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MG Midget and Sprite Technical - Trunnion Bolt Material Grade

Hi folks, back again with another obscure one!

Would anyone happen to know what grade the special top trunnion bolts are?

Having stripped the threads on a few in the past, part of me thinks they could be cheese grade, but it seems scary to think that such a critical fastener is not some sort of higher strength steel.

Or is there an easy way to find out? Hardness testing or something? (I am a bit rusty on my materials science...)

Thanks,
Malc.
Malcolm

If you have the old bolt to look at have a look at the top of the head,
Three radial "pips" mean grade 5 and six radials mean grade 8.
Fairly unlikely to be anything else.
Ignore any numerical marks like "8.8". You'll often see them even on SAE fasteners but it's irrelevant because they are applicable only to metric stock. FWIW though 8.8 is an equivalent to grade 5.
Greybeard

I am aware of the markings for normal bolts, but the trunnion is not a normal BS/SAE/ISO or whatever bolt. Does the marking principle still apply?

To elaborate, I am looking at getting some longer bolts made for a triangulation link kit. If I end up unable to modify an off the shelf bolt and have to start from scratch, specifying a material etc., I am wondering what I have to "beat".

It's the slightly lazy approach on my part that doesn't involve getting a calculator out and actually considering loads and safety factors!!

Malc.


Malcolm

Malcolm, I believe that Rob Armstrong triangulated his top trunnion like that. He used to appear regularly on here but then drifted off to the "K series" BBS board and now posts on the MASC Facebook pages. You might track him down there for advice.
GuyW

I've wondered this myself when thinking of doing my own triangulation kit. In the end I bought one from Peter May which uses an arrangement that works reasonably well but I think could be improved on.

You definitely wouldn't want that bolt breaking though!
John Payne

Sorry Malc, I read your post on my phone without my spectacles and didn't read it properly, but I see what you mean now.
Greybeard

Guy, thanks, I chat to Rob quite frequently. I will get his two penneth.

John, PM kit is way to pricey for me :-) Glad I am not the only one that has had deeper pnderings about the trunnion bolt!

Grey, no worries, all good.

Malc.
Malcolm

Malcolm, I kind of shy away from making things when there is a good ready made option available, especially when it’s from a low volume and innovative place like PM. It is expensive but if you were to make one to the same standard then it would take a lot of time and effort. I felt the same with the panhard rod kit.
The trouble comes when you buy what is a nice kit and it doesn’t really fit as it should and you look at it and think with a few tweaks it could be perfect!

One thing I would say is that it is worth the effort. My car used to wander left and right under braking (I mean hard braking on track) and the triangulation kit has stopped it completely. I coupled it to new Caldwell adjustable dampers so there was last years budget blown!
John Payne

The only reason the threads strip is they are either overtightened or the nuts removed when they are covered with rust and crud. The nut do up against a shoulder so there is no point in overtightening and the clamp bolt stops it turning or coming out. If you are making one, a cut thread is not as strong as a rolled thread. I have made loads of studs using EN8 steel, quite strong enough.

Mike
M J Pearson

"Three radial "pips" mean grade 5 and six radials mean grade 8.
Fairly unlikely to be anything else.
Ignore any numerical marks like "8.8". You'll often see them even on SAE fasteners but it's irrelevant because they are applicable only to metric stock. FWIW though 8.8 is an equivalent to grade 5."

What a fantastic piece of knowledge! The 3 radials finally make sense! I knew that I used them to differentiate between UN and metric but never thought to myself, why are they there?
Tarquin

An update on this - I managed to find a place to do a hardness test on a new, reproduction trunnion bolt. It was the kind of old fashioned place with a miserable foreman that uses far too many swear words and makes everything seem like far too much effort... I digress... at least they did it for nothing!!

Hardness testing suggests it is not cheese grade, but not quite what you would expect from a Grade 5 high tensile fastener.

The test showed a Vickers hardness of 230, which translates to an estimated tensile strength of about 740 MPa. (For reference, Grade 5 is normally quoted at 820 MPa minimum, Grade 2 around 510 MPa minimum).

