Author Topic: Big end shells  (Read 663 times)

Offline Dean

  • A's Best Friend
  • ***
  • Join Date: Dec 2013
  • Posts: 130
  • Karma: 4
Big end shells
« on: 21.09. 2015 21:30 »
I am in the process of changing my small journal crank for a large journal one.
I was surprised to find that the big end shells for large journal rods are narrower, by about 1.5mm than those for small journal rods.

Does anybody know why this should be?
Never tell people your troubles. Half of them are not interested and the other half are glad you're getting what's coming to you.

Online KiwiGF

  • Last had an A10 in 1976, in 2011 it was time for my 2nd one. It was the project from HELL (but I learned a lot....)
  • Wise & Enlightened
  • *
  • Join Date: Feb 2011
  • Posts: 1529
  • Karma: 13
Re: Big end shells
« Reply #1 on: 01.09. 2016 21:49 »
It's a bit late to answer this but here goes anyway (while my tea is brewing  *lol* ).

My guess is that BSA decided as that if the journal on the new crank was larger in diameter, hence had more bearing surface, it could then be narrower, being narrower of course reduces the bearing surface.

I've not tried to work out which journal/bearing type has the greater surface area, I doubt there is much difference.

I don't know where I heard it from but the crank design change was initiated due to small journal cranks breaking at the join between journal and web (or at least flexing too much), as opposed to big end failure, if true BSA would have concentrated on strengthening that area rather than making the big end last longer by having greater surface area.

The small journal crank seems to be generally accepted as fit for purpose provided the engine is not highly tuned eg high compression Pistons, 357 cam etc.

Time for tea  *woo*
New Zealand

1956 A10 Golden Flash (1st finished project)

1949 B31 rigid “400cc hot rod” (2nd finished project, + favourite bike)

GL1800 Goldwing, well, the wife likes it

KTM 950 ADV, cos it’s 100% nuts

1952 Armstrong Siddeley Whitley for rainy days (with wife)

Offline Rocket Racer

  • Wise & Enlightened
  • *
  • Join Date: Jun 2009
  • Posts: 1498
  • Karma: 16
  • A kiwi with a racing A10 rig and too many projects
    • NZ Classic Sidecar Racing
Re: Big end shells
« Reply #2 on: 02.09. 2016 06:13 »
I'd also point out that the issue with the small journal cranks wasnt their bearing area, but that the cranks snapped if the motors were tuned. The LJ cranks resolved crank failures not an inadequate bearing surface.
A good rider periodically checks all nuts and bolts with a spanner to see that they are tight - Instruction Manual for BSA B series, p46, para 2.
New Zealand

Online muskrat

  • Global Moderator
  • Wise & Enlightened
  • **
  • Join Date: Jul 2009
  • Posts: 8505
  • Karma: 112
  • Lithgow NSW Oz
    • Shoalhaven Classic Motorcycle Club Inc
Re: Big end shells
« Reply #3 on: 02.09. 2016 10:12 »
Yep, snapped a few. 14:1 at 8 grand  *bash* *pull hair out* *sad*
Cheers
'51 A7 plunger, '57 A7SS now A10CR,  '83 CB1100F, 88 FXST .
Australia
Muskys Plunger A7