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'71 750 Commando Rebuild

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'71 750 Commando Rebuild

Postby ELMZY » Thu Apr 21, 2016 9:56 pm

Hi everyone.
In preparation for my return to this classic motorcycle I've absorbed a lot of valuable information from this forum. With that in mind and having recently secured a project bike - it's time to contribute (first post).
I had a MK 111 850 when I was a young bloke so my original plan was to find another one (at a reasonable price) for a complete rebuild. Easier said than done here in Australia.
The search was draggin' on.
Then a 750 "barn find" U.S. import turned up locally.
Reality bit.
Now there's finally a Norton Commando in my back shed - and I'm happy.
It'll probably be a slow thread due to funding - and my bias towards shiny bits and attention to detail (too much time ogling the cNw image gallery). As far as the process goes, my mechanical expertise is limited so the heart of the matter (motor) will be rebuilt by a professional and I'll have a crack at the rest. And there will be mods. I guess that makes me more of an assembler than a rebuilder.
I'll post Day 1 photos when I figure it out.
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Re: '71 750 Commando Rebuild

Postby Danno » Fri Apr 22, 2016 3:25 am

Don't sell yourself short. those pinheads at Orange County Choppers (Paul and Pauler) claim to build bikes when they do little more than make tanks and fenders other than wiring and assembly. Almost all their real work is farmed out to specialists and catalog parts houses. And they have a TV show.

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Re: '71 750 Commando Rebuild

Postby grandpaul » Fri Apr 22, 2016 5:19 am

Welcome to the forum.

Plenty of excellent resources here.

You'll get on fine.
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author "Old Bikes"
too many bikes to list, including:
952 Prototype, MkIII Interstate, Combat Dunstall, Triton
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Re: '71 750 Commando Rebuild

Postby Torontonian » Fri Apr 22, 2016 3:26 pm

Make some money and be careful who you give it to and for what.
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Re: '71 750 Commando Rebuild

Postby ELMZY » Fri Apr 22, 2016 6:49 pm

Thanks for the encouraging words fellas.
Here's a few pics.
Image
The fibreglass tank/side covers and that strange looking seat will be discarded.

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The back mudguard looks suspect too.

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Sadly, this bike's been left out in the weather for some time.
Most of the frame rust is surface and should clean up ok.
The only area of concern for me is the bolt area around the rear guard bracket/plate.

Image
RH side fins look ok.

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LH side fins look ok.

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Check out the side stand mount.
When I got her home I flicked it out and leant her over, the u-bolts rotated on the frame and she crashed into my thigh. I nearly went down on day 1 without even throwin' a leg over her.
On a more serious note - I don't like the look of those rust stains around the inspection holes.

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Ancient rubber.

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Apart from some of the fork internals, I'll probably only use the LH slider for the new disc front end.

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The numbers

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Match.

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Someone tried to take the head off but couldn't do the pushrod shuffle.
It's a worry - because the motor's seized.
They also "misplaced" the Amals and headsteady.

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Any Aussies out there know of an oil tank restoration/modification service here in Oz ?
Similar to what Colorado Norton Works offer ?
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Re: '71 750 Commando Rebuild

Postby htown16 » Fri Apr 22, 2016 8:38 pm

That's a 74 or 75 Commando seat and looks like with some cleaning it might be salvagable. I sure wouldn't trash it. Sell it on ebay if you don't want it or put the correct 71 cover on it.
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1972 Commando
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2004 XL1200R Sportster
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Re: '71 750 Commando Rebuild

Postby ELMZY » Fri Apr 22, 2016 9:16 pm

Yeah, I agree htown. I won't chuck it - it's in pretty good nick.
I don't think it's off a '75 though. From memory they were a lot thicker and had a hinge on them.
In any case, it's not relevant to this build. I've got "ducks disease" - so I'll be doin' the modified Corbin Gunfighter thing.
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Re: '71 750 Commando Rebuild

Postby htown16 » Sat Apr 23, 2016 7:55 am

Well it's definitely a 74. I have one just like it. I wasn't sure if the 75 was the same or not.
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Re: '71 750 Commando Rebuild

Postby ELMZY » Sat Apr 23, 2016 10:26 am

I managed to take the head off and it looks bad in there.
I soaked it in WD40 for a day or so and gave it a light scrub with a scotch pad .
Most off the gunk came off to reveal a badly rust pitted bore. It's no wonder the pistons won't budge.
Image
Looks like whoever attempted to remove the head had trouble with a stud.

Image
The left side is the worst.

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Fugly.

