General Bike Theory/Purchasing Questions

Congratulations.
Don't be bothered about the high mileage, my old Mk3 850 has passed 160,000 and still going. It's had a few rebuilds, threw down the road a couple of times, constantly getting modified, could do with the frame re-coating, needs rewired, and she's broke my heart plenty of times. But! I don't think I'll ever part with her.
Now, the thing to remember is no one usually parts with a good Commando, there might well be an annoying fault that the previous owner struggled with. This should not be a problem for you as in my opinion the members of this site are most helpful in short the best, and will get you through.

L.A.B. said:

That number would appear to be considerably higher than the last known (October 1973) 750 production model = 230935.


Unfortunately the production numbers given here (or anywhere else for that matter) cannot be guaranteed as being 100% accurate, so should only be looked upon as a guide.

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L.A.B.--

I definitely read that chart wrong.... and I completely missed the last production model #, which, you are correct, my number considerably exceeded. Combine that with the fact that a couple members on this list e-mailed me and suggested the engine was likely older than a '73.... it was time to contact the seller for verification. He wrote back yesterday that he had indeed got the number wrong (very nice fellow, wasn't intentional in my opinion. Instead the number is: 20M3 148544. That places the engine build time between May - October of 1971 (keeping an eye on your noting that production numbers cannot be guaranteed as 100% accurate).

Now I'm in a bit of a conundrum.... bike is beautiful, comes with some great accessories (not that big of a deal), appears to be kept up, etc., etc. But I've read that the '73 750 engine was the best of all the 750s, and I was bidding accordingly. "This work makes the 1973 745 cc engines the best of the bunch but," Roy Bacon writes in the Illustrated Norton Buyer's Guide, "as any engine can be brought up to scratch, this is more of an academic point now." [Kudos to geo46er for lending the book]. Alas. I'm not sure Mr. Bacon's point helps(!) Is really merely academic? Since this is a bitza bike, maybe indeed it is. Hmmm. Let's see. What ARE the main differences between a '71 and a '73 engine?

Bluntly, I guess the biggest question is whether it matters enough to back out of the purchase or not.

Thanks L.A.B, per usual for your help.

kevin

* C O N G R A T S ! *

THANKS, Werner. I'm very impressed by what you have done to your bike and will be looking at your build list frequently.

Kevin

CommandoRoadster said:

Kev-
Maybe somebody already said this. If so, I missed it. Don't ever sit on your Commando on the sidestand. You can break off the pivot tab, and sometimes take part of the frame with it! Don't ask me how I know this!

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Thanks CommandoRoadster. That sounds painful.

If your bike doesn't have a centrestand, get one. If it does have one, take it off, and inspect it for cracks in the welds. Maybe someone on here can post some pics on where to add the gussets so it doesn't crack again. Norton fixed this on the 850s, so maybe it's already done for you. Check it out.

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I don't think this bike has a centerstand. I appreciate the advice. I was on the British Only site just yesterday pricing them. They have the genuine Norton version (higher priced) and a reproduction version for slightly less. geo46er has made it clear I should look into it as well.

Is there a place on this site to post pics, or must one link to another site (like Yahoo! photos, or the like)?

Thank you CommandoRoadster!

kevin

cash said:

Congratulations.
Don't be bothered about the high mileage, my old Mk3 850 has passed 160,000 and still going. It's had a few rebuilds, threw down the road a couple of times, constantly getting modified, could do with the frame re-coating, needs rewired, and she's broke my heart plenty of times. But! I don't think I'll ever part with her.

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Thanks cash. One thing I keep noting here is everyone's complete devotion to these bikes. Great to see.

Now, the thing to remember is no one usually parts with a good Commando, there might well be an annoying fault that the previous owner struggled with. This should not be a problem for you as in my opinion the members of this site are most helpful in short the best, and will get you through.

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I get the feeling that you are 100% correct.... the seller, while very helpful and (I think) genuine, has said he really does not know much about the bike (only that he bought it from a 'professional'). That seems like an ideal set-up to me(!).... again, I don't think he's purposely deceiving me about anything, but may not know himself why the bike does this or that, only that he doesn't want to fiddle with it much. I'll keep my fingers crossed if I get the bike (probably will). And you can count on hearing from me countless times with questions and shameless pleadings for assistance.

