I recently picked up yet another 7, but unfortunately it came with busted shear pins in the collapsible steering shaft. The dealer says it will cost a ridiculous amount of money to order more nylon shear pins, and I was wondering if anyone knows of a cheap alternative that would still shear in an accident. I value my ribs, and prefer not to go with the more common method of riviting the shaft. I'm considering stopping by the hardware stores and other various places and testing different materials to find something that would work, but I figured I'd ask you guys first in case I missed something in my searches. Any Ideas??

 

Best one I've heard so far was using nylon machine screws of appropriate diameter, pressed in carefully; should preserve the shear function relatively properly.

Far as I know, you simply CAN'T order shear pins - - there's no part number. They are thermal-fused into the shaft at the factory, and the only 'legit' way to replace them is to order a new entire gearbox... so sayeth the FSM.

 

Nylon machine screws? Where would I look? And regarding the factory shear pins, the guy at the dealer today said he could order some for $112, unless we misunderstood each other??? He said they could be pressed in after the old was drilled out--but I agree, in my experience mazda always wanted you to buy a whole new gearbox.

 

Nylon screws & are commonly found at electronics supply stores, and at hobby shops that cater to the RC Plane community. radio Shack used to have them, but you usually need someplace bigger than that to find them now. They're used where non-conducting fasteners are needed.



See if you can get the dealer to reference a Mazda part number. Maybe this is something that became available as a service part, after the cars' production run ended?

 

The shear pins aren't really removable/reuseable - - there's no real way you can "transfer" good ones from one unit to another without ruining the pins in the process.

 

Ok, thanks Divin, I'll check that out. I'm certainly not going to pay $112 for a couple plastic pins if I can help it. One of the reasons I want to bother repairing this shaft instead of replacing it with one from one of my parts cars, is that my gearbox is very low mileage and has almost no play. I'll call the dealership back later and get a part number for you guys, but honestly you can get a new, play-free, gearbox and shaft for the same price.

 

I wonder if aluminum rivits would give the same sheer strength as the nylon?

 

Aluminum's shear strentgh is much higher than that of nylon (depends on alloy/formula).

6061-T6 aluminum: 28,000 PSI (28ksi)
Nylon: 9,000-11,000 PSI

So, even if you could find an aluminum rivet as small as the nylon shear pins (they're quite small), you'd be increasing the resistance to collapse significantly - - reducing the safety of the system.

If you end up using a larger rivet, you're increasing shear resistance by orders of magnitude. Shear strength would increase as a multiple of the square of the area multiplied by the strength differential.

 

https://www.rx7club.com/1st-generation-specific-1979-1985-18/does-your-collapsible-steering-column-telescope-752361/

 

I'm betting Billy's aluminum rod pins start life as welding rod...? Wonder what alloy. Probably something softer than 6061, which is fairly hard as aluminums go.

I stand by the safety caution: Unless someone has empirical data on required column collapse force with the original pins vs. aluminum pins.

Peened aluminum rivets would have been cheaper than Mazda's original solution, so I have to wonder why they didn't go that solution originally.

Might be as simple as nylon's superior elasticity and fatigue resistance. Might be one of the engineers had a cousin who worked at a nylon plant.

 

Exactly, I'm trying to keep the safety factor, basically get the same shear strength as the factory setup. I'm also trying to figure out how to do this easily and cheaply for the benefit of everyone who reads this thread. I'm well aware of the re-speed setup with the permanent aluminum pins, I've installed a couple of them. Which brings me to another reason I feel the need to find a safer alternative. Anytime you're going faster, you need better safety provisions. The collapsible shaft doesn't just protect you from flying forward into the wheel, but also protects you in a serious frontal impact if the steering gear were to get pushed back towards you. Who knows, maybe it would save a life, if not some serious hospital bills.

And Divin, I like that you quote actual data on the shear strengths, it makes me wonder, how much engineering have you studied? I'm a mechanical engineering student here at UAH and I'm a bit of a perfectionist so I love to see actual numbers instead of "it oughta be close."