Any suggestions on how to free it up and remove the barrels ?
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Re: '71 750 Commando Rebuild

Postby htown16 » Sat Apr 23, 2016 11:52 am

Mix 50/50 automatic transmission fluid and acetone. Fill the bores with the mix, cover them and let sit for a few days.
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Re: '71 750 Commando Rebuild

Postby dave M » Sun Apr 24, 2016 3:39 am

This looks like a good project, things missing like exhausts and carbs you would likely be replacing anyway. If you plan to put a disc setup on it the lower legs are both different to the drum setup, they have a more oval shape and more meat on them. Why not keep the drum to begin with, RGM Motors have some new brake shoes that make these work quite well. If you do decide to convert it the drum backing plate is worth a decent amount of money on the 2nd hand market. A bit of heat from a propane torch and some kind of drift with a hammer will help to budge the pistons and remove the barrels after leaving them sitting with penetrating fluid as suggested by htown16.

A couple of special tools like a clutch compressor, clutch holder and crank pinion puller might come in handy.

Good luck with this, you will enjoy the journey.

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Re: '71 750 Commando Rebuild

Postby ELMZY » Sun Apr 24, 2016 5:44 pm

htown16 :
Thanks for the heads up on the ATF/acetone mix.
It wasn't as bad as I thought. After a few more days with the WD40 I decided to give the pistons a wack with a rubber mallet and they moved about 20mm - so no rust below. I couldn't kick them up back past the rust though, so I knocked them back down and hand honed the top of the bores with 240 wet/dry and Inox and up she came.
There's no discernible lip at the top of the bores so, with good contact onto the walls, I took a few preliminary measurements (estimates) with a vernier caliper (don't have a proper bore gauge) and highest estimate was 72.92mm.
The pistons are stamped STD so it looks like a stock bore (73mm?). Hard to believe on a 45yo bike.
Anyway, it gives me hope that a rebore will take out the rust pits.
dave M :
Thanks for the reminder about the fork slider differences - I remember reading about that on this forum a few years back (debby).
I'm pretty keen for the disc front end so it's lookin' like 2 more sliders.
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Re: '71 750 Commando Rebuild

Postby ELMZY » Sun Jun 05, 2016 6:37 am

Dismantling process has been slow.
Seems life has it's way of interfering with hobbies and other selfish pursuits.
Some assessments:
The rear axle was rusted into the hub bearing spacer and difficult to remove (yes, it was disconnected from the stub axle).
Instead of using some kind of mechanical extractor I decided to short cut it and use vice grips and a hammer.
I measured the process consistently and achieved 2-3 mm increments until it was out.
Further inspection revealed that I MAY have inadvertently damaged the swingarm in the process.
The drive side axle slot/lug is bent and the torque stop slot is out of shape.
I didn't check/notice it before so I'll take the rap for it.
On a positive note - the bike gets a new, stronger, gusseted swingarm upgrade.

Image

Image
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Re: '71 750 Commando Rebuild

Postby ELMZY » Sun Jun 05, 2016 7:58 pm

There's been a few shots on this forum of a modification to the upper rear corner of the gearbox cradle opening (kickstart side) to allow gearbox removal without undoing any engine mount bolts.
There are those that believe it may compromise the structural integrity of that area of the framework.
And there are some reputable rebuilders that do the mod with no apparent problems.
I like the idea so I decided to do a dummy run with the bottom end of the motor in place during disassembly, to see what I was up for.

Image
The gearbox needs to be rotated about a quarter turn anticlockwise to position the bottom mount in the bottom righthand corner of the opening and up against the back of the crankcase (above).
This puts the top mount diagonally opposite and reveals the need for at least a 15mm cutout to clear the top mount (below).
Image

No problem thus far, until I factor in another mod I've had in mind - the cNw/Comstock crankcase breather.
With it mounted on the rear of the crankcase, the gearbox will be pushed back a bit further and the chunk out of the cradle will be a bit bigger.
Anyone out there have both mods ?
Any info in this area would be greatly appreciated.
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Re: '71 750 Commando Rebuild

Postby ELMZY » Sun Jul 03, 2016 1:51 am

I bought a new "Hybrid" gearbox/engine cradle from AN.
Image

It has the Mark 111 cotter pin swing arm spindle fix housings (above).
And pre drilled oil filter mount holes (below) - absent on the '71.
Image
Image
I finished the gearbox removal mod (above)
The piece of 12mm ply wood, bottom right, is to simulate a cNw/Comstock crankcase breather.
And the cut out, top left, for the top gearbox mount - with 5 or 6 mm to spare.
I also drilled another hole for a double sided primary chain/belt adjuster (below).
Image
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