Best--

kevin

The 71 cases are nice and beefy under the jugs but a little light around the output bearing. Still many out there running long miles. If it were mine I would be crack checking the case behind the primary cover some time soon. and checking the end play on the crank at the same time. Don't worry be happy.

norbsa48503 said:

The 71 cases are nice and beefy under the jugs but a little light around the output bearing. Still many out there running long miles. If it were mine I would be crack checking the case behind the primary cover some time soon. and checking the end play on the crank at the same time. Don't worry be happy.

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Good stuff. Needed to hear those last four words. Let's get on with it. I'll be 'checking the case behind the primary cover'..... as well as the 'end play on the crank,' I promise. Thanks for the pointers. Mucho appreciated.

Best--

kevin

kevster™ said:

But I've read that the '73 750 engine was the best of all the 750s, and I was bidding accordingly. "This work makes the 1973 745 cc engines the best of the bunch but," Roy Bacon writes in the Illustrated Norton Buyer's Guide, "as any engine can be brought up to scratch, this is more of an academic point now." What ARE the main differences between a '71 and a '73 engine?

kevin

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I've been lurking as a guest here for a time. Have to say this is a great group of folks. But I DID want to see some responses to this question that kevster™ asked. (I dunno myself, I do appreciate Kevin's new level of Zen of his last post, but don't want him to stop askin' questions). If you are of the dark side (i.e. a Norton owner), one must ride on. And inquire. And find a way. Few of us here are immune to being compulsive when it comes to bikes, is my guess. And that's a good thing.

750 Commando engines by model year. Engine breather is one. What else?

mo_fo_commando

mo_fo_commando wrote - "750 Commando engines by model year. Engine breather is one. What else? "

Here's a start..

'68-'69 Fastback - Distributor behind cylinders
28.5 mm intake ports
30 mm Concentrics
Tach drive from timing cover
Ball and roller main bearing

'69 'S' - Points on end of camshaft
Tach drive on front of crank case

'70 Fastback and Roadster - same as 'S'

'71 all models - bolt on blank plate behind cylinder
where starter was intended.

'72 Combat - 32 mm ports
32 mm concentrics
10:1 compression
double roller crank (early superblend)
2 S camshaft (some bad nitriding)
stronger cases
relocated oil pickup (bad!)
relocated breather
spin-on oil filter

'72 low-compression - 28.5 mm ports
30 mm concentrics
original grind camshaft
8.5:1 compression
all else as Combat

'73 Hi-compression - 32 mm ports
32 mm concentrics
original grind camshaft
even stronger crankcases
improved superblend roller bearings
9.3:1 compression
all else as Combat

'73 low-compression - same as above except
8.9:1 compression

Any additions or corrections?

Just a couple of small extra points.

The '73 timing side halves are recognisable by having a full casting behind the timing cover, where the earlier versions had the blanking plate.

Neither the '72 or '73 (All 750 engines from 200,000 onwards) have the large sump strainer but it was reintroduced for the 850s

Mix and match engines are not impossible.

Your mention of the aborted electric start is interesting. Can you imagine trying to bring a cold Commando up to compression using a motor driving through the old mag drive chain ? I dread to think of the mess that would have occurred when it inevitably let go :shock:

There seems to be some confusion on when the blanking plate disappeared. Dave Comeau claims a 20M3S prefix case (pre-'72) with no machined starter hole and a later one with it. (http://atlanticgreen.com/engcases.htm) I wonder if they had more than one source for casting cases or possibly more than one pattern at the same foundry.

Of course, none of this really affects the performance other than the sump position.

I understand there was quite a debate about the aborted starter. Maybe Frank can weigh in here with some insight. You're right, it would be hard to imagine and effective starter mounted there.

Ron L said:

There seems to be some confusion on when the blanking plate disappeared. Dave Comeau claims a 20M3S prefix case (pre-'72) with no machined starter hole and a later one with it.... I wonder if they had more than one source for casting cases or possibly more than one pattern at the same foundry.

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I don't quite follow your reasoning Ron L?
When the plate *disappeared* would seem to have no bearing on either of the two 20M3S cases shown your Dave Comeau website link.
As the 20M3S blanking plate type cases were not (apparently) used until engine number 133618 (or 136618), that date corresponding to approximately April 1970 production, which places it well after the start of 20M3S production (supposedly number 131257) from March 1969, so I would not expect *early* 20M3S cases to have the blanking plate?

Presumably the blanking plate disappeared at some point after number 200000 when the strengthened cases were used from approx. Jan. 1972?


