Author Topic: 70mm bore on an A7 bottom end  (Read 957 times)

Online spanersc

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70mm bore on an A7 bottom end
« on: 19.05. 2010 21:02 »
Hi to all you technical members. As I'm about to start the engine rebuild on my 1954 shooting star, I was wondering if anybody has tried or any knowledge of running a 70mm bore on an A7 bottom end? Is it possible? if so, what pistons are suitable? std A10 or an alternative?  Does the conbustion chamber need opening out? Does the head and barrel require 'through' bolting to potect the lower flange?  Any advice or findings (positive or negative) greatly received. If someone can point me in the right direction, I think it could turn out to be a worthwhile venture. Regards, Peter
Peter C    Cambridgeshire. UK   1935 Blue Star. 1937 Empire-Star,  1939 M23 Silver-Star. 1950 A7 Rigid.  1952 A7ST.   1953 A10 Super Flash.  1954 A7ST.  1955 A7SS. 1956 A10RR.  1962 RGS.  1962 DBD34

Offline A10Boy

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Re: 70mm bore on an A7 bottom end
« Reply #1 on: 21.05. 2010 11:35 »
As far as I know, and others will correct me if I'm wrong, your 54 A7 bottom end is the same as the C A10 bottom end. That is to say, the pre 58, pre D A10, before it went large journal and thick flange.

So, given that both had identical small journal cranks [same stroke ?] and the cases were identical, you could fit an A10 top end onto your A7 bottom end.

Personally, I would use std a10 pistons and an A10 alloy head, but I know others believe that an A7 head is ok on an A10 - I cant comment on that as I've never tried it.

You dont need through bolting that is only necessary if you are going racing, you could always use thick flange barrels.

Hope this helps


1958 Super Rocket
Harley Super Glide Custom
Yam XJR 1300

Online Brian

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Re: 70mm bore on an A7 bottom end
« Reply #2 on: 21.05. 2010 12:42 »
The A7 and A10 have different strokes, the A7 is 72.6mm and the A10 is 84mm. I think the cases are the same but you would need a crank, rods, barrels and head to turn it into a 650cc. You can modify the A7 head but it would be a shame to butcher a good head unnecessarily.

Why not put it together as a A7 and keep a look out for a A10 motor.

Offline muskrat

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Re: 70mm bore on an A7 bottom end
« Reply #3 on: 21.05. 2010 14:37 »
G'day Peter,
                    I once had similar thoughts. Brian is right about the strokes. So I thought about lopping off 5.7mm from the top of the barrels. On close inspection and measurement it was deemed too risky. The next idea was to punch out the A7 barrels  but this would leave the skirts too thin. So now, like Brian suggests, it's an A10. More cubes, more power, more fun.
'51 A7 plunger, '57 A7SS now A10CR,  '83 CB1100F, 88 FXST .
Muskys Plunger A7