71 frame and steering yokes

This is a brief explaination as I understand it of the peculiar matter of the 1971 750 frame (part 06-2002) and steering yokes. I'm working on a basket case 750 with the usual bits missing. The frame for '71 has the side stand bracket welded into the inside corner of the lower cross tube at the left lower frame rail. The stand mount is a downward peg with a circlip groove. These are allegedly weak and can fail, probably from the rider heaving down on a reluctant Combat motor!

The 71 introduced the new style of top and bottom steering yokes. In any parts book, the only listing is 06-1916 bottom and 06-1915 top for 71, 72 and 73 when the 850 introduced the ANG stamped yokes. According to good friends long associated with Commandos, there was a short production run of yokes that are only suitable to the 71 frame. Something to do with the steering angle. This correct yokes are 06-1916 bottom yoke and 06-1917 for the top yoke. Two friends have authentic 71 750's, one bought new a real one owner bike. Both confirmed the stamping 06-1917 in the underside of the top yoke. With that I found a pair at the local British bike wreckers, dirty and rusty but the right parts for this rebuild.

The issue apparently is that fitting other yokes can seriously disturb the handling, and the original fitment didn;t handle too well either. At speed above 100Kph / 60Mph the bike feels twitchy, a sort of mild weaving sensation. Better tyres and a steering damper can sort this out, but at the time the Norton factory soon got rid of the 06-1917 top yoke. So the outcome is never fit 850 yokes to a 750, and if its a 71, beware of the difference.

Mick

Thanks for posting this. My 750 has a build date of 10/71 and it always puzzled me why the triple clamp part numbers did not match the parts book. On my bike, the upper triple has the part number NM 061917 cast into the lower surface, and the lower triple clamp is marked NM 061916. The NM prefix is kind of puzzling too for that matter. The frame on this bike had the inboard pin-style sidestand mount, which I had removed and replaced with the Old Britts weld-on mount so I could fit the later sidestand.

Do you know what the exact difference is between the 061917 and 061915 parts? My 750 is not the greatest handler. It has a moderate case of the 35 mph headshake and will go into a full-on tankslapper if ridden hard on rough pavement (i.e, hit some bumps in the middle of a fast corner). It seems stable enough out on the highway though.

My 850 started out as a parts bike I bought on ebay. It was a bitsa, with a 750 frame and a 74 850 motor. I just checked the triples on it and it has the 1971 part numbers, just like my 750! I replaced the frame because the DPO had tried to replace the sidestand mount but butchered the job and ruined the frame. The frame I have on it now is one of the Italian-made ones, but I have no idea whether it's a 750 frame or 850 frame. I don't even know if the Italians made any 750 frames. There are no part numbers visible on it so I don't know what it is.

So now I'm wondering what the best triple clamps would be for each bike...

Debby

ML said:

In any parts book, the only listing is 06-1916 bottom and 06-1915 top for 71, 72 and 73...

.....correct yokes are 06-1916 bottom yoke and 06-1917 for the top yoke.

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But, changing steering/fork geometry by altering the yoke dimensions wouldn't really be possible by changing only one of the yokes, would it?

As the two pairs of stanchion bores in the yokes would not then line up with each other if the relative fork centre positions in one of the yokes was moved?

ML said:

when the 850 introduced the ANG stamped yokes.

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Are any of them "stamped"? I think at least the 850 ANG markings and numbers are cast in.

And "casting" numbers can often differ slightly from actual "part" numbers. Part numbers would be far more likely to be stamped on the part, rather than cast in.

My 850 MkIII yokes has them cast in, and the numbers are, top yoke = 065721, bottom yoke = 064035, [Edit: Correct number is 064083] neither of which are the book part numbers for the MkIII yokes. Both those yokes I believe are unique (certainly the top one) to the MkIII model.

Good points LAB, I've never got to what could be the definitive truth on these odd yokes. For example, the basket case I am working on has a 207 stamped frame number, which indicates around mid '72. It just happens to match the motor number as well. But, according to my guru friends (one a former Norton dealer / service agent) the factory had a habit of doing some random acts which confuses things. For example, this frame is definetly a 71 build, which may have been surplus and put into stores. Later on, during a production run it may have been put into assembly and the frame number stamped with its matching engine.

As to the yokes, the angle or the alignment of the bores is questionable. I have here both a 70 Fastback with the original yokes and two 850's. If I can get around to it, I will pull a set off each and put them into our machine shop for measurement of the respective yoke alignments. That should physically evidence the differences between the 3 models.

Mick

Debby wrote:
The frame I have on it now is one of the Italian-made ones, but I have no idea whether it's a 750 frame or 850 frame. I don't even know if the Italians made any 750 frames. There are no part numbers visible on it so I don't know what it is.

