TOPIC: Drive Train-Commando

The 1949 Norton Heavy Twin went through many evolutionary steps even before the 1968 Commando. Both 500 and 600 engine cases had both dynamo/early and alternator/late versions, Then 1960+ both 650 and 750 are alternator only. However all commando 750/850 are the Desaxe version, which is where the piston bore grew only toward the rear to prevent interference problems with the cam/valve train.

These common production NHT engine families are installed into the series 1, 2A, 2B, 2B-ES commando rolling chassis
The engine/primary/gearbox drive train variations DO NOT cleanly change at rolling chassis or year changes.

20M3
20M3S
200000
300000
325000+
factory race shop and production specials not addressed.

20M3: These are a twin chain timing chest carryover from the atlas with minor changes, rear points/coil, and routing the over pressure valve vent back into the oil pump inlet feed.

20M3S: This is where the NHT changed to the single chain timing chest. Ignition is run off the end of the cam. This is the last series to use the thick cam bushings.Twin and single chain are very substantially different and swapping parts require extreme familiarity, skill and modifications. There is even a bit of evolution within this family. This series is actually where Norton was going to put an electric starter. These bikes were out fitted with 6 volt coils in anticipation of an E-start, Trials failed miserably and the project was postponed but the coils stayed..

200000 Should be called "Combat Breather". First of the new thin cam bushing engines. Original state of tune was unchanged with the same 28.5mm port heads, 1S cam and 8.9 compression as early commando. Very quickly it was given 10:1 high compression, hot rod 2S cam and 32mm big ports. I call it "combat tune". Combat tune was quite troublesome and production was reverted to standard tune, and two different heads were used until the completion of this series.

300000 850 (828cc) bigger head bolt pattern, from the 750, and was offered with 73&75 32mm intake ports with 74 having 30mm intake ports, otherwise similar engine package form and fit as all earlier 750 commando engines.

325000 MKIII Electric Start 850 (828cc) "similar" to 300000, but in reality it is very different in MANY ways. Not usable in earlier drive trains OR rolling chassis without a very significant amount of re-engineering.

20 marque 1 and 20 marque 2 engines were utilized in the slimline era, The slightly modified 20 marque 3 was repurposed for the isolastic Commando frame.
NHT 20M3
This variant 20M3 was entirely used in series 1 commando rolling chassis. The drive side case machining was a smooth single flat surface to mate with the cast aluminum primary.
The 20M3 NHT has a new version of twin chain timing chest cover with a carryover from the atlas with cam end tachometer and rear points/coil, with minor changes like routing the over pressure valve oil flow now venting back into the oil pump case feed cavity.

The 20 Mark 3- S.was the next 750 NHT evolution and was slated to be the first Electric Start version. As a result of single chain timing chest conversion to points on the end of cam, and the initial machining of the intended starter mounting hole on the rear of the engine case,


The attempt was almost instantly terminated with frequent blow ups on the prototypes. The "starter prep machining" was still carried over to several thousand 200000 series engine cases. This machining is NOT for magneto or rear points.
You will note the 20M3S cases are the last where they buffed the top of the case above the timing chest to match the timing cover. The timing chest probably did not get stamped with matching case + cover set # like earlier engine versions.

This new 200000 series of 750 NHT cases was a very modified crank case despite the vast majority of internals being mostly carried over from the predecessor. It's design and execution were quite flawed. Initially the standard/stock "state of tune" version was begun.
A very quick "upgrade" high performance version to "COMBAT TUNE" which gained much attention both good and very bad.
Following are the characteristics and production changes and evolution.
The engine case had a new weir on the back internal wall. This blocked direct crankcase venting back up to the oil tank. On extreme wet sumping or high RPM windage oil accumulation, the vent would still flow back to the oil tank bypassing the oil pump.
Further internal oil flow discussion is addressed here: nneno.org/forum/nht-engine-drive-train/711-nht200000-d-d-b

The early 202xxx case also had the "stress riser" on the rear of the cases.



Your 207xxxcases might be after the corrected machining to move the cutter tool pass and remove the stress riser sharp cut???

This change was to a slightly more robust cases. It was a step back to before the 200000 combat breather case disaster. It retained the internal rear oil weir and reintroduced the screened sump pick-up.
The barrels were redesigned to solid "slab side" with long internal allen bolts replacing the visible 750 base nuts. The 73-74 cam bushes are a new thin type and were the best used for all 200000-325000
Very likely the best single chain NHT commando engine.