The BSA A7-A10 Forum

Technical (topic titles must be descriptive) => A7 & A10 Engine => Topic started by: andy2565 on 15.07. 2011 20:56

Title: rephasing
Post by: andy2565 on 15.07. 2011 20:56
take a look at this,i've just been reading it with interest,but dont know if it would make any difference,
unless you want to sound like a harley,i can unerstand the logic and the debate about vibration,has anyone tried it with an a10 ?
Title: Re: rephasing
Post by: MG on 15.07. 2011 21:44
Haven't tried it nor do I know anyone who has, but they are done for A10s a well: (
Title: Re: rephasing
Post by: andy2565 on 15.07. 2011 21:54
you got to have money to burn at that price.
Title: Re: rephasing
Post by: muskrat on 16.07. 2011 06:47
The old saying "how fast do you want to spend" comes to mind.
Mark was a contributor here on the forum a while back, haven't heard from him in a while. I think his A65 has an offset A10 crank in it. He lives a couple of hours south of me and his bike is well known.
The great Phil Irving recommended 76 degrees as the optimum offset for a twin. Have a look here
Title: Re: rephasing
Post by: andy2565 on 16.07. 2011 09:36
hi muskat hope the bones are healing ok,if i were an engineer i would entertain doing the work,guess i should go back to night school,i first read about it on the xs650 website.
i'll keep my eye out for the parts.
Title: Re: rephasing
Post by: trickytree on 16.07. 2011 09:58
Not done it yet but will do with the plunger A10 cases. Ive got a 82mm Norton crank to use as its bolt together so just have to make a new flywheel. Using a A50 head with custom ally barrels. SRM have A65 ofset cams in stock (Spitfire grind).

Just got to decide wether to start this project or the J.A.P. engine *conf*
Title: Re: rephasing
Post by: andy2565 on 16.07. 2011 10:02
what japanese engine you got ?
Title: Re: rephasing
Post by: MG on 16.07. 2011 12:09
J.A.P. = John Alfred Prestwich of Tottenham (I assume)  ;)
Title: Re: rephasing
Post by: andy2565 on 16.07. 2011 12:54
sorry MG,i didnt mean japanese,i just wondered which J A P ENGINE AS IN S/VALVE OR OHV,as i own and race japanese,and did you know j.a.prestwich made more money out of making pencils than engines,cheers andy
Title: Re: rephasing
Post by: MG on 16.07. 2011 13:20
man, don't baffle me like that   *lol*

I had heard they made projectors and cameras, but I wasn't aware they also fabricated pencils. Interesting.
Title: Re: rephasing
Post by: Brian on 18.07. 2011 02:14
Some very interesting reading in that lot, especially the link you put up Musky. Interesting to note that Phil Irving recommended 76 degree as the optimum angle yet the motor he designed and of course was famous for started with a 47 degree angle and ended with a 50 degree. Maybe he wanted 76 but manufacturing or other factors meant they ended up with the 47.

I reckon there are several was of looking at all this. There is no doubt a parallel twin vibrates, they always have and without a counter rotating shaft, they always will. The only thing that can be changed is just where (as in what rpm) they vibrate at. I can see the advantages in these crank modifications but the big question is why ? You can go and buy a 270 degree crank (almost) vertical twin in the form of a TRX Yamaha, or there are lots of 90 degree twins on the market, Guzzi's, Ducati's etc etc. So why butcher up a heap of BSA parts to make something you can get easily. Maybe there doesnt have to be a reason ?

I'm a big fan of V twins, I have a 1000s Guzzi that I bought new in 94' and have done 86,000 k's on so far. Its a terrific bike but has one minor problem, the faster you go the smoother it gets. At around 5,000 rpm which is 100mph it is totally smooth, not the slightest hint of a vibe but its difficult to explain this to a Policeman ! Maybe its because I have a V twin I dont have the desire to build something out of a BSA. I think the comment " you would have to have money to burn" is very fitting as all this would involve a lot of machining, you would have to be very skilled and have access to a lot of machinery to do something like this.

