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Commando - best capacity

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I'm building a Commando engine from scratch. In terms of rider experience, my 750 Commando is the best of the 30-40 bikes I've ridden over the past few decades.

But what of the alternative capacities? What are experiences of the short stroke 750, 828, Kenny Dreer's 880, 920 and 1000? 

Which should I choose?

Advice and experience please.

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Are you going to need an electric starter in the future?, and a lower compression to be able to kickstart?. These are the sort of questions I would ask myself. 

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Consider also the gearbox originally designed two decades before the first 750 Commando came out - won't keep the same reliability with too much more power piled on.

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hello well to my experience  the  best capacity is the 650cc   as after this  the motor is to stressed out,  and the gearbox is having a hard time too handling the power, along with the clutch ad other drivetrain components  and no mater how much you try beef up the components    the life span of these components  is some what shorten  and the motors reliability   is some what shortened too and maintenance needs to be done on a regular basis  were has the 650cc is a much more sweeter motor  and has the similar  power out put too the 750,   but more a balanced motor to the 750  even Doug  Hele  at bracebridge street warned the management of the day  that the 750cc was a bridge too far,    the real problem with the 750 is the balance and weight of the crankshaft  and oil presser and oil circulation    and a set of crankcases to handle the power out put,   now  Steve Maney knew all this  That why he makes his own cranks and crankcases  and dose his own crankshaft balancing, and so far his motor have been more reliable  , So see Steve Maney and His web Site,   yours  Anna J     

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Once again AJD hasn't got a clue, the Commando was neve made in 650.

As far as the gear box is concerned I ran a 750 Commando for 100.000 miles without any cush drive either, no issue. Although I did double up the sleeve bush early on. I didn't bother with oil changes much either. So often one mans failure is anothers success. 

Actually the gearbox in the Commando was originally designed some time in the 1920s, by Sturmey Archer / Raleigh.  Norton bought the design rights and added an external positive stop mechanism in '34.  It got some mild titivation post WW2, and then a significant improvement with the 'horizontal' box. 

The AMC box which you refer to got a much neater positive stop mechanism, and other detail modifications, but it was still closely derived from the previous Norton gearboxes.  So no, the Commando box is much older in design than you give credit for - and it coped remarkably well with at least double the power it was intended to transmit.

Commando 750s were prone to cylinder base flange fracture. To cure this, the 820 barrel had four cylinder base studs extended to the head joint face.  That makes for a much stronger motor, but then other aspects became more marginal.....

Lots of people have had great success in converting British parallel twins from 360 degree cranks to 270 degree.  They are MUCH smoother.  But I'm not sure the mild rocking couple will totally suit the Isolastic frame. 

Paul

I'd be quite concerned about introducing a rocking couple into any frame not designed to cope with it, but perhaps particularly into a Commando Isolastic system. The frame is hardly over-engineered, and the design surely assumes vibration strictly parallel to the vertical centre line.

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There is probably no single 'correct' answer to this question. Will it be installed in a 750 or 850 rolling chassis ? 750s feel 'livelier' but there are several reasons for this, not all engine-related.

The bore to stroke relationship of the 750 is rather special...but the 850 actually produces more torque. The short stroke 750 had a bad name for power delivery and not being as easy to ride. The 1970s big-bores produce more power but some aspects become marginal.

What sort of bike is being built ? One for trackdays with a light rider or a two-up tourer for a large couple ?  If you like the 750 power delivery then there is no doubt that a strong one can be built using 850 cases and the through-bolted barrels now sold for 750s.

If it's for a road bike then I'd stick with the capacity used at the time. Registration of 'Historic' vehicles is only going to become more restrictive in the future.

 

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You have the answer - Borrow an 850 and follow a 750 - all will become clear, beyond 850 a lot of work will be needed to have a comfortable to ride fast road bike. Don't see much point going over 850 with the existing crank, stroke, pistons etc. Change these then you are into the race territory of performance and handling. 

There was a series on YouTube recently about a project to fit a Yamaha XS 650 twin engine into a Commando frame.  I got bored after starting to watch...  Is the Yam engine a 360 or 180 degree crank?  I wonder how he got on

Paul

Paul, the series on YouTube was actually a  Yamaha XS 750 engine in a Commando frame. The XS 750 is a three cylinder engine.

The Yamaha XS 650 twin has a 360 crank but conversion to 270 is quite common.

 

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