The BSA A7-A10 Forum

Technical (topic titles must be descriptive) => A7 & A10 Engine => Topic started by: Fast 58 BSA on 08.04. 2012 15:33

Title: Fin repair - dual carb head
Post by: Fast 58 BSA on 08.04. 2012 15:33
In my quest for building the ultimate a10 I acquired the much talked about dual carb alloy head some time ago. It is in desparate need of som fin repair, though. I found this guy on EBay offering fin repair (see link). Anyone have any experience with his repair services? Or better yet, can anyone recommend a skilled shop for this type of work? I am based in Norway, but can ship the piece to UK or Europe. Any ideas of typical cost for this repair? On large fin and two smaller pieces broken off.

http://www.ebay.co.uk/itm/BSA-cylinder-head-repaired-/320880179898?pt=UK_Motorcycle_Parts&hash=item4ab5f2faba#ht_802wt_922
Title: Re: Fin repair - dual carb head
Post by: Rocket Racer on 08.04. 2012 22:24
You really should be looking for a good local alloy welder. Talk to your local clubs, doesnt need to be BSAOC, any vintage, racing or car club for a reference.

Personally I would not consider shipping offshore.

I run a twin port head on my racer. Don't know if it makes 1 iota of difference but suspect it may be better over 5000rpm but it certainly looks very bling with a pair of amal 10TT9 carbies wink2
Title: Re: Fin repair - dual carb head
Post by: Fast 58 BSA on 09.04. 2012 06:33
Rocket Racer,
A local welder would absolutely be the best. I'll do some checking with the local clubs.  Yes, there is definitely a bling factor with the dual carb head (I got the 67-1106), performancewise I am not sure about any serious gains. I will be running monoblocs, 357 cam, lightened valvetrain, and 9,5:1 pistons. Not a racer, but hopefully a modest powergain over the meager 34 hp in the standard version *smile*
Title: Re: Fin repair - dual carb head
Post by: Rocket Racer on 09.04. 2012 08:33
You're going to need some decent fuel to avoid knocking on 9.5:1, might even pay to run a race gas blend. - manual timing advance would be useful.
A mate runs a 357 in his single carb iron motor on the road with flattops (7:1) and its very smooth. My race bikes on 8:1 and very tractable, runs through to 7500 effortlessly.
What size monoblocks? would pay not to oversize as you end up losing air/gas velocity. Nothing wrong with small ports.
But very hard to put metal back once someones opened the ports up. 

Might even pay to ring round some local engine reconditioners to see if they can recommend an alloy welder who might work with an old engine.
 
Title: Re: Fin repair - dual carb head
Post by: Fast 58 BSA on 09.04. 2012 08:44
You don't think 9,5:1 pistons would be ok on premium gas (98 octane)? The forged omegas are they 9:1? Pistons aren't bought yet, so any tips are very welcome *smile*. I have a matched pair 389/689, 1 1/8". That's just over 28,5mm bore size. Pretty big, but I think they should work ok, given the right jetting and some careful port matching....
Title: Re: Fin repair - dual carb head
Post by: Rocket Racer on 09.04. 2012 08:59
9:1 was as high as BSA fitted to the final RGS models and late super rockets. The earlier 40hp road rockets got by with 8:1 and a single 27mm carb.
High compression tends to make engines fussier. You may be fine with 9.5:1, but wouldnt be my first choice.
A pair of 28.5mm carbs will be ample for road use.
Having previously raced a norton; Paul Dunstalls norton tuning guide quotes the maximum carb size for a 650ss racer as 30mm (twin carbs).
I'm not familiar with the Omega's, but would certainly be looking to keep my pistons light, as adding reciprocating weight doesn't make an engine smooth. If you ever get your hands on some 2nd hand genuine BSA 8:1 pistons you will notice how light they are. May be worth considering radiusing the pistons on the sides to lighten
Did you mention what o/s you are looking for?
Title: Re: Fin repair - dual carb head
Post by: Fast 58 BSA on 09.04. 2012 09:07
Yeah, weight is an issue. I do not know how much the omegas weigh compared to oems. Cylinder is bored .40. The bores look fine, no scoring. Just a little light honing would probably suffice.

 Btw, do you have any thoughts on manifold length? There are manifolds available, but I am considering having some made up for a better fit. Longer manifold increases gas velocity, no? How much low end power does one sacrifice with longer manifold? Is it well worth to have a balance pipe between them?
Title: Re: Fin repair - dual carb head
Post by: Rocket Racer on 09.04. 2012 09:36
My initial thought would be to get some wiseco's from Cake Street Classics.
Personally I wont touch JP pistons with a barge pole. Don't have an opinion on GPM, Mondial or Omega.
I always keep my eyes open for old stock at swap meets. Quite often find bare genuine BSA pistons- either 8:1 (top looks a bit like a shallow volcano). or 9:1 (which have round top).