 

I know that the aluminum is stronger and wouldn't allow the shaft to collapse easily but what if you only used 1 of them? the shear strength of nylon is low but there are 2 pins and I think they go all the way through making it 4 shear points which would make using aluminum rivits a decent repair with only 1 shear point.

 

I make my living as an engineer, though not a mechanical engineer (I do systems design).

But I'm curious about nearly everything, and as a result have picked up a fair amount of facts as well as being a careful student of scientific and engineering methodology.

In other words, I'm one of those guys with "the knack."

I'm currently a few credits "short and holding" of finishing a physics degree; once it's done, I may give MechEng a look; it's always interested me.

 

Well after a short run-in with the flu bug, I went to the home depot and found some nylon bolts as you described and they worked perfectly. I tested the assembly to see how much force it took to collapse, and it seemed to be almost identical to the strength of the factory shear pins (I've broken enough of them to know). The whole process took about 45 minutes, if anyone wants pictures I would be glad to post. Total cost: $0.83

 

I had to speak up once I saw this. The pins supplied is a very soft alloy, 3003-0 Temper. They are no more permanent than the stock setup. The collapsible nature of the inner shaft is retained with the use of these pins and steering kit. For anyone who has actually had to dis-assemble a working stock setup you know the force needed to shear the stock plastic.

The stock plastic is melted into the joint. Not only do you have the shear strength of plastic that flows into the holes but you also have the friction strength of that entire surface area between the two shafts. Pin replacement only has the shear of the pins to contend with.

The shear strength on nylon (we can only assume what stock is) = 8120 - 11000 psi
The shear Strength on 3003-O = 11000 psi

I feel posting "we need to find a safer alternative" was a bit hasty.


-billy

 

Many thanks for jumping in and providing hard numbers; there is NOTHING better than having actual data!

 

It would seem my 84 GSL-SE has shared pins, although I have no idea where to find them. Where are they located on the column?

What was the final consensus on fixing this:

- factory replacements - part #?
- Home Depot nylon bolts - part #?
- replace entire steering column
- some other replacement

I'm a bit lost at this point, but need to get this resolved so that I can get my car on the road.

Thanks

Dave

 

my question is why would the shear pin cost a ton of money from the dealer? i highly doubt it is completely unreasonable. some parts are and some are not terrible.

 

nice thread resurrection!

theres a few other old threads about this:
https://www.rx7club.com/showthread.p...steering+shaft

copied this from one as well, bu thvent fixed mine yet:
he best suggestion that I've heard came from Dave Barniger at KD Rotary, and it can be done with the column and steering gear in place: turn the steering wheel so that the wheels are straight ahead and position the wheel so that you have the desired clearance between the wheel and the plastic steering column covers. From the floorboard, measure 7.5" up the column, and then drill a 1/2" hole into the bottom of the steel column jacket. Then, through that hole, drill a quarter or eighth inch hole through the collapsible shaft. Install a quarter or eighth inch ALUMINUM pop rivet into the hole and you're done! The idea behind using an aluminum rivet is to retain the safety factor of the collapsible shaft. I should note that I've never done tried this repair, but it sure sounds like a great tip. Dave says he's done it probably twenty times, and it only takes 15 minutes or so.

 

Sealing up the shaft cover somehow would be a good idea before you declare "done"; the steel shaft is not painted or otherwise protected from rust, and the seal at the bottom of the cover would guarantee any trapped moisture would end up attacking the shaft where it goes through the seal bushing at the top of the end cover.

But why drill a NEW hole through the steering column, when there are already two holes (for the original pins) that could be used? With proper measurements, you could target them for your access hole(s).

Using the original holes also guarantees you have the shaft in the right position.

Also, depending on how badly the shaft got collapsed, the green electrical wire that bonds the two sections together electrically and provides ground for the horn may have broken; that may need fixing too.