Gearbox rebuild problems (2014)

timsha said:

Would you buy both gears even when the 2nd gear lay is what one would describe as almost unused condition...i personally don't think these gears have seen much action at all...my suspicion is that were dealing with an aborted rebuild...and since then the bike has just sat there.

I will get on to Andover Norton tomorrow and get dimensions of the 2nd gears that they're selling before i order anything.

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It's down to you what you decide, as long as you ultimately end up fitting a matched pair of 2nd. gears, which is more important than "dimensions". So you need either the 04-0418 & 04-0019 18/24 standard pair, or the 06-4639 & 06-4640 18/23 high ratio "grooved dog" pair.

Just for the record, I believe that the change to this gear set (changing the second gear ratio so that the Commando could pass noise tests more easily) came in in September, 1973. The first 850 serial 300001 was built in April of 1973, the gear swap was serial number 306591, which is identified as the serial number for the first 850 Mk1A. Since gearboxes were not specifically built to exact serial numbers, I think it's likely that some 850s prior to 306591 had the later gears and also that some after that serial number had the earlier type (but care would have been taken that only the later type was used in the 850 Mk1A). (Oddly, despite building over 6500 Mk1 and Mk1A 850s in six months from April - September, 1973, there were only about 720 built from September, 1973 until the end of December 1973; the first 850 Mk2 was built in early January, 1974 with serial number 307311.)
Please note that some sources call the original gear ratio parts "1973" and the later, amended gear ratio parts "1974" but this does not appear to be strictly correct. Records show that the gearboxes with the higher gears were built into 850 motorcycles for three months or so of 1973. The amended-type gearboxes were also used in Mk2 (and 2A) gearboxes in 1974 and continued into the production of 850 Mk3 motorcycles starting in early 1975.

Besides the gear change, the Mk1A had the "black plug" silencers and an early version of the plastic air filter box -- all of this was to meet the rapidly- changing more stringent noise tests that were required in markets around the world, starting about 1973.

Since the last 750 Commando is recorded as 230935, built in October, 1973, it's likely that a small number of very late 750s were built with the higher-gear second gear pair (note - we know that there were a few "ghost" 750s built with 235*** serials numbers but I have no dependable information on these). As has been noted, when trouble-shooting a Commando gearbox or replacing gears, it is vital that the second gear components (layshaft and mainshaft) must match as regards the "circular groove" markings machined into the dog ends of the individual gear. Both must have the groove or both must be the type without the groove.

The plan was that the new gear ratio-type gearboxes would come into production and from that date, all 850 Commandos (and a few 750s) would have that type, regardless whether they were the "A" variant models or not. Of course, there is no way to tell what parts might be inside any Commando gearbox today as many changes could have been made in the field during the past nearly (or more than) 50 years.

Blewdy Yaink said:

Just for the record, I believe that the change to this gear set (changing the second gear ratio so that the Commando could pass noise tests more easily) came in in September, 1973. The first 850 serial 300001 was built in April of 1973, the gear swap was serial number 306591, which is identified as the serial number for the first 850 Mk1A. Since gearboxes were not specifically built to exact serial numbers, I think it's likely that some 850s prior to 306591 had the later gears and also that some after that serial number had the earlier type (but care would have been taken that only the later type was used in the 850 Mk1A). (Oddly, despite building over 6500 Mk1 and Mk1A 850s in six months from April - September, 1973, there were only about 720 built from September, 1973 until the end of December 1973; the first 850 Mk2 was built in early January, 1974 with serial number 307311.)

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Funny you mention that 2nd gear set with the same subject being in a recent FB post.

You were there, what was going on I ask.
My Mk2A stamped DEC 73 had that altered gear pairing but how could there be Mk1A's and Mk2A's being made in the last few months of 1973 (There are Mk2A members here with bikes stamped SEP73 to DEC73)
The internet says the MK2 and Mk2A were not made until 1974.

Could the Mk2A's with these 1973 stamps actually be unused Mk1A frames that had the late 1973 stamp on the already affixed steering head data plate but engine/gearbox number box unstamped and were then used in 1974 for Mk2 and Mk2A's then getting the RH10 engine etc with the odd date stamp.

It would be totally illogical to be building Mk1 A and Mk2 A models side by side.
I do not know how Mk2 bikes fit into that time period of late 1973.

I replaced the 2nd gear set with new A parts but did consider using the previous ratio. (1968 to 1973)

Blewdy Yaink said:

Just for the record, I believe that the change to this gear set (changing the second gear ratio so that the Commando could pass noise tests more easily) came in in September, 1973. The first 850 serial 300001 was built in April of 1973,

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850 production began in January 1973. This is known from Andover Norton records...

