A7 head fitted to A10

[old53]

My profile picture shows my A10 and as mocked by a few colleges BSA stands for bits stuck anywhere, which may best describe this bike. It has A7S frame fitted with what I believe to be a 1953 model A10 engine. Other things include Laverda fiber glass front mudguard and collection of Bsa parts from various models foot pegs reverse and modified to allow for folding Suzuki  kick start  and custom break leaver . Stainless steal swept back pipes. Thick flange cylinders with big journal crank  and modified twin carby alloy head with 32mm concentrics which is where my story is heading.  It is pleasant to ride but suffered a couple of major breakdowns. First the cylinder flange separated from the barrels I was advised from friends that that was not uncommon for BSA twins! I replaced the cylinders only to have a similar problem some months later when the barrels broke around the bottom of the first fin and parted company. On investigation it appears that my A10 has actually had a A7 head modified and fitted to it. I suspect causing my problems. I have purchased another head off ebay 67-1126 and shall now fit this hoping to overcome my problem. I need some info on the 67-1126 head. I wish to know:
1.   what the valve guide tolerances are so I can determine if they need replacing.
2.   what type of carby would suit and jet sizes if I am not running an air cleaner.
3.   are jets for Monoblock and Concentric the same sizes for the same carby throat size  
Thanks old53

[bsa- bill]

Lots of head info here atlanticgreen.com/a10dph.htm
I believe A7 heads were sometimes used for racing - to raise compression - might be a clue to your barrel problems

BTW all A10 frames were numbered A7.............

You could probably get a good price for your twin carb head !!!

All the best -Bill

[MG]

Hello old53 and welcome to the forum!

If the A7 head had been fitted to your engine without either changing the pistons or modifying (enlarging) the combustion chamber, this had very likely been the cause for your broken cylinders. If an A10 engine with a geometric compression ratio of 8.5:1 is fitted with the smaller diameter A7 head without further modification the CR will rise to approx. 10.7:1.

Ref your questions:
1. Hard to give an exact number for valve to guide clearance. This is a matter of feeling and experience, moving/rocking the valve in the guide and judging whether the play is still ok or too much. If you are in doubt, it is best to show it to someone who is more experienced.
2. Currently don't have the brilliant Roy Bacon book at hand, but I'm sure someone else can look the original carb setup up for you. If not, I can do so this weekend.
3. Needle and main jets are fully interchangeable. Some Concenrtics do have a removable pilot jet, others don't, depending on the year they were manufactured. From my experience the main jet can be chosen one or two sizes smaller when using a Concentric (compared to a Monoblock of the same size).

Hope that helped a bit, Markus


edit: Looks like Bill was faster on the CR issue  

[muskrat]

G'day old53,
                 your just down the road from me.
Been there done that with barrels, but running 14:1 on methanol and 8000 rpm. Cured it by through bolting head to cases and a jack bolt from frame to rocker box.
It appears the head has been modified around the comb chamber to match the bore, so will give the same volume as an A10 head. I have the same on my cafe with 10.5:1 pistons, still through bolted.
 Two 32's might be a bit over carbed. 30's would be max for intake velocity. I'm running two 30mm Kehin PWK flat slides. Looking at the book a '72 lightning used 2 X 30mm concentrics with 200 main 3 slide needle position 1, .106 needle jet. A good starting point.
 As to guides, put a valve in and place at max lift position. If you can rock the valve any more than 1mm replace the guides with bronze ones and then re-cut the seats.
Hope this helps. We will have to meet up, say at the Steampacket Pub about half way mark between us.
Cheers

[old53]

Thanks for your replies it all helps. Through bolting an A10 Muskrat was that a complicated procedure, have you a post about how you went about it and the 30mm Kehin PWK are you satisfied with these carbies?  As for meeting half way sounds good it means I’m on the road again regards old53

[muskrat]

G'day old53,
                it's quite a job but did hold the top end on better. Then started snapping cranks (small journal).
Here's the post http://www.audioworld.net/BSA/forum/index.php/topic,1806.msg11925.html#msg11925
The PWK's are very nice but a bugger to set up. I've done over 1000 miles and now am getting very close to having them set right. Changed every jet/slide/needle at least twice.
I like the look of those splayed inlet manifolds. Where did you get'em ?
Cheers

[old53]

The manifolds were purchased from Modak Motor Cycles. Elizabeth St, Melbourne but it was about 15years ago. They were cast as a single manifold. I think to convert single carby triumphs to twin carbies. The web connecting the inlets was removed to get the right spacing for a BSA.  Modac has lots of parts they may still have some? I’ve been looking at Webco manifolds for Triumphs. As I have mentioned before I am replacing this head with a 67-1126  and are undecided whether I should run a single carby or convert it to 2 x28mm  and  I agee with your comment that 2x 32mm is over carbed .

[muskrat]

G'day again mate,
                        2 X 28's would be a good choice especially if running a 356 cam for good low/mid range. If you wanted to go single carb a 30 is nice. I have a manifold, but you still won't be able to have an air filter in a plunger frame.
We'll have to hookup and chew the fat.
Cheers

[old53]

Muskrat just checked out your through bolting link. 
Its a really nice job. Ill be in touch regards manifold
- Cheers

[olev]

I heard a rumour that Arthur in Geelong passed away a few weeks ago.
If thats the case there might be a lot of alloy heads on the market soon.
There is a picture of some of his stuff in the the latest OBA.
cheers

[trevinoz]

If that is true, I'll bet the vultures are circling!
You will need a fat wallet to get that lot.
   Trev.

[old53]

At this stage I am thinking of running a single carby on 67-1126 head and yes the plunger frame prevents running an air cleaner and I came across these specs
389/037 BSA 650cc 1958-59; Road Rocket Twin - less air filter - 1 1/8" 30 3.5 400 0.11 3
I understand 1 1/8" is throat size and 400 main jet can someone help me with the other specs. Would they be the same for  a concentric or best to fit monoblock thanks

[muskrat]

The 30 is pilot jet, 3.5 is slide and 113 is needle jet. Just personal pref I use concentrics as I have numerous jets/slides etc from racing days.
Cheers

[old53]

Thanks  Muskrat can you email me regarding getting your single  carb manifold