Bonnet hinge bolt size and thread depth

[Durango2k]

Hi,

I just received my bonnet hinge bolt flat steel bars.

Originally, they are 1/4 inches thick, and the bolts are 5/16 inches which is almost 8 mm (7.96mm).

I did not get the bolts nor any of the bars with the car so it‘s all home-made stuff based on catalogue research and asking others.

Now, this means that originally they went for 80% of the bolt size as the thread depth. That‘s ok, in DIN we consider this as about the lower boundary for a given bolt / depth size.

In my case, I wanted to cut a Metric M8 thread, but as the bars are just 6 mm thick, I cannot reach the 80%, let alone 100 % as it would require an 8 mm thick plate. 80% of an M8 bolt would be 0.8x8=6.4 mm minimal plate thickness.

At 6 mm, it just feels too thin. Is it ?

I thought I might either drill and tap them, then fit a bolt and a nut, then weld the nut solid on the bar ?

Or drill and tap for M8 as beforen, then weld a thick washer with a 6 mm hole on the hole and re-tap through the washer ? (Can be done in one go by welding it first then drill‘n tap) ?

An M6 looks too weak for the bonnet. I could go for M7 as I have a set (as well for M9, don‘t you just love the french), but I think a hardened M8.8 is what is needed for the huge item.

Suggestions ?

Carsten

[Series1 Stu]

Hi Carsten

The standard nut thickness, per ISO4032, for M8 is 6.8mm so don't be too worried about using quarter inch thick plate. Yes, you increase the possibility of stripping the thread, rather than snapping the bolt in fault conditions but it's not a likely problem. Especially if you don't use full rated torque figure.

If you use the 5/16" tightening torque, instead of the M8 figure you should be fine. Remember to use a spring washer under the bolt heads.

All of that said, I struggle to countenance using metric fasteners in an E Type but it's not my car.

Regards

[Durango2k]

Hi Stuart,

I am not using a quarter inch plate - all I have is a 6 mm plate. Thus my worries.

Regarding metric bolts, it‘s the only place on my car, and it is waaaay easier for me to get 8 x M8.8 bolts, and I already have a metric M8 thread cutting set.

Imperial would mean I have to order hardened bolts from elsewhere, plus a cutter set for this one singular job, plus a drill. Hence my resistance...

Carsten

[Series1 Stu]

Hi Carsten

I understand. Here, we can buy taps and dies for either metric or Imperial quite easily and relatively inexpensively. It results from us only ever really being half committed to the metric system. A set of UNF/UNC taps and dies is useful to have to clean up threads when reassembling the car anyway. Ask Santa to bring you some.

Even at 6mm thick you should still be fine. If you're concerned about the possible reduction in the assembly clamping forces, is it possible to add an extra fastener to compensate for the lower tightening torque figure?

Regards

[JagWaugh]

Hi Stuart,

I am not using a quarter inch plate - all I have is a 6 mm plate. Thus my worries.

Regarding metric bolts, it‘s the only place on my car, and it is waaaay easier for me to get 8 x M8.8 bolts, and I already have a metric M8 thread cutting set.

Imperial would mean I have to order hardened bolts from elsewhere, plus a cutter set for this one singular job, plus a drill. Hence my resistance...

Carsten

6mm is close enough for this application.

Please don't sub metric for imperial.

The original 5/16 weren't hardened, only grade 5 iirc.

I thought I included some 5/16 NF taps in the care package I sent you?

Really, a set of NF & NC taps and dies isn't all that expensive, and belongs in your toolbox. A set of chasers too, but you can get by without them.

[Durango2k]

Hi Andrew,

I‘ll have a look into the care pack - good idea !

What would use as a pre-drill then ?

Carsten

[JagWaugh]

Hi Andrew,

I‘ll have a look into the care pack - good idea !

What would use as a pre-drill then ?

Carsten

6.9mm (Not an odd size - even Bosch makes them)

see: http://www.fairburyfastener.com/tap_dri ... l_inch.htm

[Durango2k]

Hi Andrew,

Yes you sent a 5/16, but....NC, so I had no bolts for it.

M8 it is now, and it looks and feels good.

I painted the parts with primer, but when a real snowstorm started outside,
I stopped working in the shed.

Next stop- assemble the front underpan, but as the welding gas bottle (a recycled fire extinguisher) is currently empty, I have to get some CO2 first...).

Carsten

[Durango2k]

Surprise. Surprise.

I cut M8, but when assembling, I found I got the holes in the wrong places. By about 5 mm each.

Reason ? I somehow tried to follow the holes in the cages, and felt clever. But I should have followed the holes in the hinges !

Now, I made 4 new bars, cut 8 more holes, because on the old ones, the holes would interfere with the new holes needed, so I had to put them to scrap heaven.

I did not cut M8, but decided this time to order a 5/16 UNF cutter plus a set of stainless bolts. Job done, 15 Euros, will arrive on Wednesday.

I already pre-drilled 6.5 mm, will need a 6.9 drill then....

Carsten

[abowie]

Hi,

Suggestions ?

Carsten

Have you considered buying yourself a UNF bolt and nut assortment from Ebay?

Sometnig like https://www.ebay.com.au/itm/Demon-Tweek ... JIEUwtacjw

[Durango2k]

...yes I did, about 8 years ago when I started. Then, I zinced every nut and bolt that came off the car, and only took what was missing or damaged from the pack, but my supplies are drying.

Furthermore, the bonnet came with cut-opened retainer cages, large metric M14 bolts and about 100 thick washers, enough for a lifetime.

Carsten

P.S. if my wife is allowed to stay away that long from her work, we‘ll go to Australia in Juli 2018, and start in Adelaide (she has relatives there).