Tech Tip: BSA A65 Cranks - Early vs Late

Tech Tip: BSA A65 Cranks - Early vs Late

By Marc Bergeron (Posted by CBS)

In 1966, BSA 650 model line up (and A50 500 models) where fitted on the drive side with a roller bearing (68-0625) which replaced the earlier deep groove ball bearing (67-1240). The increase radial load capacity of the rollers was selected to keep up with the gradual overall engine performance increases over the years. Replacing the bearing also meant switching from a one-piece bearing to a two-piece part, which also required a thrust washer (part no 68-0685) on the timing side to center the crankshaft and protect the crankcase. This required a handful of other changes that are mostly forgotten today (2019).


The first well-known difference between 62 - 65 ball bearing cranks (68-0175/68-0179) and 66-and-up roller bearing cranks (68-0734) is that the earlier cranks are narrower. But why? Since the crankcase dimensions remained the same throughout the years and the width of each bearing type is the same, this is a bit counter-intuitive. If anything, the later cranks need extra space to fit a thrust washer against the right-side shoulder and need more space to allow this.


What BSA did to make up the space needed was two-fold:

  1. The original timing-side bush (68-0015) had an outer lip to fit into a machined section in the right-side case. This lip was reduced from .140" to .075" on the bush introduce in 1966 (68-0657), and a section of the cases machined by .075" to create a channel for the thrust washer to sit in.  
  2. The left-side case half was machined in two places: 1) the face of the case was machined by approximately .050" to allow the crankshaft to shift to the left, away from the timing side bush, and 2) the oil seal lip, which the outer bearing race rests against, was machined from the inside by approximately .050", allowing the bearing to sit deeper in the primary drive side.

Now, with an extra .050" of space gained on the left and .075" on the right, a 1/16" thrust washer (approximately .062") could be added. This, however, resulted in the crankshaft center line and drive side sprocket and alternator rotor also shifting to the left. BSA addressed these issues this way:

  1. Before 1966, both crankshaft counterweights were the same thickness. In 1966, the left-side counterweight gained .060" to re-center the rods in the cylinder bores.
  2. On the primary side, the shaft was shortened by .065", from 4.732" to 4.666". This was achieved by shortening the drive sprocket shaft section.

 

So, in the end, there isn't so much a ball bearing versus roller bearing crankshaft (either can be used with any bearing without issues). There is, however, a thrust washer versus no thrust washer issue.


For early crankcases, it is not feasible to fit a later (wide) crankshaft regardless of bearing type, and neither is fitting a roller bearing and thrust washer on an early (narrow) crankshaft; not without machining the cases to allow for the extra space. There is no easy way to machine a later crank to fit early cases. Removing material from the left counterweight would make the most sense, but any changes to the width will impact the alignment of the shafts and could impede oil flow or cause problems with primary chain alignment.


In later, post-65 crankcases, however, any crank and any bearing option can be fitted as long as it's properly shimmed and centered.

 

-Marc

 

 NOTE: all measurements below were taken over a period of many months under differing conditions. It is possible that the changing temperatures affected some minute measurements. My goal was simply to illustrate the differences between the early and later motors, and better understand why certain parts would or would not function together.


Description

Pre-66 (narrow) crankshafts

A

Overall width

13.55"

B

Outside counterweight shoulder width

5.830"

C

Inside counterweight shoulder width

4.570"

D

Drive side (left) counterweight width

0.630"

E

Timing side (right) counterweight width

0.630"

F

Timing side shaft (bearing, pinion, and nut)

2.988"

G

Drive side shaft (full length)

4.732"

H

Alternator rotor nut threads

0.911"

I

Alternator shaft section

1.335"

J

Drive sprocket shaft section

1.675"

K

Drive bearing section

0.720"

 
BSA Crank Specs

Early crankcase, left half (drive side), "as cast"

Early Crankcase - BSA A50 / A65

Case, left side, distance to gasket surface: 3.095”, 3.221”, 3.185”

Bearing seal lip: .275"

Later crankcase, left half (drive side), machined

Late Crankcase - BSA A50 / A65

Case, left side, distance to gasket surface: 3.151”, 3.262”, 3.235”

Bearing seal lip: .218" - .2238”

Words from CBS

In regards to the crankshaft thrust washer and later wider cranks..

For me, I can only assume that BSA added a crankshaft thrust washer (1966 - on) which was primarily used to prevent the timing side bushing flange from wearing on the crank face and to also center the crank (as Marc suggests) since the crank would have "end float" based off the roller bearing and open-end bushing 

 

Regardless of my thoughts I will lay them to rest as the "the proof is in the pudding"

Thanks for reading today

Big shout-out again to Marc for submitting his post to share with CBS and our readers

This topic has been discussed prior on forums but without a definite answer or with much data

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Feel free to comment in the section below and join in on the conversation!

Ride safe! - CBS


9 comments

  • Nicholas Hoag

    What is the @3" “distance to gasket surface” by the photos at the bottom of the page?
    What gasket? And from where?

  • Nicholas Hoag

    Sorry, I find this confusing. If the counterweight gained .060" but the shaft lost .065" wouldn’t the late overall crankshaft be narrower?

  • Nicholas Hoag

    In the second paragraph it says the “earlier cranks are narrower”. Shouldn’t that say “wider”?

  • Classic British Spares

    @David – come on man! Triumph’s have there fair share of problems too……..

  • Classic British Spares

    I agree with Sluggo

    It all comes down to “preparation”

    A lot of folks do not have the time or tools to perform the work needed on a BSA A50 / A65

    A needle bearing conversion IMO is not really needed. Line ream the bushing, run it tight, change the oil and call it good

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