Author Topic: A50/65 Heads  (Read 1143 times)

Offline Nourish

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A50/65 Heads
« on: 28.12. 2013 00:58 »
The head gaskets of the unit and pre unit twins don't look too dissimilar so what would be involved in fitting a unit head onto pre unit barrels? As the cranks will swap do I presume that the bore spacings are the same?

Offline Rocket Racer

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Re: A50/65 Heads
« Reply #1 on: 29.12. 2013 06:55 »
The stud patterns are very similar ... but different.
Same pattern but spaced further apart for the bigger bores of an A65.

The more interesting option would be to fit A65 barrel and head to get the 74mm bores, but this moves the issue down to the barrel flange studs and the cam followers

So neither is simply a bolt on option.
as you'll be aware the cranks are interchangeable with A10 cranks a popular mod for unit models so basically compatible with effort...
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.
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Offline Nourish

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Re: A50/65 Heads
« Reply #2 on: 29.12. 2013 11:14 »
I was thinking that perhaps plugging and welding up the stud holes would be easy enough, Using an A50 head would even give a little bit of room for a squish band. But would the bores and push rods line up.
Are A10 barrels not able to take larger pistons?

Offline wardleybob

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Re: A50/65 Heads
« Reply #3 on: 30.12. 2013 13:30 »
Hy nourish I used to put Datsun cheery pistons in my a10 .whitch worked out at 2 1/2 mm over size .it worked
All right but the liner was very thin .wardley bob

Offline Briz

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Re: A50/65 Heads
« Reply #4 on: 31.12. 2013 01:32 »
Building up an A10 case to take A65 barrels takes a huge amount of weld! tried it once years ago. Never finished it; the case distorted badly after about 2 hours welding!

An A10 barrel is best bored to a max of .060"........080" is possible. Went to 73mm (+.120") on one once, but the cylinders got a bit thin and nearly seized on one occasion due to localised hot spots.

As you say, an A50 head would be best; the 75mm bore A65 head would be too big for the bore.

If you do get an A50 head on there, remember that the pushrod /cam lobe order is different; the outer 2 are exhausts on the A65 but inlets on the A10. An A65 cam is not a straightforward fit.

Offline Nourish

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Re: A50/65 Heads
« Reply #5 on: 31.12. 2013 11:31 »
Thanks for the replies.
I read somewhere on the net - god knows where- that someone back in the days raced an a10 with an a65 top end, they milled off the top of the crankcase and then welded on an Alloy plate and machined it to fit the A65 barrels - easier than building it all up but still a lot of work. So to use an a50 head you'd have to use a unit cam? And the differences on the cam fitment?
Cheers

Offline Briz

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Re: A50/65 Heads
« Reply #6 on: 31.12. 2013 19:21 »
Cant really remember; its over  30 years now!
But I know the vertical distance from cam to barrel flange is different, probably the horizontal distance from the crank centre is too. I think the cam follower angle is different.
Only sensible way to proceed is to measure all parts & compare.
A custom-ground cam is probably easier.