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Main Bearings Clearances for Series A Singles and Twins

A_HRD

Well Known and Active Website User
VOC Member
Post-war Vincents are generally accepted as needing C3 fit main bearings so as to allow for the radial clamping action of the crankcase on the bearing outer ring - causing a slight tightening of the bearing clearances. The C3 looser fit takes this into account and minimises the chance of early bearing destruction.

However, what is the general view regarding the need for C3 fit bearings in the Series A?

Don't forget we have the same basic principle on the timing-side of the 'A'; that is the bearings press-in to the alloy of the crankcase (although in my experience there appears to be less interference here than with post-war Vincents).

On the drive side, the 2 x bearings are housed in a fairly thick steel sleeve that is a push-fit in to the crankcase. Again, the bearing fit in to this sleeve is little more than a firm push-fit. Moreover, the drive side roller (MRJA1) has a thrust face on the outer sleeve making it an L shape (in section) which would better resist any tendency to closing-up radially. So I contend that the need for C3 clearance bearings in the Series A, particularly those with original 80 year old crankcases, is not essential. But I would welcome other views.

Peter B
 

Robert Watson

Well Known and Active Website User
VOC Member
All you need to do on the drive side is get a new housing from Conways which was "in stock" and only took 3 months to get shipped and then instead of having a nice interference fit for the bearings had a few thou clearance, no need for a C3 there....... \
Oh, just a minute.
 

clevtrev

Well Known and Active Website User
VOC Member
Doesn`t the ball race slide to and fro in the housing also ? If so, that negates the use of C3.
 

vibrac

Well Known and Active Website User
VOC Member
All you need to do on the drive side is get a new housing from Conways which was "in stock" and only took 3 months to get shipped and then instead of having a nice interference fit for the bearings had a few thou clearance, no need for a C3 there....... \
Oh, just a minute.
Is that the same 'in stock' as the Honda Clutch conversion?
 

peter holmes

Well Known and Active Website User
VOC Member
To be fair the Conways website does state they are currently out of stock due to supply issues, bearing in mind that some of the major components of the clutch conversion kit come from an obsolete Honda motorcycle I can only surmise that they might continue to have supply issues. I think I remember Colin Jenner telling me that he used to source the required components by scouring motorcycle breakers yards, but I will be happy to be corrected on that.
 

MartynG

Well Known and Active Website User
VOC Member
Is that the same 'in stock' as the Honda Clutch conversion?
To be fair the Conways website does state they are currently out of stock due to supply issues, bearing in mind that some of the major components of the clutch conversion kit come from an obsolete Honda motorcycle I can only surmise that they might continue to have supply issues. I think I remember Colin Jenner telling me that he used to source the required components by scouring motorcycle breakers yards, but I will be happy to be corrected on that.
The Conways Clutch Kit for Comet uses a clutch from a:
  • Honda CB360 G/T 1974-1976
  • Honda CJ360 T 1976-1977
EBC Brakes are able to supply suitable clutch parts if needed: www.ebcbrakes.com

The EBC Friction plate set is EBC part # CK1132
The EBC Clutch Spring Kit is part # CSK082.
EBC can also supply a special tool used to allow positive holding of the clutch basket and center assisting in removal or refit of the clutch basket center securing nut.. It is EBC part # CT009

Plain Metal drive plates are Honda part # 22321-357-0 10

Barnett Clutches in the USA also can supply replacement parts suitable for the Honda clutch used in this conversion. www.barnettclutches.com

The plain metal drive plates are Barnett part # 401-35-047002
The friction plates are Barnett part # 301-48-10001
The clutch springs are Barnett part # 501-66-04010
 

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