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Can a B50 motor swap to a A50 motor

Posted: Wed Dec 23, 2009 3:56 am
by 1962mike
Hi I'm new to this club, I have a nicely restored 71 b50ss which constantly wrecks kick start quadrants.
Its a cow to start , but once running it rides great, but it broke another quadrant tonight , enough's enough.
I know that a triumph 500 twin will go in this frame, however I'd like to stick to the BSA brand an A50 motor is available to me, has anyone done this conversion .
I know this will upset some people, this not my intention, I've had many BSA motorcycles A65T's mainly A10 golden flash,B44 and a B40 I love them .

Re: Can a B50 motor swap to a A50 motor

Posted: Wed Dec 23, 2009 9:41 am
by steve m
hi Mike,
welcome to Rickard's forum.
most of us have more than likely had your problems. it's about the ride - not what happens before it. you say yourself you enjoy the ride, yes?
so, why create a headache for yourself. you have a good motor already fitted in your frame, so what can we do to fix your problems?
if you have been repairing the old quadrant then, try a different tactic. P.E.S sell a quadrant that is to the '73 spec (larger diameter shaft, but fits std. kicker and has the same needle roller for the gearbox shaft), with this item you need to buy the bush as well (it's just thinner so fits shaft and std. timingcase). the std. quadrant was crap (i reckon), so it's not worth fitting a N.O.S quadrant 'cause same thing will happen - who wouldn't get frustrated with that !
also , buy a good spring from P.E.S as well (but ask Mark to fit it to the quadrant first).
the starting problem could be anything. i would suggest, just make sure the idle speed screw (throttle stop screw) is not set too low - allow yourself a generous idle speed (say 800 - 1000). i believe some of the starting issues come from setting the idle speed as low as possible and boy... is that low. and don't touch the twistgrip when kicking. most of the time it will start if it's going to start.
never heard of anyone doing the swap you are thinking of (which doesn't mean anything).
the one thing you need with these bikes is persistance. then enjoy a great ride.
hope i've helped and good luck.


Re: Can a B50 motor swap to a A50 motor

Posted: Wed Dec 23, 2009 10:41 am
by SteveS
Hi Mike

Welcome to the forum.

I don't know anyone who has done the conversion you are considering but don't let that stop you. See the thread here viewtopic.php?f=3&t=2916#p21924 - Mr Mike based in North Carolina, US is considering the A65 engine route.

Whilst I agree with Steve that it may be worth perservering with the B50 lump if your mind is set on experimenting with the A50 engine just go for it. Obviously you will need to check the basics about chain alignment and the physical size of the motor - but anything is possible :thumb

Keep us posted with your progress

Regards Steve

Re: Can a B50 motor swap to a A50 motor

Posted: Wed Dec 23, 2009 11:07 am
by Mark Cook
Sorry to hear about your kickstarting troubles, when I've had this problem reported to me it's normally turned out to be the owner kicking the bike through compression. The kickstart gear is just not up to this. There is a note and many posts on starting procedures on this forum.

Thanks for the plug Steve.

Happy Christmas to you all.


Re: Can a B50 motor swap to a A50 motor

Posted: Wed Dec 23, 2009 4:20 pm
by kommando
I have a picture of an A65 engine in a pre-OIF B25/B44 frame. It fits but only just and looks odd as its tilted backwards to fit.

Re: Can a B50 motor swap to a A50 motor

Posted: Wed Dec 23, 2009 10:26 pm
by minetymenace
Welcome 1962mike (young man!!) <135

Its your bike, you can do what you like with it, you won't upset anyone here!!

The following procedure is recomended:
1. Remove the old engine and crate it up nicely.
2. Send it to me.
3. See if the A50 crank cases can be made to fit...
4. See if I will give you your engine back.

Seriously, the A series are quite different, most of the B series problems have been sorted (see this forum, or ask). It would be an interesting conversion, but not an easy one I let us know how you get on.

Re: Can a B50 motor swap to a A50 motor

Posted: Thu Dec 24, 2009 3:14 am
by Richard Hunt
Welcome 1962mike to the forum

The link that Steve S has put in above reffering to MrMike interested in putting an A65 engine in a B50/B25 wet frame was me being flippant. Definitely NOT serious.

However I am intrigued to see how you progress with your project


Re: Can a B50 motor swap to a A50 motor

Posted: Fri Dec 25, 2009 3:10 pm
by 1962mike
hi, thank you for your comments which I'll be looking into .
It looks to me that it'll be a very very tight fit but I'll look into the kick start first then the engine swap.
happy Xmas.

Re: Can a B50 motor swap to a A50 motor

Posted: Sat Dec 26, 2009 12:36 am
by steve m
i remember 33 years ago, when i was riding my first B50. i had the first 3 teeth worn off the quadrant.
it's not good to persevere with a situation like that but a part work-around is to take the piston further past top dead than you normally would. so at least when you are at the bottom of the stroke on the kicker the piston is roughly in the same spot as if the quadrant was complete - you are only (only !) losing momentum and the piston will still get to the firing point. worked for me ;-)


Re: Can a B50 motor swap to a A50 motor

Posted: Sat Dec 26, 2009 7:17 pm
by beat
hi al,
I think, we are here once more at the point of this ugly kickbacking of the engine wheilst starting.
this process is able to bracke the quadrants gears as well injuring badly your leg as we know.

my way of getting off this problem is to set the ignition timing for the starting process to a other point.
this other point is by 4 ore 5 degrees crank later !
and it is easely to do on the autom. Timing Advancer.
if you look on the ATU from the rear side, at first you se the stamped nombre wich says the degrees of advancing this unit is doing.
for a B50 used on the road, it should be a 15° one fitted if I am right. ( 15° on the unit makes 30° Crank)
as now most of us are setting the timing to 32 ore 34 degrees Crank by fully advanced , it fires wheilst kickstarting by 2° ore 4° bevore TDC !
if we look on my ATU, it shows this
Slots prolonged to 9.6mm in direction of  - fully advanced-
Slots prolonged to 9.6mm in direction of - fully advanced-
so, I have simply set a new lenght of the two slots to get more advancing - ore say more retarding!
I have prolonged the slot at the FULL ADVANCE side where the arrows shown it.
( as my 15° one is a bit worn out, I have done this with a 5° one wich is in good condition to the same mesurements )

so, if you set up the timing as usual to 34°Crank position by fully advanced ATU, with a slot lenght of 9.6mm it has no ( 0° ) advancing wheilst kickstarting, and this ugly " backhammering " is go on...
I think, a eye has to be kept on the spring tension of this ATU, as the springs needing be able to pull the bolts al the way back in to the zero - position as to se on this ATU.
it works for me, - maybe it is a litle help for a other one two..