Engine size question

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2Xbad

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Hello there,

I have a question about the engine size on my 1974 Dunstall replica commando. It's headplate is stamped 10/73 and serial number 309405 which according to Roy Bacon's book makes it clearly an 850, in fact the book shows all serial numbers after 300000 to be 850s. But this bike has 750 barrels on it.

I've seen other bikes with earlier serial numbers and 850 barrels so not sure how this could happen.

Did they just have extra 750 parts lying about and were building both 750s and 850s at the same time? Does it have an 850 case but a 750 top end?

Maybe it is as simple as my first question but wanted to get some thoughts.

Thanks
 
2Xbad said:
It's headplate is stamped 10/73 and serial number 309405
Click to expand...

Is that the plate and the engine number?


2Xbad said:
Did they just have extra 750 parts lying about and were building both 750s and 850s at the same time? Does it have an 850 case but a 750 top end?
Click to expand...

Likely somebody has put a 750 top end on it if it's an 850 bottom end.
 
Yes, engine number and headplate match. As does gearbox.

Is that the likely scenario? That there is an 750 top end on an 850 case?

Or is it more likely that they were still building 750s in late 73, and this one is just a straggler.

I have it listed for sale on Bring A Trailer. Take a look and let me know what you think based on the photos there. I can also add more here if you need to see something else.

bringatrailer.com

24-Years-Owned 1974 Norton Commando 750

Bid for the chance to own a 24-Years-Owned 1974 Norton Commando 750 at auction with Bring a Trailer, the home of the best vintage and classic cars online. Lot #46,394.
bringatrailer.com bringatrailer.com
 
2Xbad said:
Yes, engine number and headplate match. As does gearbox.

Is that the likely scenario? That there is an 750 top end on an 850 case?
Click to expand...

Considering how non-standard it is, I'd think the top end has been changed sometime in the past as they are 850 cases so the cylinder head is probably also 750.

2Xbad said:
Or is it more likely that they were still building 750s in late 73, and this one is just a straggler.
Click to expand...

Although 750 models continued to be built in '73 the very last of which had 750 top ends on '850' crankcases they had '230xxx' serial numbers (the official last 750 being 230935) so I very much doubt this has anything to do with the factory. One thing that's for certain is that it's not an actual 828/850 as described in the ad as boring a 750 barrel to 828 is either not possible (or not safe).
 
2Xbad said:
Hello there,

I have a question about the engine size on my 1974 Dunstall replica commando. It's headplate is stamped 10/73 and serial number 309405 which according to Roy Bacon's book makes it clearly an 850, in fact the book shows all serial numbers after 300000 to be 850s. But this bike has 750 barrels on it.

I've seen other bikes with earlier serial numbers and 850 barrels so not sure how this could happen.

Did they just have extra 750 parts lying about and were building both 750s and 850s at the same time? Does it have an 850 case but a 750 top end?

Maybe it is as simple as my first question but wanted to get some thoughts.

Thanks
Click to expand...
Looks like a '72 750 Combat head with the narrow gap between the bottom head fin and the top barrel fin. Is the head stamped with a C?
 
Ok, interesting. My (English) mechanic thinks they used up 750 parts at the factory but were already into 850 numbers.

So you also think the case is an 850 case not a 750 case?
 
Unfortunately I can't tell if there is a "C" because the head steady covers that part of the head
 
2Xbad said:
My (English) mechanic thinks they used up 750 parts at the factory but were already into 850 numbers.
Click to expand...

Well, I'm doubtful, for one thing, if it left the factory as a 750 then why is it being advertised as an "850" so I'd guess it probably left the factory as an 850.
As the present owner has had it since 1997 then a significant part of its history is unknown.

2Xbad said:
So you also think the case is an 850 case not a 750 case?
Click to expand...

Yes, it's definitely an '850' case.
 
I'm the one advertising it as an 850. That's my listing. That's the point, it's a mistake. I called it an 850 based on Roy Bacon's book. It took some other guy to point out that the barrels are 750 barrels, at which point I realized my mistake.
 
2Xbad said:
I'm the one advertising it as an 850. That's my listing. That's the point, it's a mistake. I called it an 850 based on Roy Bacon's book. It took some other guy to point out that the barrels are 750 barrels, at which point I realized my mistake.
Click to expand...
I see. I got the impression from your first post you'd just bought it and then missed the "I have it listed...".
Doesn't it have the cc on the title document?

"the seller notes that the Vehicle ID Number on the title (850F104368) is not present on the motorcycle itself."

Probably is there but can be covered with paint (and the fairing bracket?).
 
Last edited:
L.A.B. said:
I see. I got the impression from your first post you'd just bought it and then missed the "I have it listed...".
Doesn't it have the cc on the title document?

"the seller notes that the Vehicle ID Number on the title (850F104368) is not present on the motorcycle itself."

Probably is there but can be covered with paint (and the fairing bracket?).
Click to expand...
Nope, the 850F104368 is nowhere on the bike. The headplate, motor and gearbox all match to the 309xxx number.
There is no cc on the title.
 
2Xbad said:
Nope, the 850F104368 is nowhere on the bike.
Click to expand...
Well, it's would normally be stamped on the headstock at the RH side of the plate so would be mostly under the fairing bracket.
As the number is on the title then it ought to be there.
 
When a fairing was fitted at the factory the plate was relocated to under the seat so it could be read, so a fairing bracket covering up the 850F number is not unknown.
 
2Xbad said:
Hello there,

I have a question about the engine size on my 1974 Dunstall replica commando. It's headplate is stamped 10/73 and serial number 309405 which according to Roy Bacon's book makes it clearly an 850, in fact the book shows all serial numbers after 300000 to be 850s. But this bike has 750 barrels on it.

I've seen other bikes with earlier serial numbers and 850 barrels so not sure how this could happen.

Did they just have extra 750 parts lying about and were building both 750s and 850s at the same time? Does it have an 850 case but a 750 top end?

Maybe it is as simple as my first question but wanted to get some thoughts.

Thanks
Click to expand...
Hi I am sorry I cannot offer any suggestions as to your machine. I do have a question re frame. My 850 310528 is stamped 11/73 but does not have the shock mount reinforcing, however yours does. LAB do you know if there is a good reason for this? Just idle interest it does not freak me out or anything.
Thank you,
Martin
 
kommando said:
When a fairing was fitted at the factory the plate was relocated to under the seat so it could be read, so a fairing bracket covering up the 850F number is not unknown.
Click to expand...

It's a Dunstall (type?) fairing and the plate is in the normal place.
Engine size question
 
Last edited:
holtcorseaux said:
Hi yes it is F103393
Click to expand...

I see your bike on Greg Marsh's list.

My bike is the next month after yours (12/73 - 311032 -850F105226 ) but has the extended inner shock mount plate but maybe the F number does have input on that.
Perhaps an oddity is the F number and the engine that was installed seemed to show no relationship as on that list in some cases and do know that Dave Swanson's Mk2A has a 50 higher engine/gearbox/tag number but seem to recall my bike with a lower number has a higher F number.

?
 
Time Warp said:
I see your bike on Greg Marsh's list.

My bike is the next month after yours (12/73 - 311032 -850F105226 ) but has the extended inner shock mount plate but maybe the F number does have input on that.
Perhaps an oddity is the F number and the engine that was installed seemed to show no relationship as on that list in some cases and do know that Dave Swanson's Mk2A has a 50 higher engine/gearbox/tag number but seem to recall my bike with a lower number has a higher F number.

?
Click to expand...
Hi yes must be that. Thank you all

Martin
 
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