The Italians did make some 750 frames. I have a late 750 ('73) frame (metric tube, larger spine, wider rear loop). I don't know if they made them any earlier than that.
Ben

ML said:

the factory had a habit of doing some random acts which confuses things.

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Yes I agree, and we've certainly had a few of those here!

ML said:

As to the yokes, the angle or the alignment of the bores is questionable. I have here both a 70 Fastback with the original yokes and two 850's. If I can get around to it, I will pull a set off each and put them into our machine shop for measurement of the respective yoke alignments. That should physically evidence the differences between the 3 models.

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I did however, remark in your previous thread on this subject that I thought I'd read of a fork geometry change around this period, and after looking through various books I did find it was mentioned in Steve Wilson's Norton book and also Mick Duckworth's Commando book, both saying that the geometry was altered at the start of '71 production when the yokes changed to the non-adjustable type, but no mention of any further changes (until 850).

My 850 MkIII yokes have the letters/numbers cast in, and the numbers are, top yoke = 065721, bottom yoke = 064035 [Edit: Correct number is 064083], neither of which are the book part numbers for the MkIII yokes, which are; top = part 06-6649, bottom = 06-6228 .

LAB,

I agree this is more of a Sherlock Holmes exercise into how the factory went about things! My current theory is that following the earlier yokes prior to '71, the factory decided upon a new steering geometry. It can't have been much more than 1 degree one way or the other, but suspected to be less rake for quicker steering but less high speed stability.

Possibly what they did was to design and cast the new 1917 top yoke. Then given the budget engineering constraints, instead of making a new matching bottom yoke they took the 1916 bottom yoke and gave it a new bore alignment. Not long after they decided it wasn't a good idea and scrapped the 1917 top yoke and went for the new 1915 geometery and reverted the 1916 to realign with that.

It is only a theory and it would be great if somone had the real facts.

Anyway, as mentioned before, I am going to measure the pre'71, the odd '71 and the 850 ANG versions to see what is what.


Mick

I am trying to get the yokes apart from my MK 111. The nut on the bottom took some leverage to undo (must have been done up by someone on steroids!)
Anyway the # cast on the top one is 065721 but the bottom # is 064083 (not 064035) but it looks the same age as the top one and they are both stamped ANG. It is a 77 model so I am hoping it is not a misfit .
I can not get the bottom yoke off. The manual does not indicate any special tool required, I have given it some solid thumps with a rubber mallet but no joy yet.
I am thinking of making up a pulley but would be really pissed if I damaged or broke one of the yokes.

Jed

Jed said:

I can not get the bottom yoke off. The manual does not indicate any special tool required, I have given it some solid thumps with a rubber mallet but no joy yet.

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No special tool is required-but the lower yoke is often rusted onto the stem, so use penetrating fluid and keep knocking it, not just downwards but try supporting one yoke and then knock the other one sideways. You may find you need to use someting more substantial than a rubber mallet.

Jed said:

Anyway the # cast on the top one is 065721 but the bottom # is 064083 (not 064035) but it looks the same age as the top one and they are both stamped ANG.

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Both "ANG" numbers are correct for a MkIII. Note that they are casting numbers not part numbers. [Edit: The 064035 number I quoted above is incorrect].

The archives have a fair amount of information on the topic of what yokes belong on what frames. Unfortunately, much of it is people repeating what they have read or been told, often without deep understanding of what or why. Such as never use 750 yokes on an 850 frame, cause Roy Bacon wrote it. For the record, I claim no deep knowledge on this topic.

In the last detailed thread I recall, after the best minds were done, there wasn't agreement on the physical differences between the frames and the yokes, the physical differences of combinations of frames and yokes, nor the actual handling results of the different combinations. The notion that slowly emerged (for me anyway) from that thread is you can probably throw anything on anything and it will probably work as well as anything else. The main reason being, as best anyone can determine, all the variations were within specs found on many other bikes that handle well.

It would be cool if somebody produced a table by year that showed the stock frame and yokes with resulting rake and trail.
And then continued the table to show what different combinations of frame and yoke produced in terms of rake and trail.
And the crowning jewel in this table would be a consensus driven column where people with that actual combination commented (and agreed) on how it actually performed. Course it'll never happen.

In the meantime, in my humble opinion, looking to the yoke and frame combination for an answer to handling issues such as mild weaving is problematic. Many stock bikes on this list with correct frame and yoke combinations have handling issues, and there is seldom agreement on how to solve this either (tires, rim diameter, wheel bearnings, spindle wear, head bearings, the list goes on). Never mind tossing into the mix the elusive topic of frame-yoke combinations.

