Grade specification of bolts for brake parts

[Vali]

Hello,

for my GTS project currently I´m assembling together the suspension and brakes.

Is it sufficient enough to use Grade 5 or better use Grade 8 bolts?
In detail I would like to know the bolt grade specifications for:

- Front brake disc to alloy/bronze hub
- Front brake caliper to aluminium caliper mounting brackets
- Rear brake caliper to upright

[RichardHawkins]

Vali.

I assume you still have the original fasteners. If so you should be able to find a letter such as R, S, or T. These letters refer to the strength grade of the fastener as specified in an old British Standard now obsolete. I do not have a copy of the standard nor do I remember it’s number, but I still have the booklet which came with my first torque wrench that I bought in the 1960’s which seems to be an extract from the standard and gives the yield strength of the fasteners as follows.

A. 15 Tons per square inch.

P. 22. Ditto

R. 34. Ditto

S. 38. Ditto

T. 44. Ditto

X. 63. Ditto

I hope this helps, but you do need to be careful, stronger fasteners are not always better as they can be more brittle and adversely affected by corrosion. Perhaps someone with specialist knowledge will comment.

Richard Hawkins

[Vali]

I got the car/parts in pieces so I don´t know which "original fasteners" are supposed to be there.
I know that on the rear it´s supposed to be lockwired.
Yes, I know that with higher grade it becomes more brittle which I believe is more of a concern at 12.9 grade bolts.
I have a sortiment of Grade 5 (8. bolts, but if it has to be Grade 8 (10.9) I would have to buy them.

[HCA]

This was a favourite argument, maybe still is, in the American muscle car forums. There were two arguments - one, that a G8 bolt will snap without warning when it can take no more and two, a G5 will bend first giving you warning it is about to snap.

I always wondered, but never questioned, how a G5 bolt would bend if highly torqued in say, a situation of holding a shock absorber to a wishbone...

However, I do remember that the consensus of opinion was to 'use G8 on anything suspension, or where you value the load and G5 keep for fender bolts'

If you are starting from scratch, and for the little difference in cost, why not just get Grade 8 for everything..? Or, if money is OK and you want the best, then I am sure ARP would be happy to specify..!

[2cams70]

This was a favourite argument, maybe still is, in the American muscle car forums. There were two arguments - one, that a G8 bolt will snap without warning when it can take no more and two, a G5 will bend first giving you warning it is about to snap.

What a load of rubbish - the G5 bolt will have broken long before the G8 bolt broke. Who cares if the G5 bolt bent before it broke once the caliper is bouncing along on the road! By that logic a G5 bolt is what you need on a conrod cap so you can hear the bearing knocking as a warning before the conrod exits the block!

Replace like with like is safest. The bolt will have been selected by engineers based on it's size (as constrained by the part it's used in) and the anticipated loads.

[HCA]

Hey! I was merely stating my experience on the question on other chat rooms.

I also qualified it by not understanding how a bolt can bend if well tightened...

So please do not have a pop at me talking rubbish! My only input was to use G8 for everything, and yes, I guess to a pure engineer, this could be a misguided comment, but not rubbish.

[2cams70]

So please do not have a pop at me talking rubbish!

Please don't draw the straw that it's me having a go at you personally!! It's merely me having a go at the source of where you got that information - i.e "muscle car forums". It's amazing how many old wives tales that there are about on different topics to do with cars especially on the internet. One does need to take a critical mind to it all!

[HCA]

Ah! OK, I see

[USA64]

While we're on this, do old bolts need to be replaced? Do they wear, maybe the threads, or get brittle or fatigue?

[mikealdren]

High tensile bolts need to be replaced if they are tightened to stretch point but that is rare, especially on older cars, apart from some head bolts. Bolts can also fatigue but steel is pretty immune to that in normal use. Most bolts in auto use are reusable.

Really you should be looking at the physical condition of the bolts, are they damaged or rusty especially where they are used in a critical area: drive train, steering, suspension, brakes etc.

[fotsyr]

I allways use AN hardware. Grip length bolts. If its good enough for planes its ok for me.

chris

[2cams70]

One other point on this. It is not always wise to replace a bolt with one of a higher grade than originally specified. The higher grade bolt will require a higher torque setting in order to achieve enough stretch to prevent loosening. This higher torque may damage the part being clamped or put excessive stress on the female threads. Always best to replace like with like.

The original bolts used on English cars were very high quality - typically manufactured by GKN. Apart from maybe using a few ARP fasteners in the engine if it's a non-standard build these GKN fasteners are perfectly fine to use elsewhere and are typically of far high quality than what is available in the aftermarket.

[rgh0]

The design of bolted joints is a complex engineering exercise with many many variables to consider. The structural bolted joints in the brakes and suspension of the Elan all used originally GKN Grade 5 bolts as far as I am aware, which were perfectly adequate for the service. I would stick to Grade 5 unless you know how to assess the suitability of any alternatives around all the relevant variables. Just make sure you are getting genuine Grade 5 bolts ( normally with the 3 radial lines on the head) from a reputable source.

It is unlikely using grade 8 bolts would cause any problem if tightened to the same torque as the original specifications but I have not done a detailed assessment of each bolted join to confirm that, as there is no need to make the change

cheers
Rohan

[Vali]

Thank you all.

Thanks Rohan, I will stay at the Grade 5 bolts.

In the Elise most suspension bolts are also only 8.8 (Grade 5), only at the Rear ToeLink and the steering arms have been at some point a bulletin to switch over to 10.9 bolts. Bolt dimension also only M8 and M10...