Odd bolt size

I have just bought a cheep ebay engine hoist and the bolts that hold the castors on are M8. The odd thing is the head and nuts are 14mm across the flats.

I am pretty sure they are metric as they have specification 8.8 on the heads.

Googling bolt dimensions says they should be 13mm.

Anyone have any idea why they are 14mm?

Various cars and motorcycles use 14mm heads on bolts and matching nuts. No idea why. I have had to buy a 14mm socket or spanner to complement a set that came without it more than once over the years.

There seem to be various standards of metric, ISO, ANSI, DIN and a Japanese one. And Chinese manufacturers seem to have a pretty freeform approach to such things. They may have their own standards but they exist out of sight on the other side of the language barrier.

Edited By Hopper on 31/03/2019 12:45:57

14mm on M8 is quite common but don't know why, maybe so all the old fogeys with 9/16 spanners like that size

You've never worked on Japanese cars then?

the 8.8 is a strength designation. The reason for having different sized nuts is because a normal spanner set does not include two 13mm spanners. So if you want a spanner at each end to tighten it it is extremely helpful to have different sizes. The same is found occasionally in imperial bolts on cars that have been designed by someone who actually does his own servicing.

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... it lets them get away with using bigger clearance holes

MichaelG.

The sensible reason for the bolt head being a different size to the nut is so that only one set of spanners need be used for maintenance. Nuts usually get damaged, so replaced more often than the bolt, so they are the standard size and the bolt head is sensibly one size larger.

I've got some 14mm M8 nuts so that rules out just the bolts being bigger.

Seem to remember one item I bought had four M8 flange nuts - two were 13mm and two 14mm and they fitted onto studs so only one spanner needed

The reason i noticed it was that i am in the middle of relocating and of course my 14mm spanner is at my other place 160 miles away. At least screw fix have some cheep spanners.

Will be great when the workshop build is finished and I can move it all.

the head size was reduced way back in the 70s maybe 60s to save metal, what you have was probably made on old presses, spanners were 10x14 now I think 10x13 is more common

If you want odd bolts, try working on pre-war French cars! They made up their own sizes and pitches!

Back in the 70s/80s Ford perfected this approach to the extent that you could undo all four bolts on a typical part simultaneously because they all had different heads...

Neil

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Assuming that you were quadridextrous

... one-man-band ?

MichaelG.

Edited By Michael Gilligan on 31/03/2019 23:36:41

repaired a Chinese motor scooter no so long ago and quite a few of the nuts and bolts had 5 corners around the engine and ancilleries.....

standard threads tho.....everything got replaced with Stainess..

had fun getting the seized fixings undone......hahaha.........well funny now........

Hi 13mm for an 8mm bolt is only a standard. I've seen 8mm bolts with 14mm heads a number of times and I've even come across them with a 12mm heads. Other standard sizes also have variables, like 12mm with 18mm heads, 10mm with 16mm heads. It is often due to their usage for a manufacture to which size heads are used and often down to cost if you produce equipment using many thousands of bolts per year.

Regards Nick.

Modern French cars are doing this to stop no dealer mechanics working on them!

Apparently, 14 mm A/F was an old DIN standard, but they are still available

My impression is that, as a rash generalisation, European cars tend to use 8, 10 13, 15, 17 mm A/F hexagons while oriental seem to prefer 8, 10, 12, 14, 17 in many cases.

The idea of of using different A/F sizes for bolt head and nut sizes, obviating the need for two spanners of the same size is a sensible and practical design feature.

The different sizes will not affect the clamp (tensile / compressive ) load applied since this is produced by the torque applied to the fastener. Given the same material and friction conditions, a M8 capscrew will produce the same clamp load as a 13mm A/F headed M8 bolt when the same torque is applied.

A screw thread can exert quite large forces. A thread with a 1 mm pitch can equate to a very long lever.

Driven by a spanner 100 mm long, ten revolutions will lift a load by 10 mm. But the end of the spanner will have travelled (100 x pi x 10  mm so effectively, the load has been lifted 10 mm by a lever which is 3141.6 mm long. A Mechanical Advatage of 314! A 3 metre long crowbar will lift quite a load

Howard

It doesn't work quite like that. Most of the work done turning the spanner goes into friction between the male and female thread and the face of the head against the abutment. This is why torque settings on bolts aren't that reliable, change the friction coefficient and you change the clamping load. Finer threads are more affected as the rotation for a given axial movement is more.

Some open ended spanner and ring spanner sets did have two of each size, except for the smallest and largest sizes, but not true for most combination and socket sets. Although I do have a socket set that does duplicate sizes having 6 and 12 point sockets.