engineering help on how to size BSW bolts please

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GPO_Van
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Joined: Tue Sep 28, 2010 10:58 pm
Location: Wirral NorthWest, CH60
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engineering help on how to size BSW bolts please

Post by GPO_Van »

Hi All
I have purchased on eBay some 1/4" whit bolts for the front seat mounting, I was expecting to get a head size that would fit a 1/4" spanner, on checking they fit a 3/16" Whit. and far to thin to go into the captive nuts ( I have bought longer bolts as they are going through a bracket to take different seats )
If you wish to check the advert it is eBay item number: 192244796002.
Was I incorrect expecting them to fit a ¼” Whit (5/16BS) spanner.
His reply below.
Thanks
Reg

hi Reg
Whitworth as with all bolts are measured across the bolt not the head
there are two head sizes for whitworth bolts
"whitworth" which is before the war and "British Standard Whitworth" after
to save steel in the war they dropped a head size by one bolt size so all the spanners would still fit
this is why some older spanners are now marked incorrect and latter ones have two sizes marked on each end and new spanners are marked as BSW
cheers
stephen
taupe
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Re: engineering help on how to size BSW bolts please

Post by taupe »

Hi

I think you will find that you need 5/16" BSF screws and not 1/4" BSW, where did you get that size from?

Taupe
GPO_Van
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Location: Wirral NorthWest, CH60
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Re: engineering help on how to size BSW bolts please

Post by GPO_Van »

My spanner is marked as 1/4 whit and 5/16 BS on the same end.
philthehill
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Re: engineering help on how to size BSW bolts please

Post by philthehill »

Nearly all body bolts on a Minor have BSF threads and Whitworth heads.
The threads in the captive nuts for the seat are 5/16" BSF and the bolts/screws require a 5/16" BS - 1/4" Whit spanner.
A 5/16" BS - 1/4" Whit bolt head is 17/32" across the flats.
There are thread and spanner charts at the start of the mechanical threads which will give you the appropriate thread spanner size.

IslipMinor
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Re: engineering help on how to size BSW bolts please

Post by IslipMinor »

BSF/BSW spanners always fit 2 sizes of bolt/screw thread. For example a spanner marked 1/4" BSW / 5/16" BSF fits the head size of both bolt sizes. The correct head size for a BSW bolt is one size larger than the same thread diameter BSF bolt. BSW bolts supplied today are a mixture of the correct +1 head size and the BSF head size. It sounds like the 1/4" BSW bolts that you bought are as described 1/4" BSW thread, but with the incorrect 1/4" BSF head size.

When BSW and BSF bolts were in their heyday, pre-WWII and just after, spanners were often only marked with a single bolt size, and this would be the BSW size. It was expected that the person using the spanner would know than when using it for a BSF bolt, to select the spanner end one size smaller than the size shown.

The picture below shows a typical pre-war Austin spanner, as in a friend's 1927 Austin 7 toolkit, with only the BSW thread size marked:
Ebay Spanner.jpg
Ebay Spanner.jpg (179.86 KiB) Viewed 1109 times
This spanner fits a 3/16" BSW / 1/4" BSF bolt head one end, and a 1/4" BSW / 5/16" BSF bolt head the other.

I am not aware, but stand to be corrected, of any BSW threads of 1/4" BSW or larger on the Minor? I would take it that all the body screws and bolts are BSF and select the spanner size accordingly. BMC originally fitted 'BS' bolts and screws to the body, and the 'body end' of suspension fittings, but many of the aftermarket offerings are sometimes now Unified, or even Metric, which the Minor never had (Series MM and 14mm spark plug threads excepted, of course!).

The true classic example of the pedantic BMC thinking on the Minor is the lower front suspension arm - the inboard (body end) threads are BSF, including the eye bolt (9/16" BSF), but the wheel end threads (mechanical) are UNF!

The rear axle is similar - the front spring mounting and rear shackle pins are all BSF, but the rest of the axle is Unified.

The first post under 'Mechanical' - 'Thread Sizes' - lists most of the bolt and screw thread sizes used on the Series II and M1000, together with the appropriate spanner size required - see Page 1, about halfway down, of the post.
Richard


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