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An expensive mistake - over-reamed taper pin hole for 7A nose gear.

david.perl

david.perl

Well Known Member
I knew my build was going to well. Ive just finished the canopy with no issues and was working on drilling out the nose leg on my 7A for a taper pin.

I've gone too deep with the reamer (broke the first one and in my excitement to get the hole finished with the new reamer i didn't check the depth often enough. I had the first reamer marked with tape to warn me but in my haste didn't bother with the new reamer. Like others, reaming the hole was a pig and than it suddenly started cutting and i got carried away in my excitement at making chips.

Its about 1/8 inch too deep but looks like an expensive 1/8 inch. I got that horrible sickening feeling when you know you've screwed up big time.

Ive got a quote from a manufacturer of taper pins of $900 for one pin (ouch) and am awaiting a local machine shop to quote. The Cadmium plating alone has a minimum charge of $160 (UK prices). Im looking at Stainless Steel as an option.

Im wondering what other options are there? At this rate a new engine mount and nose gear will be cheaper. Can i get a new nose leg and weld the holes in the engine mount and re-ream the mount? Can i just enlarge the hole to take an AN6 bolt?

Any thoughts welcome and hopefully my mistake can help others.

Main thing is no one got hurt.

cheers

David
 
I would think you have multiple options, all but the last of which you have already mentioned, and listed here in order of probable cost: 1. weld the hole on the engine mount, buy a new gear leg, and start over (I don't think you can weld the gear leg without changing the stress characteristics of that steel, but am not entirely certain of that); 2. Enlarge for a close-tolerance AN bolt (would check with Vans to make certain you don't enlarge over a safe margin; 3. buy two new parts and start over; 4. get a larger/longer taper pin; 5. convert to a TW (had to add that one:D).

Best of luck,
Greg

p.s. I've done #2 for my gear legs when the original bolt holes got a bit wallowed out.
 
taper

David, Are you using a standard 1/4 inch per 12 inch taper? If you are , 1/8 in. to deep is less than .003 in. dia. McMaster-Carr has stainless in lengths like 6 or 7 in. long. You could get one (1) size bigger x long and use the section you need. A stainless #7 x 6 in. is $9.76 in the U.S. Big end is .409, small end is .284. Has a breaking strength of 54,000 psi. & rockwell B85 hardness. Hope this helps Ron
 
You'll be fine. Stop, think, and do not spend $900! I don't know what hardware you're using, but taper pins come in a vast array of sizes. Any reasonable machine shop should be able to make one up, or better yet, modify one that is too big. The material here is quite important. Consider that carefully.
 
Thanks Ron, Ive used the number 3 B&S taper reamer as recommended. This has a taper of 0.500"/ft

The ACS supplied pins have tensile strength of 125,000 PSI. does this mater if its less? So your suggesting i get a longer length of tapered SS and cut threads on the end? Sounds like a good idea.

Also McMaster-Carr wont mail to the UK.
 
Don't Know if Stainless is a Good Idea

I'd definitely check with Vans on that one. Arbitrarily swapping out for another material with less than half the tensile (and shear) on a part that helps hold up the front end of the airplane is something I'd want to get an approval on.
Terry, CFI
RV9A N323TP
 
Heres the response from Vans:

You can weld a patch on the engine mount and drill a new hole but that could change the shape of the socket out of round and require you to have it honed again. The other alternative is if the hole was drilled in the center of the nob on top of the gear leg you can go up to an AN6 bolt. The only problem with going to the AN6 is if you have a problem again you will have to weld up or replace the mount and gear leg (gear leg can not be welded).


Im apprehensive about drilling the hole out to AN6 size in case i mess it up.Any suggestions on how to minimise errors in drilling a tapered hole to a parallel one? Ill also get a quote from a smaller local machine shop more setup to smaller runs
 
I want to make sure I understand what you said in the original post.

I read that the hole is too large by an amount that allows the pin to penetrate 1/8" further than desired.