That was my bit of engineering fun for the day :-)

Malc.
Malcolm

I've got a few originals, so I'll pop one in the car and see if I can get it tested on my travels. If not, if I manage to get in to the office - a rare thing, these days - we should have a hardness tester in the showroom.
Dave O'Neill 2

Malc,
I know you're sorted now but I've just watched these vids that might interest you and being a professor you'll appreciate some of the wording used.

If I'm right vids are shot in Canada, perhaps close to US border as lots of US references.

Inform, educate, entertain -

. Home Depot Bolt vs. USA Made - Offshore Suprise! - https://www.youtube.com/watch?v=GPAbUZABcu0

. Busting Nuts, Exotic Screws Titanium, Inconel, Bronze - https://www.youtube.com/watch?v=TmLn5LwB2A4
Nigel Atkins

My uninformed thoughts. BMC and Healeys for reasons of cost control would only design and get made special fittings or components when they really needed to, and very likely not to be of too fancy spec materials (grade and treatment) if through design (e.g. increasing a diameter of bolt/stud/screw) could use less exotic material spec, as well as having as few ‘extra’ manufacturing steps to make such components.

An example of this is spec of crankshafts for A Series engines, including differences in standard ones plus different spec ones that were offered through Special Tuning as well as any Morris Engines experimental parts, works comps dept.

Plus when you look at Mk1 Sprite, or Mk1 Mini, how many components can you recognise that are from other vehicles as well as more widely used. What has been unkindly called ‘Parts Bin Engineering’ for efficient innovation in design and production engineering, not to be confused with the BMC/BLMC/BL marketing led ‘Badge Engineering’.

Cheers
Mike
M Wood

Don't forget that hardness doesn't necessarily mean stronger--The harder a bolt is the more brittle it becomes, specially sideways load
The trunion bolt may well be softer than grade 5 but it would take more abuse without breaking
An example of this was using L9 fasteners through the torque arm on a Franklin QC axle, the bolt going through the top centre link kept snapping and we kept replacing it often as a precaution, but the risk of it breaking was always there
A friend suggested going back to a softer/tougher bolt so went Gr5 and never broke another one

willy
William Revit

A colleague has tested a standard (original) pin for me. He said it was around 10 HRC (Rockwell), which appears to be 180 HV Vickers.

His exact words were, "it's like butter".
Dave O'Neill 2

I don't normally disagree with Willy but I have to in this case. There is a direct relationship between hardness and Ultimate Tensile Strength in steels. At the range of UTS's we are talking about they are all pretty ductile, typically 20% elongation to failure.
I have to say I have never come across the grading scale Greybeard and Malcolm are talking about. In my time as an Engineer imperial fasteners had the following strength grades :-
R, UTS = 45 tons/in2 = 695 N/mm2
S, UTS = 50 772
T, UTS = 55 849
U, UTS = 60 927
V, UTS = 65 1003
W, UTS = 70 1081
X, UTS = 75 1158
In metric fasteners
8.8, UTS = 800 N/mm2, yield = 0.8 x 800 = 640 N/mm2
10.9, UTS = 1000 0.9 x 1000 = 900
12.9, UTS = 1200 0.9 X 1200 = 1080
Anything above 10.9 is often frowned upon in Automotive circles because this requires heat treated alloy steel and there have been instances of brittle fracture at very low temperatures.(depending on alloying elements)
I would have thought 10.9 or equivalent would be fine for this trunnion pin. You shouldn't find any fasteners below 8.8 on a vehicle today.
Paul Hollingworth

Paul
Can't believe you haven't seen bolt head markings, they've been about since Adam was a boy
And believe me the L9 bolts are much more brittle with sideways load than a grade 5
Some reading -

https://www.bolt.com.au/media/how-to-define-what-type-of-bolt-youre-using/

Also earlier someone mentioned 8.8 on imperial bolts and to ignore it---not so
The bolt in question was probably a bolt manufactured with imperial thread by someone that usually makes metric bolts and has used that grade of steel 8.8 is that close to Gr5 that it makes no difference really
8.8 = 800mpa tensile and .8 yield
William Revit