So I guess what I am saying is leave the BSA alone and enjoy it for what it is, if you want a V twin, go and buy one. Just my opinion, mind you the "other half" says I'm not allowed to have one of those.............
Title: Re: rephasing
Post by: bonny on 18.07. 2011 03:21
I am currently doing an offset crank for a triumph pre unit engine using a norton commando crankshaft. Its has an 89mm stroke (triumph standard stroke is 82mm) which combined with morgo 750cc barrels gives 820cc, the problem i am told is the norton central flywheel to which the crankhalves bolt is cast iron and above a certain amount of revs explodes.
I machined a new one out of en24t steel , which is very impact resistant and tough. The drive side big end journal is 90 degrees behind the timing side one. The camshafts i was lucky enough to get from dave newman camshafts , they are spitfire profile and nitrided for 80 hours. Srm does them for the bsa twins i am told.
The ignition is the standard boyer triumph/bsa twin electronic ignition slightly modified, i was told how to do this by ed collins who runs the offsetcrank website and he was very helpful.
I am currently waiting for map cycle in florida to machine up a batch of the "stroker" conrods, the problem
is the triumph rods cannot be used because the big end journals on the triumph rods are too small and the
rods too long. The norton rods cannot be used because they are too short and the small ends are smaller
then the triumph gudgeon pins.

This sounds like a whole heap of work but it has so far been easy and things have fallen into place so i have continued with this project. The point of this project, apart from the increase in capacity, is that the vibration levels are drastically reduced, somewhere in the region of 45%-50% , this is because the pistons are not stopped at tdc and bdc together , when either piston is at tdc or bdc the other is at maximum speed and contributes a flywheel affect. The crank can also be lighter and this all gives the main bearings and
crankcases an easier life.

I have been told that this design was recommended to all the three big brit bike makers in the 50's by irving ,triumph especially and none of them could see an advantage in employing it, they couldn't sell their standard 360degree cranked bikes fast enough already and one set back with this design is it really needs twin carbs or one cylinder tends to run lean and the other rich with balance difficult to achieve.
If you enter bsa 883 into youtube there is a aussie chap on there with several beezas converted and they
sound fantastic.
Over on the man who knows these conversions inside out is 76-degree-triumph, he races several of these converted bikes ,he is based in australia and is really decent bloke.
Title: Re: rephasing
Post by: andy2565 on 18.07. 2011 10:52
yes i read also that they discounted the idea due to them already being set up,and not wanting to change.
someone has given me an old xs650,and this was what made me take notice of the subject,this conversion is readily available and seems quite alot cheeper for the yam model,and it certainly has the V-twin sound about it,if you watch it on youtube.
Title: Re: rephasing
Post by: bonny on 18.07. 2011 13:24
The costs for me so far are pretty much the same as rebuilding a 360 degree engine, i would have had to buy the conrods, cams and electronic ignition anyway, the steel for the crank cost me around ?50,i am lucky enough to have access to machine tools though , but even still this is not a massively expensive proposition.
I know with the xs the crank is pressed up and is easier to do, i have seen someone in germany supplying the relevant parts to do this as a kit and it wasn't expensive at all. Go for it.
Title: Re: rephasing
Post by: bonny on 18.07. 2011 19:36
This chap here did a norton crank into a bsa a10 engine , the 90 deg conversion wouldn't be much more difficult.
Title: Re: rephasing
Post by: trickytree on 24.07. 2011 10:20

........ one set back with this design is it really needs twin carbs or one cylinder tends to run lean and the other rich with balance difficult to achieve..............

Ive been pondering this for quite a while as I would much prefer to fit a single carb. The chap at Ofsetcrank reckons single carbs are OK and if you think about it just about every American V-Twin uses a single carb. Perhaps the trick is a plenum chamber...something a (Harley for example) sort of has with the shape and lenght of the inlet manifold?
Title: Re: rephasing
Post by: bonny on 24.07. 2011 21:07
It can probabily be made to work out ok with a single carb, but most people building these engines are doing so for cafe racers or scramblers so twin carbs are the business anyway , if you look at the videos on youtube of xs's with offset cranks running , the lack if vibes is amazing, they don't hop about on the centre stand ticking over.