I put taper pins in also but I don't have a picture in my build log and I'm not at the hangar.
Please look at the link below and let us know if that image is close.

Nose strut taper pin install (from matronics)

If it is, then I suspect your unthreaded end of the pin remains functional by a good margin even though it is further in by 1/8". My path might be to simply make a different washer/spacer on the threaded end and call it a day.

Obviously if the unthreaded end of the pin is radially inward of the gearmount outer surface, then you have the problem discussed in the thread. If not, I'd build on.

Can you insert the pin into the gear and mount and post a pic?
 
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Thanks Bill
the wider end of the taper pin is flush with the gear weldment, id be happier if it were sticking out a little
 
Hi Bob

Its an AN386 4-13

I can order a slightly longer AN386 4-15 (will add another 1/4 inch to the length) but the issue is the shorter end is not to protrude any further than 1/16 inch from the weldment. Mine protrudes about 1/8. It could be packed out with a washer but this would not be compliant with the spec of these bolts.

the next size up taper pin is significantly larger - too large i think.
 
If you can get an AN386 4-15, then that is the obvious solution in concert with a properly made spacer on the threaded end.

When I did my gear legs, I searched for days for the -15 and never found any that I could get in less that 100 piece lots.

The -15 would work much better on the main gear legs because they are bigger in diameter than the nosegear. I used -13s and had to be very careful just to keep the big end flush. Did. They work fine but to your point they are not to spec. Thank goodness its an experimental.
 
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david.perl said:
...but the issue is the shorter end is not to protrude any further than 1/16 inch from the weldment...
Click to expand...

Says who?

I can't think of any good reason for the restriction you cite. Sure, it's a bit untidy, but not $900 untidy. What I'd do is make a cupped washer that accommodates the extra length and call it good. Consider asking if Vans would sign off on that for your authorities.

Thanks, Bob K.
 
Bob Kuykendall said:
Says who?

I can't think of any good reason for the restriction you cite. Sure, it's a bit untidy, but not $900 untidy. What I'd do is make a cupped washer that accommodates the extra length and call it good. Consider asking if Vans would sign off on that for your authorities.

Thanks, Bob K.
Click to expand...

+1 - - Untidy, but not necessarily unsafe! As long as the taper grip length is fully engaged through the piece, then it would seem the excess "unused" length is of no consequence structurally. As long as the contoured washer free floats, then full installation stresses would be generated. Good luck!
 
A more radical solution that I used once is to get some SS shim material, about .005, and make a shim that wraps around the taper pin. (No overlap) Structurally it is the same as the original pin. With the shim you might have to ream it slightly more.

Or you could have the original plated. Or get a larger pin and have new ends, including threads, machined on.

I can't think why Vans would not approve the next size up in this application.pin
 
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how much play is too much?

while we have all this in discussion mode, I noticed the play at the end of my nosegear this annual.
I would guesstimate there is 3/16" movement, in a sort of arc, at the big nut on the end of the gear....so back at the firewall, should I be just attempting to tightening the nut on a tapered pin?....or is the gear leg the limiting element, and I have some elongation of the socket hole?...requiring reaming of the whole thing.
 
I found a longer AN386 pin in the UK.

Im going to experiment with machining up a thicker spacer from stainless steel or try skimming off a little of the metal from the threaded end of the taper and extend the thread with a 5/16 - 24 die. Im not sure which is preferable
 
Go for the thicker spacer unless you understand the processing and heat treatment of the original pin. Some heat treat and/or hardening processes can only have a depth of .010" and the best (strongest) threads are rolled.
 
If you remove metal from the threaded end of the pin to get the required clearance you may not have to extend the threads - there is probably enough margin. If the nut does bottom out when tightening it you could just put another washer between the nut and the spacer washer.

I think this is easier and safer than trying to extend the threads using a die.
 
Thanks Bill, yes this makes sense, ive ordered some 18mm stainless rod and will machine up a thicker spacer.
 
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