Splitting cases on a commando

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Hortons Norton

Oct 12, 2007
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Just wondering if I will need to use special tools to split the cases? Also this is a 72 and I have read a few post on this and other sites that have me a bit concerned, I have read you want to do the crankcase mod. and also read you can reinforce the cases? I would like to keep the matching #'s but would it be better to go with differnt cases? I really don't ride that hard and just want the bike to have a bit of pop but also last a bit. Possible? Thanks guys and have a great day, Chuck.
You will need a number of special tools and if this is the first time then you should also invest in advance in a Factory service manual which will tell you when and how to use them.

A clutch spring compressor and engine sprocket puller are needed to dismantle the primary drive. For the top end you will need sockets and spanners slim enough to fit the head and barrel fasteners. In order to split the cases you will certainly need a mainshaft pinion puller (there's no room for a universal type) possibly a puller for the cam sprocket (not always) and if you have Superblends, a bearing puller for the inner race.

For reassembly, you will need a taper for the camshaft oil seal.

I don't think that you need to reinforce the cases for road use but the breather and oil scavenge mods are a good idea. At the very least, you should read everything you can find on the subject and perhaps consider having the work done by someone who has experience of the modifications.
The engine is already out of the bike and the engine sprocket has been removed, I have all the tools to get me to this point. I have seen crankcase pullers but have also heard you really don't need it, True? Chuck.
I'm not sure exactly what you mean by a crankcase puller.

I wouldn't attempt removing the mainshaft pinion without the special three-jawed puller which engages in the three cut-aways on the backing spacer.

Doing the job without will result in damage to pinion, spacer and quite probably the crank itself.

It's a quick and easy job with the correct puller. It might be easiest to bring the motor to someone who has one.

The cam sprocket can usually be carefully levered off.
No, a crankcase puller should not be necessary, especially if Superblend bearings are fitted.
Thanks a million, I now see the tool you are talking about, I will need to get one. The crankcase puller I was talking about is one that Old Britts has for sale. I want to do this right and not damage anything as I go, I have done a lot of work on cars and machines and understand special tools are made for a reason. I must have overlooked this tool, It's raining here today so I was hoping to get this done but I have other things I can do too, Transmission for now, Thanks again, Chuck
Hey Chuck, I've looked at the Old Britts site and now see what you're referring to. :) As L.A.B. says, if your motor has Superblends then there is no resistance there to separation.

All I can say is that I've never seen one of these tools in use and never felt the need to make one up. I've never found the need to apply anything more than a couple of taps from a hide mallet backed up by a length of wood tapped inside the crankcase mouth as per the Factory drawing, to keep things square.

Removing the studs in their entirety helps as well. There are no substantial spigots on the flange (more's the pity !) and I have never come across the problem Old Britts refers to of rusted dowels.

I would definitely say give it a try without and if it sticks then jury-rig something.
So, has anyone actually needed to use any kind of similar tool to get the cases apart?

If I were selling them for $160 a throw then I think I'd be able to come up with a reason to use one!

But to be fair the dimensions of the tool are given (not that anyone would be incapable of working them out) but why the need to make such an exquisitely crafted tool that may only be used the once, when one could be made from a piece of scrap steel plate and a length of threaded rod for next to nothing, I really don't know?

It would look good hanging up on the workshop tool board I suppose?
L.A.B. said:
So, has anyone actually needed to use any kind of similar tool to get the cases apart?

It would look good hanging up on the workshop tool board I suppose?

My 850 cases were hard to split, in spite of the Superblend bearings, due to corrosion on the dowel pins. I used a block of wood, a heavy hammer, and an assistant to knock them apart, exactly as described in the factory service manual. The cases put up a good fight, resisting separation every millimeter of the way. Brute force, carefully applied, finally won out however. 8)

I'd be concerned about Fred's tool possibly pulling out the helicoils on a difficult one like this.

I'm sure it would look great hanging on the toolboard though! :lol:

more pop.

More pop, hunh? Well, I would do it by lightening up the bike. You can run without a battery or battery box, fit alloy rims and stainless spokes, and make thinks like spacers and whatnot out of alloy. Then lower your final drive gearing. I don't think that hot rodding the engine should be necessary for more "pop", although a camshaft with slightly more lift and a little different timing might compliment the 750 engine. Lighten the valve train, so that the internals aren't as stressed when you get over 6000 rpm.
Work with the strong suit of the long stroke engine....mid range torque. I would rather fit a five speed gearbox and a 19T final drive sprocket for street blasting than to build a conventional high compression cammy hot rod engine. Keep the revs down, but an extra cog in the gearbox will keep you in the sweet spot.
Alloy rims give you many advantages...less mass on the ends for better mass centralization, less mass to spin up to speed, less mass to accelerate linearly, less mass to slow during braking, less mass for the suspension to control, less mass to have to move out of the rotational plane when cornering...so easier steering. Plus they look great and don't rust. Start there, I suggest. Also, you then have the option of using 18" rubber on the rear, if you relace with a WM3X18 rim. Check http://www.buchananspokes.com/
If you fit the 18" and use a 110/90/18 tire, the overall diameter will be slightly smaller than the 4.10 /19 stocker, slightly lowering the gearing, so try that before changing sprockets.
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