G12 vs BSA Road Rocket A10

Information relating to the Matchless G12 or AJS Model 31 650cc twin
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saltbox alf
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Re: G12 vs BSA Road Rocket A10

Post by saltbox alf »

"Funny set up" might be that the oil is equally distributed to the big ends on the AMC machine where as the BSA is end fed to the crank like most of the other twins of the day. The BSA also has a plain timing side crank bush and all though generally lasts, when it wears , pressure is lost and a kit form engine is created. You can have a conversion to a proper roller bearing and quill oil feed to the crank on the BSA, much better set up.

The earlier cranks, mehanite steel, alternator type have been known to snap, apparently in 3rd gear at full blat, just as you change to 4th gear, do think there is a bit about that in the tech articles.

having had a super rocket and now a CSR 31, my fav is the CSR, but only just! Very little difference between em. I find though that I trust the CSR, having done the odd long trip!
In the end it's a matter of choice and luck :beer:
Allons-y, amis de Matchless et AJS
Ib-Denmark
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Re: G12 vs BSA Road Rocket A10

Post by Ib-Denmark »

The late twins have a double capacity oil pump.
Is that an improvement?
Or just a sales trick?
Double width pump does not provide higher pressure, just larger flow.
Regards Ib
TrevorCSR
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Re: G12 vs BSA Road Rocket A10

Post by TrevorCSR »

"You can have a conversion to a proper roller bearing and quill oil feed to the crank on the BSA, much better set up."
Most motors would use a ball race on the timing side to control end float.
On the A7, A10, A50 and A65, you cannot have a proper roller bearing fitted; there is not enough room for one. The 'conversion' is to fit a small un-caged needle roller bearing. Just think how fast that small needle roller is rotating at 6,000 rpm, whilst rubbing the outside and inside race. Not to forget counter rotating with the needles on BOTH sides of it. That needle bearing was complimented with a small ball race to control end float, both of which are inadequate for the task. Nor any better than the standard setup. I've rebuilt a BSA A65 engine, one of the earliest Tamworth conversions, and the bearing was knackered
"I'm not 100% across his meaning of the 'funny setup for oiling' and I'm not familiar in detail with the pump etc. for the twins, I think he meant that the oil has to be pumped though crank then to bearing internally?
I'll ask him next time I'm talking to him."
All crankshafts have to have oil pumped through them to get to the big end shells. AMC fed oil to the centre main then through the crank to both big ends. Basically no different from the BSA which fed oil to the outer timing main bearing. AMC supply was equal, though what difference that makes, I'm unsure. When those AMC centre main shells wore, the big ends were starved of lubrication, same as when the BSA bush was worn.
Nearly all British bike parallel twin engines feed oil to the big ends through one end of the crank.
From what I've read, the later AMC ‘high flow' gear pumps pumped too much (volume) of oil. So that possibly was a marketing exercise.
Most British bikes used a gear oil pump, save for possibly the most successful twin of all, the Triumph, which uses a plunger pump.
Unfortunately, over the years, so many butchers have been inside our engines, and that label includes so called professionals. I've had crankshafts ground wrong, and their answer is 'we've done hundreds like that'. Then British bikes get a bad name!
All of that aside, these days we need to make sure we change the engine oil regularly. Fit an external oil filter to halt sludge build up in the crank drill ways. Modern oils are so much better, and our old engines will (SHOULD) not be thrashed as in the day. If you want to go fast get a modern mount.
Groily
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Re: G12 vs BSA Road Rocket A10

Post by Groily »

My pennyworth on preference as a long-time A10 owner and even longer 'our twins' owner is - forgetting about state of tune of engine and precise designation, which is often misleading at this remove anyway . . .
On the STD Beesa box, tough as boots though it be, the gap between 2nd and 3rd is too wide for me (and they whine in third).
On the AMC box on my '61 650 I reckon the spacing is about perfect. Ergo, I prefer the Mod 31 on my sorts of roads.
However, I'd give the later 4 spring Beesa clutch the edge on the three spring AMC - but anything on earth practically is better than the six spring BSA unit which is I think universally-acknowledged to be poor.
Both types of engine will last well if assembled carefully, although of course a floating crankshaft, as found on Beesa p/unit twins from the day they swapped out the ball- for the tougher drive-side roller bearing, ain't a great idea.
Top-end assembly on pre-unit Beesa twins is a pain, many say and I agree, but then so is it on Nortons.
AMCs are the best parallel twins of all in the rocker gear area I reckon. But then you have to live with separate barrels and heads . . . as you do with REs. Easier to assemble, ideal maybe on the original 500s, but less rigid and more leak-prone.
Both bikes are pretty reliable I have found. You takes your choice.
You can luxuriate in the joy of alternator electrics with some AMCs though - which you can't with a Road/Super Rocket or similar unless you have bits of the rare police versions to add on.
If you like rpm (and I do), the AMCs have to be a nudge ahead with their willingness to spin fast.
I wouldn't part with either one of my 650s in a hurry, as I think they both do what they say on the tin, and do it pretty well. My 'Super Rocket' that isn't (due to iron head and 7.25:1 pistons) is good for a steady 65mph, is extremely comfortable at 60 for infinite periods, and will obviously do a fair bit more if pressed. But why press it? My Ajay (8.5:1 etc) is geared high for cruising, is very happy at 70mph and is a lot quicker in second and third than the Beesa. Not quite such good handling as the Beesa - but better forks for all that. I don't know how fast either of them could go, as I hate picking bits of metal out of ditches.
Take the pair of them out for 100 mile run, and they both arrive at the same time near enough. Funny that! And they are both quite comfortable.
Matchymarty
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Re: G12 vs BSA Road Rocket A10

Post by Matchymarty »

Thanks Groily!
Much appreciated words of wisdom.
I seem to enjoy the Matchy's and love them.
Most difficult to find someone that will let you take their pride and joy for a 100 mile run, but yes, I see your point.

That said, I think I'll start looking for a G11 or G12 project. An entire(ish) bike I can restore ground up properly.

I totally get these things don't want to be pushed to 100mph, but, as mentioned, would love a bike that is comfortable to give me 60 all day. That way I can tour on it and take my self back to 20year before I was born to live life as it should be lived. Simple and clean. Life of the 50's.
Groily
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Re: G12 vs BSA Road Rocket A10

Post by Groily »

Dunno about wisdom - can't be wise to do as many miles on these things as some of us do!! But seriously, you've got 60mph all day cruising on tap with either make if you want it, and you can make it silky smooth as well if you pick the sprockets to make your chosen cruising speed vibe-free.
Many here would also say the G11 is probably the smoothest thing on two wheels (from those days) but I can't say anything much as I only have a 500 and a 650. Should be good for 60 all day too though, I'd think. My 500 will do it, but is better at 55 to be honest, and hasn't got the torque for higher gearing (I found when I tried an extra tooth on the 'box) - but that's partly because I am large. I'd like a G11/Mod 30 to be truthful, too!
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