There is an amusing account given by Norton test rider Bob Rowley included in Mick Duckworth's 'Norton Commando' book of one of the experimental timing chest starter's mechanism failing dramatically when he demonstrated it in front of a somewhat sceptical Dennis Poore and other members of the Norton management.
As Bob Rowley had told them that it would break, the management did not believe him, a demonstration was arranged and after he operated the starter three or four times the management were just about to walk off, so he hit the button one more time, the starter mechanism broke, the chain going through the timing cover in the process, and as a result the timing chest starter was never put into production.

Ah, so pre-'70 20M3S cases would not have the blanking plug. I was under the impression that the blanking plug was on all 20M3S cases. I guess I should have checked the two '69 'S' models my buddies ride.

Thanks for the clarification.

mo_fo_commando said:

I've been lurking as a guest here for a time. Have to say this is a great group of folks. But I DID want to see some responses to this question that kevster™ asked.

(I dunno myself, I do appreciate Kevin's new level of Zen of his last post, but don't want him to stop askin' questions). If you are of the dark side (i.e. a Norton owner), one must ride on. And inquire. And find a way. Few of us here are immune to being compulsive when it comes to bikes, is my guess. And that's a good thing.

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No worries mo_fo_commando(!)... nice name, by the way. Most excellent. So many folks turned in some great responses and I am highly appreciative. You can count on me asking plenty of questions once I have the bike in hand. And nothing better than sitting back and reading the likes of Ron L, 79x100, and L.A.B.s input (see latest posts) on 750 engine differences. (I know when to keep my mouth shut occasionally (although not enough)). I've only entered pre-school when it comes to Commandos. I'll be reading the archives (and present posts) here for some time in an attempt to get up to speed.

Thanks again!

kevin

kevster™ said:

I've only entered pre-school when it comes to Commandos. I'll be reading the archives (and present posts) here for some time in an attempt to get up to speed.

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Kevster, we are ALL still l learning here. Good luck with the bike! Let us know when you get it and how it all turns out. I remember my first Commando like it was yesterday....

THANKS to you guys for clarifying regarding engine differences. I had no idea.... Could anyone recommend a good book or two that goes into detail on model year differences? There seems to be a few Norton books out there on the Commando specifically, but some of them have spotty reviews. I could really use a good solid reference source. Thanks in advance for any help.

mo_fo

kevster™ said:

I've only entered pre-school when it comes to Commandos.

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Me too, well maybe kindergarten but you get my point. There is a lot of great advice from some very friendly people here. Afterall, they have been putting up with my annoying questions and crazy ideas for a while now.

The problem with most books is they aren't always 100% accurate. That being said, my favorite is Norton Twins by Roy Bacon. Unfortunately this is out of print and you'll need to find a used one somewhere. Lot's of good information, some inaccurate information.

Also worth mentioning is Bacon's Norton Twin Restoration, and Norton Illustrated Buyer's Guide. Mick Duckworth's Norton Commando is also pretty good.

mo_fo_,

As Ron L says there isn't really any one Commando 'bible' (if there is such a thing then this website is the nearest there is to it I guess?) as most of these books cover many of the Norton models not just Commandos, so they all contain a certain amount of useful information. The books Ron L recommends I would also recommend, and also 'Norton Motor Cycles 1950-86' by Steve Wilson, and the Norton Owners Club Service Notes (small booklet that contains much tech info but now getting a little out of date).

There doesn't seem to be any problem getting a copy of Roy Bacon's Norton Twin Restoration off the shelf in the UK at least as I think it may have been re-printed:

http://www.motorbooks.co.uk/showsect.asp?id=131

Britcycle others seem to have it as well:

http://www.britcycle.com/Products/roy_bacon.htm
http://www.motolit.com/1855790432.html

It was just reprinted, only in the last couple of months. Just don't follow it 100%, all Roys books are suspect in a few area's but are right in just enough to fool you that they are 100% correct.

Ron L said:

The problem with most books is they aren't always 100% accurate. That being said, my favorite is Norton Twins by Roy Bacon. Unfortunately this is out of print and you'll need to find a used one somewhere. Lot's of good information, some inaccurate information.

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Thanks, Ron L.

Just so has it, I found the book at a terrific bargain! Just can't afford it right now, but I thought I'd provide the link for anyone who might be interested. Here's the link. Snatch it up folks:

http://product.half.ebay.com/Norton-Twi ... 3QQtgZinfo

Thanks for the other book recommendations. Will be checking into them right away. And your point, that these books are 100% accurate is duly noted.

Thanks!

mo_fo