemble them and subject them to a pressure test prior to kitting them up for shipment. Just part of his PMA approval.

Now, it may be that the only application for this part is your helicopter; and the only folks to ever spec and order these bolts was the helicopter company. If that is the case, you might be SOL. On the other hand, if this was a Hughes 269C or some such, there are probably quantities of parts stashed around the country, waiting to be discovered.

05-30-2008, 07:28 AM

Wouldn't having grease in the center of the bolt, add to its integrety? Grease is still a liquid and has a secondary outlet, but the restriction of the perpendicalar hole would creat some resistance wouldn't it?

Rob :)

05-30-2008, 09:36 AM
Chithook losing parts-


Warning(annoying music in link):D

05-30-2008, 09:56 AM
Does the classic definition apply?
"An assembly of nuts and bolts flying in loose formation".

05-30-2008, 10:28 AM
I see what you were thinking now, while it is a NAS bolt once modified modify it then it as their own part number. Likely I could find some used with service life left but unless it was from the factory I would not be confident that the time life was accurate or the stress they were put under before removal, as recalled the factory will not sell used parts to individuals only service and repair stations.

Never thought about that, seems doubtful though.

05-30-2008, 01:19 PM
(Disclaimer, this is US only)
If you check the FAR's you'll find that an "owner" can manufacture parts for "his own aircraft" as long as they are approved by an A&P/IA. The parts have to conform to the original (how this is determined is left as an exercise to the owner). AC43 references are probably helpful. AOPA has some useful information here as well. It's not necessary for the "owner" to actually make the part, but the "owner" must be involved in the process.

Essentially, one cannot manufacture parts for "resale" without either (a) holding the original type certificate and manufacturing approval or (b) holding a PMA or (c) holding an STC with manufacturing approval.

What happens when a type-certified aircraft containing an owner produced part is resold, that's a nice grey area.

With this part being a fairly critical application, you might want to get an experienced structural engineer involved, and/or a DAR, preferably one that has worked on STC or PMA approvals in the past.

For my Archer, I just bought the bolts from Piper ("paid the Piper", so to speak). Since the Archer is still in production this was a non-issue.