Stephen Hill
Victoria, BC

Re:

ML said:

For example, the basket case I am working on has a 207 stamped frame number, which indicates around mid '72. It just happens to match the motor number as well. But, according to my guru friends (one a former Norton dealer / service agent) the factory had a habit of doing some random acts which confuses things. For example, this frame is definetly a 71 build, which may have been surplus and put into stores. Later on, during a production run it may have been put into assembly and the frame number stamped with its matching engine.

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The frame is not stamped on the "VIN" plate. Highly probable that someone replaced the original 72 frame with a 71 and transferred the plate. Better explanation that the factory did it.

The numbers that are cast onto the underside of the yokes are the casting part numbers. The finished part number does not appear anywhere on any of the yokes.
There are only two different Commando steering geometries, the older with the fork tubes parallel to the steering stem, and the 850 with the tubes angled back.
Of the parallel types, there are the adjustable Atlas-style yokes used on early Fastbacks, the R, and S-Type. These are not interchangeable with the next, non-adjustable pair with the stem fixed in the top yoke. Later versions of the bottom yoke had two 5/16" holes to mount the front fender of the SS and also for the option steering damper kit.
Of the angled types, the first 850 looked very much like the non-adjustable 750 pair but had ANG cast on their undersides for identification. They must be used as a pair and should not be interchanged with the 750 pair. To maintain wheelbase, and clearance of the fender at full 'bump', the frame head angle changed at the same time. All 750 frames have the same head angle; all 850 frames have the same head angle, but different to the 750.
The Mk111 used the same bottom yoke as the non e/s 850, but a new top yoke casting for the Neimann steering lock and bosses for the warning light console.
I may not have said this very well, but I do know from first hand experience what I'm talking about.

Stephen Hill said:

Unfortunately, much of it is people repeating what they have read or been told, often without deep understanding of what or why. Such as never use 750 yokes on an 850 frame, cause Roy Bacon wrote it.

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Poor Roy Bacon, he seems to get the blame for everything! The fact that this information is given in the factory workshop manual is his fault too I suppose?

To help the establishment of the yoke data base for 1975 MK3 models:

Top Yoke: 06-5721 casting number-----ANG casting letters ----M&B casting Letters
Bottom Yoke: 06-4083 " " ANG " " MB " "

These are the casting numbers and letters on my 75 MK3 yokes---just verified---just got them back from the powder coating shop.

I guess the 75's can be now certified as to which yokes are correct for this model/year. ( see replies noted previously )
Now for the 1974 model year. I have several and will check the numbers to post later to help solve this mystery.

303 fmj said:

To help the establishment of the yoke data base for 1975 MK3 models:

Top Yoke: 06-5721 casting number-----ANG casting letters ----M&B casting Letters
Bottom Yoke: 06-4083 " " ANG " " MB " "

These are the casting numbers and letters on my 75 MK3 yokes---just verified---just got them back from the powder coating shop.

I guess the 75's can be now certified as to which yokes are correct for this model/year. ( see replies noted previously )
Now for the 1974 model year. I have several and will check the numbers to post later to help solve this mystery.

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Reasonably accurate offset measurements of both upper and lower yokes supplied along with the casting numbers would be useful.

teeb said:

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Ok L.A.B. here are my measurements on the MK3 yokes;

Top yoke--casting no. 06-5721-----offset is 2.880" from my measuring method. Distance between tubes---7-3/8" ( 7.375" )

Bottom yoke--casting no. 06-4083---offset is 2.660" ~ 2.665" Distance between tubes---7-3/8 / 7.375'

Looks like a bit of rake built in.

303 fmj said:

Top yoke--casting no. 06-5721-----offset is 2.880" from my measuring method. Distance between tubes---7-3/8" ( 7.375" )

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Thanks, that tallies with my own findings for the MkIII upper yokes at least, and the lower yoke offset I didn't know.



There have been a number of previous discussions about yokes, but as yet, we haven't managed to get offset dimensions for each yoke type so it would be nice if we could finally get the full set of measurements as that information could help resolve a few arguments.

Hi L.A.B.

I am starting on the rebuild/restore of one of my 74 850's soon---1 of 2 I have---both are untouched from new as far as I know. I've had them since 1985 or so.
I also have a 73 850 as well--also all original as known.
Ditto for the 750 models I have.
I will record all the specs from these and add to this thread as time allows. Perhaps with other contributions this fork yoke dilemma will be finally set to rest.

Thanks again L.A.B

Sideways knocking increasing in intensity over a period (a couple of days between Easter eggs & goog cheer) applying inox repeatedly did the trick.
I was advised to mark the fork leg relative to the bottom yoke as a precaution otherwise there would be complications later if something was out of whack
Thats all good
I wish I could help with the technical detail in previous comments but that's a bit like greek to me at this stage
No doubt it will prove to be relevant later on

Cheers and hope all had a great easter
Jed