Author Topic: Plunger frame dimension conundrum  (Read 271 times)

Offline Nourish

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Plunger frame dimension conundrum
« on: 15.10. 2019 10:59 »
I have 3 frame drawings here, a rigid, Super Flash and the sprung frame. The dimension from the centre line of the seat tube to the wheel spindle on the rigid is 18.125", the Flash 18.50" but the Sprung frame is 15.50"! - Is this correct as the Flash/Sprung sub frame looks to be the same item (part No's?)?

Cheers

Offline manxman

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Re: Plunger frame dimension conundrum
« Reply #1 on: 15.10. 2019 11:47 »
If you examine the drawings carefully you will see that the  distance on the superflash frame is measured from the rear gearbox mount while the other two are measured  from the verticle frame tube.  That and the kink in the Superflash tube would account for the difference.

Offline Nourish

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Re: Plunger frame dimension conundrum
« Reply #2 on: 15.10. 2019 11:57 »
I believe you're mistaken there - it's an 1 1/2" from the seat post centre to the gearbox mount.
But cheers anyhow
Martin

Online Swarfcut

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Re: Plunger frame dimension conundrum
« Reply #3 on: 15.10. 2019 12:04 »
From my 1947-1953 Parts Book...


   Frame Front    Standard Frame....67-4056       Frame Rear   Standard Frame  67-4094 

                        Super Flash.....      67-4141                          Super Flash...      67-4142


 So, there is a difference. Need someone with a Super Flash Frame to do a measure.  The gearbox lug is 1 1/2" forward of the saddle tube centreline, but this does not account entirely for the dimension difference.

Swarfy.

Online JulianS

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Re: Plunger frame dimension conundrum
« Reply #4 on: 15.10. 2019 12:55 »
The frame diagram above for the standard spring frame is from the service sheet 711 but the dimensions have been added other than by BSA and whoever did it got the questioned dimension incorrect. Below is a straight lift from the service sheet, you will see that it gives centre frame tube to wheel spindle as 18.5 inches.

Dont know the source of the diagrams above but I would check them against the factory original before relying on them.

Offline Nourish

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Re: Plunger frame dimension conundrum
« Reply #5 on: 15.10. 2019 14:13 »
Thank you Julian - spot on.
Regarding the rear frame part No's - looking at the two drawings it looks like the differences might be the bracketry
Thanks all for your input.

Online DJinCA

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Re: Plunger frame dimension conundrum
« Reply #6 on: 18.10. 2019 16:36 »
I'm late to the party as usual, but I would think one would do well to question the dimensions on both of the sprung frame drawings originally posted, having frame dimensions listed out to 3 and 4 decimal places.   ;)

DJinCA

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Re: Plunger frame dimension conundrum
« Reply #7 on: 18.10. 2019 17:11 »
 That got me looking. All the drawings have a mixture of fractional inches and decimal inches. Maybe I'm wrong but back then, working to 1/64" would be pretty accurate blacksmithying, let alone to .001" Whether the  dimensions could be achieved, or were necessary with a thou (or less) accuracy, on a fabricated steel tubed brazed frame is questionable anyway on such a mass produced component.

 Even Julian's Service Sheet has the same mix of fractions and decimal inches, maybe the result of revisions by different drawing office cultures, or a lax attitude of those in charge at the time. All fractions or all decimals makes more sense, especially in a service and repair environment.

 Confirming dimensions from other published sources would be a good way forward. I would not be inclined to trust entirely the original drawings  that started this post.

Swarfy.

 

Offline manxman

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Re: Plunger frame dimension conundrum
« Reply #8 on: 19.10. 2019 09:22 »
 A different question, why is the verticle tube (seatpost)  different on the superflash frame ?

Online Swarfcut

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Re: Plunger frame dimension conundrum
« Reply #9 on: 19.10. 2019 09:28 »
By all accounts simply to allow the use of a single larger carb, a TT type rather than standard AMAL 276.  Presumably this was cheaper than developing a twin port head at the time. Twin carbs splayed to clear the vertical tube would have allowed the use of a standard frame. Here the only major frame component difference is the seat support lug, the gooseneck bend vertical and the top tube...all  relatively cheap and quick to introduce in comparison to developing a new cylinder head casting. The result  gave the marketing department something with a USP that was very different from the regular staid offering.

 Swarfy.