The BSA A7-A10 Forum

Technical => A7 & A10 Engine => Topic started by: Nourish on 23.01. 2014 16:05

Title: A10 Motor for Rigid Frame
Post by: Nourish 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
Title: Re: A10 Motor for Rigid Frame
Post by: muskrat 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.
Title: Re: A10 Motor for Rigid Frame
Post by: Briz 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.
Title: Re: A10 Motor for Rigid Frame
Post by: Nourish 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!
Title: Re: A10 Motor for Rigid Frame
Post by: chaterlea25 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
Title: Re: A10 Motor for Rigid Frame
Post by: Briz 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.
Title: Re: A10 Motor for Rigid Frame
Post by: Nourish 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?
Title: Re: A10 Motor for Rigid Frame
Post by: Briz 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.
Title: Re: A10 Motor for Rigid Frame
Post by: muskrat 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
Title: Re: A10 Motor for Rigid Frame
Post by: Rocket Racer 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.



Title: Re: A10 Motor for Rigid Frame
Post by: Nourish 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?
Title: Re: A10 Motor for Rigid Frame
Post by: muskrat 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
Title: Re: A10 Motor for Rigid Frame
Post by: Briz 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.
Title: Re: A10 Motor for Rigid Frame
Post by: Nourish 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.
Title: Re: A10 Motor for Rigid Frame
Post by: muskrat 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
Title: Re: A10 Motor for Rigid Frame
Post by: Briz on 28.01. 2014 20:07
Ummmm, 'ang on; Triumph bigends are only .062" smaller than LG A10 ones at 1.625"
So if he's going smaller than .070" to fit Triumph rods, he's going way past the mark!
Title: Re: A10 Motor for Rigid Frame
Post by: Nourish on 28.01. 2014 22:32
No he has the in between sizes too - not just the -0.070"!
I appreciate it would be weaker but at -0.070" it's still much bigger than the small journal crank - and I'm not likely to be holding it on the red line. Isn't someone here building a bike for the salt flats using Triumph T120 rods? - now there much smaller big ends and he'll be sure to be red lining it!
Title: Re: A10 Motor for Rigid Frame
Post by: Briz on 28.01. 2014 22:58
Ummm, yeah; thats me!
Triumph rod size would be fine, as I say, only 1/16" smaller.
But Musky's right, dont make it any smaller than necessary.
We're using triumph size because of the 89mm stroke crank - we're hoping to minimise crankcase clearance problems. The slightly smaller size buys us a bit more room.
No worries about strength as we're using a billet crank and rods.