Paul, Been doing a bit of poking about and the bolt grading system was probably US originated so maybe it didn't reach the UK till more recently--don't know, but it's been here for as long as I can remember
But I have found this nuts and bolts writeup that might be of interest--Unfortunately it doesn't show all the available nut grade identifiers
Some grade 5 nuts have a row of nicks around the points of their hex as well as their dimples and some grade 8 have a double row as well -- but not all just to make it hard

https://www.earnestmachine.com/sites/default/files/documents/training.pdf

willy
William Revit

Willy, that ASME (American) grading system didn't reach the Austin Design office in my time (84-2000). I graduated and started work as a engines stress engineer in 79. Imperial fasteners were always U,V,W etc. Take the conrod bolt 11/32 UNF is V grade, I recall.(is the head marked ?) I did stress analysis on the bottom end of A series Metro turbo in the mid eighties. Typically nuts are 1 grade lower than the screw. (larger area in shear)
Paul Hollingworth

Paul
Just happen to have a 78 MGB engine apart
No markings on the main or big end nuts and just a strange little arrow head looking mark on the bigend bolt as per pics

Cheers
willy




William Revit

Willy

Does the arrow head marking relate to assembly/disassembly directions? I know a different design of arrow over here on other items, e.g. historic stonework, can mean government property.

Cheers
Mike
M Wood

Mike

If the broad arrow mark on historic buildings that you are thinking of has a bar across the top - as in the first image, it is probably an Ordnance Survey triangulation point marker. My Geography master was thrilled that there was one on the front of the crumbling pile that was my secondary school!

Other Government Property will have the plain broad arrow mark, as on the immensely useful little shovel that we inherited with this house. It was probably 'liberated' from the ammunition depot next door by a previous occupant.

Sorry for the thread drift but the arrow on the bolt is different from a UK government property mark as these were always 'broad arrows' rather than solid ones.

Colin





C Mee

Colin, the OS mark in your photo, if I remember correctly, isnt a triangulation mark but a bench mark. I remember one being put on our farm cottage when I was a boy. The horizontal line is an exact height above sea level and I seem to remember the OS guy who did it said a contour line ran through the house.

Trev
T Mason

Willy, I've seen those arrow marks before but not for a very long time. I have a vague idea my dad said they should point towards each other but I can't remember why.
Do they have an angled surface under the heads to lock them into their seats in the end of the rods? Maybe to stop them rotating when you torque them up?
I really don't remember and Google has let me down tonight.
Greybeard

It's probably an orientation thing, I'll check
The head does go up against a shoulder in the rod
I don't normally refit them, being a bit of an ARP sucker, anything that's going to get a bit of a stir up gets new ARP bolts, otherwise they just stay put

Ok got some pics
slightly offset head
#1--arrow side has a little machined surface
#2--hardly any machined surface

So, if the arrows go together then that little machined area would sit on the step on the rod between the bolthole and the edge of the step in the cutout
Also checked an ARP bolt and it has the little machined step on both sides identically

Might have to find another engine for a look just to see which way the arrows point originally
I've got one stashed away but too much on today to go dragging that out

Cheers
willy





William Revit

Lightbulb moment--
With the ARP bolts, depending on which set you get I have to take the outer corners off the boltheads to get clearance with the camshaft--
-Maybe-? the original bolts are offset inwards a tiddle for this same reason--maybe--maybe--or not
William Revit

Bit more info use to mark government property (more widespread than I knew): https://en.wikipedia.org/wiki/Broad_arrow

I did not know about Ordnance Survey Benchmark, thanks those above for pointing this out: https://www.ordnancesurvey.co.uk/newsroom/blog/25-years-since-last-benchmark

Good old BBS, i always learn something even on my own thread drifts!

Cheers folks
Mike
M Wood

Searching for other info on the MGA Guru website and came across this fine chart on bolt and nut markings---------------Best chart I've seen for a while---well worth printing off a copy for your info--

https://mgaguru.com/mgtech/universal/ut121.htm
William Revit

Willi, If you look at the conrod bolt you will see that there is a chamfer on the underside of head on one side. This is to clear the radius in the corner of the spotface on the rod. So the arrow is to tell you which way round the bolt goes. They could of course have chamfered both edges but perhaps they were worried about there being insufficient contact area.

Paul Hollingworth

This thread was discussed between 11/02/2021 and 04/03/2021

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