Author Topic: A10 Motor for Rigid Frame  (Read 1501 times)

Offline Nourish

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A10 Motor for Rigid Frame
« on: 23.01. 2014 16:05 »
Ok so now I've got to get together some bits to piece together a 650 motor into my Plunger cases for the Rigid frame that I will hopefully be able to construct. I've just won the bidding for a Thick flange barrel and a big bearing crank so what do I choose for the cam, pistons, carb and head?
I suppose I wouldn't need more than a 100mph top end but really just loads of allround grunt.
The cast iron heads are so much cheaper than the alloy ones so just what more would an alloy head give me?
Thanks for your thoughts

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Re: A10 Motor for Rigid Frame
« Reply #1 on: 23.01. 2014 19:23 »
G'day Nourish.
Don't forget the l/j rods. An alloy head on an A10 in a plunger or rigid frame will give problems with fitting the carb due to the downdraft angle of the intake. An alloy head cools much better allowing for increased compression. They are a bit more noisy than an iron head.
'51 A7 plunger, '57 A7SS now A10CR,  '83 CB1100F, 88 FXST .
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Offline Briz

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Re: A10 Motor for Rigid Frame
« Reply #2 on: 23.01. 2014 19:55 »
If you're shooting for torque, the 356 cam will be better than the 357. Megacycle do some good ones, bit a bit expensive.
You can probably get almost as much power from an iron head as an alloy one with a bit of porting.

Offline Nourish

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Re: A10 Motor for Rigid Frame
« Reply #3 on: 23.01. 2014 20:38 »
So what sort of compression should I be looking for bearing in mind don't want a revvy and fussy motor?
I see there are small and big fin heads - I gather the big fin heads are more efficient ?
And what are the differences between the a7 and A10 heads? - I do have a small fin head - would it be just case of larger hard seats and machining out the combustion chamber (how was that done in manufacture?)- although looking at E Bay prices it might not be worth the effort!

Online chaterlea25

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Re: A10 Motor for Rigid Frame
« Reply #4 on: 23.01. 2014 21:06 »
Hi Nourish
If you desire a 3 figure speed ?
In my opinion you will need a 357 cam, 8.25:1 pistons and a head with the SR sized inlet valves ( I would go alloy)
You will need NEW rods ( old ones  cannot be trusted at this age), New oil pump too
and have the lot dynamically balanced
The new rods are different weights to the originals, As you are in USA, MAP rods are probably your best bet

On 23/43 and 19/42 gearing with a 100/90 x19 tyre my SR revs to near enough 4000 revs at 70 mph
thats 5700 revs at 100mph, I think standard safe revs is 6200/6500
My engine/bike is extremely flexible and will tootle along at low revs in top gear if desired  Or
It will rev like a bast*&d and go like stink when desired

HTH
John
1961 Super Rocket
1963 RGS (ongoing)

Offline Briz

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Re: A10 Motor for Rigid Frame
« Reply #5 on: 24.01. 2014 13:05 »
The A7 head has a smaller chamber & smaller ports/valves. Get a big-fin A10 one, they're cheap.
The advise about rods is sound. Another US source, who we used for the salt engine, is R&R http://www.rrconnectingrods.com
Keep your eyes open for a twin-carb alloy head, they seem to be more common in the US than here.

Offline Nourish

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Re: A10 Motor for Rigid Frame
« Reply #6 on: 24.01. 2014 15:56 »
But I'm here in Bristol - UK!
The alloy heads dont seem that cheap compared to the iron ones - well not on E Bay anyhow.
Do I want twin carbs for a big bottom?

Offline Briz

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Re: A10 Motor for Rigid Frame
« Reply #7 on: 24.01. 2014 23:19 »
Oops! Sorry mate! Complete brain fart! *roll*
I suggested the twin carb head to help get around the centre-tube interference problem that Musky mentioned.
Twin carb heads breathe better. I think I remember our Julian saying 18% improvement on the flow bench?. This'll help all thru through the rev range.
Just dont go too big on the carb sizes.

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Re: A10 Motor for Rigid Frame
« Reply #8 on: 25.01. 2014 07:42 »
Shhh, just quietly there's one on flebay now 221360323082
Two 30mm works well for me up top, maybe a pair of 28's for around town.
Cheers
'51 A7 plunger, '57 A7SS now A10CR,  '83 CB1100F, 88 FXST .
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Muskys Plunger A7

Offline Rocket Racer

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Re: A10 Motor for Rigid Frame
« Reply #9 on: 26.01. 2014 05:58 »
when you say you're not looking for more than 100mph, that could imply a std golden flash spec motor which had a top speed around the ton.
The hotter motors could do another 8mph.

For the road there is nothing wrong with a single carb cast iron head with small valves.
a 356 or 357 cam with a mild compression ratio of up to 8/8.25 gives a delightful motor.

A mate of mine runs an iron motor with flat top pistons and a 357 and its lovely. The iron heads suppress noise far better than the alloy ones

My race road rocket a10 runs twin carbs but my super rocket a single carb and 8:1. For the road I'd go single carb every time.



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: A10 Motor for Rigid Frame
« Reply #10 on: 28.01. 2014 08:51 »
All interesting thoughts thank you.
 If I were to use an iron head - does it matter whether it's a large or small fin?
If flat top pistons were to be used wouldn't an A7 head be a good basis, opening up the combustion chamber by machining a squish band, maybe larger valves and a little porting?

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Re: A10 Motor for Rigid Frame
« Reply #11 on: 28.01. 2014 11:35 »
I'd definitely go big fin head & barrels. Iron A10 heads aren't that expensive and would save a lot of work.
But an A7 head matched to the bore with A10 valves will work. That's a '54 twin carb A7 head on my A10 cafe.
Cheers
'51 A7 plunger, '57 A7SS now A10CR,  '83 CB1100F, 88 FXST .
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Muskys Plunger A7

Offline Briz

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Re: A10 Motor for Rigid Frame
« Reply #12 on: 28.01. 2014 13:50 »
You aren't going to get much of a squish band using an A7 head, the bore size difference is only 4mm; making a 2mm wide band. Not enough to make a difference.
And you'll have a lot of work making the ports & valves as big as an A10.

Offline Nourish

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Re: A10 Motor for Rigid Frame
« Reply #13 on: 28.01. 2014 16:25 »
Well that p*ssed on my fireworks!
As a side note I took my crank to a local engineer to discuss a re grind and he says he has shells to go down to mins 0.070" and can go lower to fit Triumph rods.

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Re: A10 Motor for Rigid Frame
« Reply #14 on: 28.01. 2014 18:08 »
The more you take off it the weaker it gets, even with a L/J crank. Just go to the next undersize. The correct radii is most important.
Cheers
'51 A7 plunger, '57 A7SS now A10CR,  '83 CB1100F, 88 FXST .
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Muskys Plunger A7