Author Topic: a7ss head  (Read 1216 times)

Offline compressorman

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a7ss head
« on: 24.06. 2014 19:03 »
Hi all
 i have just aquired a a7ss cylinder head and would like to fit it to my a10 gf
has any body out there done this and  did you have to do any modifications
any info greatly recieved
regards
ian

Offline Briz

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Re: a7ss head
« Reply #1 on: 24.06. 2014 20:02 »
Very common fitment! A7SS heads are the only ally heads that are at all affordable.
Theres more than one operator out there remachining them to A10 spec and selling them for about £300 to the RGS replicator market!
You can bolt them on as-is and it'll work. You'll get a small compression-ratio bump from the smaller chambers but the small ports & valves will negate any performance improvement.
The valve-seats are often worn out so opening the seats up to accept A10 valves is a no-brainer. If the seats are cast reasonably concentrically you can just about get the SR 37mm inlets in there. New seats are needed to get the 1.5" RGS valves in.
Of course, you're still stuck with the 28mm carb size and small ports. These can be opened up, but beware of DIY-ing this as its easy to screw the gas-flow up unless you know your head-porting onions.
Its not really worth altering the chamber size to A10 proportions, just open them up around the valve area to unshroud them a bit. Use the head gasket as a guide.

Offline compressorman

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Re: a7ss head
« Reply #2 on: 25.06. 2014 08:14 »
hi Briz
thanks for your wisdom,however if i can just pick your brains a little further?
a' if i fit a10 valves and springs do i need to replace valve guides and spring seats?
b' my head was supplied with a twin carb manifold,would this be a worthwhile thing to use being that the a7 only had the smaller carb?
c' any ideas on carb choice?
thanks again
regards
ian

Offline muskrat

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Re: a7ss head
« Reply #3 on: 25.06. 2014 10:58 »
G'day Ian. Is it a twin port head (bolt on manifold)?
I have A10 valves in mine 1.41" inlet and 1.38" ex. That's the iron head variety. Same guides, springs & keepers as A7SS.
Combustion chamber chamfered to match A10 bore.
The twin carbs I use are 30mm PWK Kehins.
Cheers
'51 A7 plunger, '57 A7SS now A10CR,  '83 CB1100F, 88 FXST .
Australia
Muskys Plunger A7

Offline compressorman

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Re: a7ss head
« Reply #4 on: 25.06. 2014 17:05 »
Hi
thanks for that,yes it is a twin port with a bolt on manifold as for carbs? did you try any amals? as i would like to keep the classic look of the bike but i dont really know which would work best for fast road use any thoughts on that would be great
regards
ian

Offline Briz

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Re: a7ss head
« Reply #5 on: 25.06. 2014 19:42 »
Amals will work fine if they're in good condition. New ones aren't too expensive..
Muskys right about the springs, guides etc. You only need to swap the valve collars & collets if you go for the super-rocket 37mm valves. Stock GF ones will work fine.
Sounds like a bit of a result; twin-carb ones are hard to find.

Offline compressorman

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Re: a7ss head
« Reply #6 on: 25.06. 2014 20:39 »
Yet again thanks
Do you know of any reference material that i can use for carb model choice jetting etc
Regards
Ian

Offline Rocket Racer

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Re: a7ss head
« Reply #7 on: 26.06. 2014 03:20 »
really depends on what you want to spend and what you intend to use the bike for.
A pair of the 27mm premium 626's would be a nice option as would a pair of 928's
Personally I would not be looking to overcarb it particularly for the road.

I run a pair of 10TT9's but possibly not the most sensible choice, but in my case on methanol bwah hah hah  *respect*

My head was originally an A7ss too!
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

Offline compressorman

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Re: a7ss head
« Reply #8 on: 27.06. 2014 15:54 »
looks cool !!! but i will just be running UKs premium unleaded,a pair of 626 should do the job though  i think  thanks for that
i am now going to price some up
cheers
ian

Offline Rocket Racer

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Re: a7ss head
« Reply #9 on: 28.06. 2014 06:36 »
Good call, as mines a track only bike, I'm not hamstrung by pump gas. I always thought the 626 was a 26mm, but certainly those available for the rocket 3 and T150 are 27mm.
Personally I like fitting chokes to my road bikes.

If you are planning to twin carb it you will need to sort out some manifolds and ideally rubber mounting to isolate vibration and heat away from the carbs.
Nothing wrong with a single carb, mind you. Chris Vincent only ever ran single carbs on his A10 outfits.
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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Online bikerboy

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Re: a7ss head
« Reply #10 on: 29.06. 2014 03:08 »
My old plunger has an A7 head on it with A10 valves and as its a bolt on manifold I made a manifold to fit twin carbs. I used 930 concentrics and it ran and ticks over just fine. I never had the combustion chamber ground out to 70mm and have to say I was worried about the pistons hitting the head so I did fit two copper head gaskets. The bike is in bits now but only for a new paint job and I ran it for over 15000 miles like that with no issues whatsoever. I did only use flat top pistons tho just in case :)

Offline Rocket Racer

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Re: a7ss head
« Reply #11 on: 29.06. 2014 22:07 »
If you are thinking flat top pistons, then there's no reason to rush into any head mods. Even valves!
keeping in mind the SS was the performance head, while everyone loves to have "big" valves a small valve motor with carbs around 27mm will be a honey.
I had a mate drop in over the weekend who owns a lightning and a single carb star twin, but I only ever see him on the star twin as its such an effortless smoothie and no slug.
People often remark on how quick my B33 is, but it is completely stock. I just rarely shut the throttle!
If your pistons are domed however you will need to radius the combustion chamber (or swap out the pistons for flatties)
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