ZFD said:

Numbers 300.000 through 300.021 bar # 300.014 which went to Berliner/USA were booked to Experimental, starting 25-10-72, against some it says "No Date", next date is 4-2-73, but #300.020 went to Bob Trigg on 23-1-73.
The regular despatches start with 300.022 produced on 22-1-73 to Berliner and real production then started on 24-1-73.

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...and the official 850 model release date in both the USA and UK was the 7th of March 1973, therefore, production would have been well underway to have had stock in the USA by that date.
MCN, 7th March 1973:
"TODAY (Wednesday) Norton launch their new Commando in this country and America...the 850 Commando..."

Blewdy Yaink said:

...the gear swap was serial number 306591, which is identified as the serial number for the first 850 Mk1A.

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Yes. The high-ratio 2nd. gear pair are listed in the Mk1A supplement, therefore, identifying an alteration to the previous Mk1 specification.

Blewdy Yaink said:

Since gearboxes were not specifically built to exact serial numbers, I think it's likely that some 850s prior to 306591 had the later gears and also that some after that serial number had the earlier type (but care would have been taken that only the later type was used in the 850 Mk1A).

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There's no evidence that the high 2nd gear was fitted before 306591. The Mk1A model was for the European market to comply with impending European noise regulations. Some Mk2As, however, were sold in the USA later in '74 (all JPNs were Mk2As so arrived earlier).

Blewdy Yaink said:

(Oddly, despite building over 6500 Mk1 and Mk1A 850s in six months from April - September, 1973, there were only about 720 built from September, 1973 until the end of December 1973; the first 850 Mk2 was built in early January, 1974 with serial number 307311.)

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You appear to have fallen into the usual trap where "build" and "model release" dates have become confused so Mk2/2A production began well before the January 1974 model release date.

Blewdy Yaink said:

Records show that the gearboxes with the higher gears were built into 850 motorcycles for three months or so of 1973.

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Yes, (or actually longer) because Mk2, and Mk2A production didn't begin in January '74.
Serial number 307091 is given in the May 1973 Service Release N3/55 (introduction of the chainguard extension) so they weren't far off the beginning of Mk2/2A production (307311) at that time.

Blewdy Yaink said:

Since the last 750 Commando is recorded as 230935, built in October, 1973, it's likely that a small number of very late 750s were built with the higher-gear second gear pair (note - we know that there were a few "ghost" 750s built with 235*** serials numbers but I have no dependable information on these).

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There's no evidence I'm aware of that 230935 was "built" in October 1973 as they were building '230' series 750s either during or before March 1973 as Service Release No.N3/39 dated March 1973 mentions batch 230536 - 230685 having been built by then.

Blewdy Yaink said:

Besides the gear change, the Mk1A had the "black plug" silencers and an early version of the plastic air filter box -- all of this was to meet the rapidly- changing more stringent noise tests that were required in markets around the world, starting about 1973.

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Yes, but in Europe at that time which is why the 850 Mk2 models with the perforated airbox and peashooters etc. were sold on the US market.

Time Warp said:

My Mk2A stamped DEC 73 had that altered gear pairing but how could there be Mk1A's and Mk2A's being made in the last few months of 1973 (There are Mk2A members here with bikes stamped SEP73 to DEC73)

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Because Mk2/2A production didn't begin in January 1974.
Both Mk2 and Mk2A had the higher ratio 2nd gear from 307311 as listed in the Mk2/2A supplement.

Time Warp said:

The internet says the MK2 and Mk2A were not made until 1974.

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The "Internet" (Facebook?) is wrong as the available Mk2/2A data has been interpreted incorrectly to mean "build", not "model release". The January 1974 release I believe could have been for the Mk2A not necessarily Mk2 as that model could have been released earlier in the US because the Mk2 was the US model and the first (approx. 1,000) built.

January 1974 edition of Motorcycle Mechanics (so the Mk2A test would have been some weeks earlier) reprinted in Norton Commando Gold Portfolio:
"Norton introduce the uprated mk2A Commando". ROU 15M was registered in November 1973.


The January 1974 amendment in the Mk2/Mk2A supplement regarding the change of chainguard and brake plate with removable rubber bungs for checking the brake linings to comply with US regulations introduced from that date indicates Mk2/2A were in production before January 1974.

Factory records for 305550 - 317847 no longer exist, therefore, wouldn't have been available to Roy Bacon and where "January 1974" probably comes from old copies of weekly and monthly motorcycle publications (MCN and The Motor Cycle, Motorcycle Mechanics etc.).