http://www.a7a10.net/forum/index.php?topic=2313.0
Title: Re: Fin repair - dual carb head
Post by: Fast 58 BSA on 09.04. 2012 10:37
Thanks for the valuable info. If I can figure how to attach some images, I'll post some of the head
Title: Re: Fin repair - dual carb head (now with pics)!
Post by: Fast 58 BSA on 22.04. 2012 12:23
Hi all,
Here are finally some pics of the patient; the 67-1106. Aquired it on ebay a few years back. In need of some serious TLC. Will be rebuilt with 1.5" intake valves, race springs, ported w/dual 1 1/8 matched 389/689 monoblocs. BTW, any good suggestions for removing seized sparkplugs?
Title: Re: Fin repair - dual carb head
Post by: Rocket Racer on 22.04. 2012 22:27
Just a word of caution, those early heads (and that looks like the 500 head) weren't designed for 1.5" valves.
I big valved my own race twin port head (a previous owner had ported it to 30mm *sad2*) and although with a plasticene build had clearance, in practice I had valve clipping issues and had to have the exhaust seats reduced so I could fit smaller exhaust valves. (I bent several valves)
I don't have the measurements to hand but recall my big inlets were more like 1.454 rather than 1.5"
I would suggest you run it with smaller valves and keep the ports on the small side if a road motor.
Opening up ports reduces flow at lower throttle settings. Big ports are for race motors where the throttle is wide open all the time...
Sort out the fins,
make sure your valves, collets, guides and springs fit.
But once you start opening up ports and valve sizes, its very hard to put the metal back and you may find it goes very well without the big valves.
Title: Re: Fin repair - dual carb head
Post by: Fast 58 BSA on 22.04. 2012 22:49
Could be that I am wrong *eek*, but looking at the head and measuring the chamber diameter (70mm) it should be for the 650? Also referenced this site http://atlanticgreen.com/a10dph.htm where my head has the cast in 67-1106, not stamped 6 over number 2 as per converted A7 heads. Then again, I am wrong more often than I am right *dunno2*, so some extra checking is probably in order. Flow is another issue, though and maybe SR intake valve is good enough? I am not planning on any massive porting, primarily blending the port openings to match custom manifolds and lightly massaging the inlet tracts. Decisions, decisions....I'd like to hear more pros and cons regarding the big valve conversion. Perhaps start another thread for that....

First thing first, fins must be repaired *smile*
Title: Re: Fin repair - dual carb head
Post by: trevinoz on 23.04. 2012 00:12
Your head is a 650 type.
It is most likely a late production head for the "speed kit" offered in the U.S. as there doesn't appear to be a date code cast near the number.
Probably the way to get the seized plugs out is a lot of heat on the plug and patience. If you destry the thread, it can be repaired. Don't forget to lubricate as you go.

  Trev.
Title: Re: Fin repair - dual carb head
Post by: Rocket Racer on 23.04. 2012 04:29
apologies, didnt pay enough attention, nor check the casting numbers. yes 650...

Nothing wrong with small valves, or small ports, or small carbs.
Title: Re: Fin repair - dual carb head
Post by: bonny on 25.04. 2012 22:50
apologies, didnt pay enough attention, nor check the casting numbers. yes 650...

Nothing wrong with small valves, or small ports, or small carbs.

Didn't the british industry go a bit overboard with valve and port sizes ? i know triumph 650's for instance had a much larger port and valve size in their late 60's machines compared to the models from the late 50's, but weren't nearly as fast, i have been told this is down to overporting/valving.   
Title: Re: Fin repair - dual carb head
Post by: Rocket Racer on 26.04. 2012 00:27
If we're talking about holding a motor flat out on the race track, then a big carb, the right cam & gas flow/turbulence into a big inlet valve will give slightly higher top speed.
But on the street we're probably losing bottom end and mid range to an engine with stock carb, and valves during roll on riding.

The big valve motors were developed for the competition mad american market *respect*
Mods do provide some bragging rights *beer* in the bar or carpark for road riders however.
For a quick road motor I would leave carb & valves stock, go for compression of around 8-8.5:1 and fit a 356 or 357 cam and some nice rods that could live with keeping the throttle open.  Buts thats just my humble opinion.
Title: Re: Fin repair - dual carb head
Post by: Fast 58 BSA on 26.04. 2012 06:11
Perhaps it is a case of bigger isn't always better....As seats aren't cut yet and no mods have been made to the head I can still opt for stock 1.41 or slightly larger 1.455 valves as opposed to 1.5. Based on feedback from this excellent forum it sounds as though I might be happier with smaller intake valves. There are hardly any roads in Norway where you can go flat out in 4th anyway....but I would lose some bragging rights going stock *smile* *smile*
Title: Re: Fin repair - dual carb head
Post by: Stephen Foster on 26.04. 2012 06:59
Better get the welding repair done first before working on the valves , may be a good idea to check mating surfaces remain flat ?

Best Wishes ,
Steve ..