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Drive Train-Commando
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TOPIC: Drive Train-Commando

20M3 Commando drive train 05 Jan 2022 10:04 #2

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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.
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20M3S- Commando drive train 05 Jan 2022 10:12 #3

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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.
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200000 AKA /Combat Breather- Commando drive train 05 Jan 2022 10:51 #4

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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
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200000 AKA /Combat Breather- Commando drive train 07 Jan 2022 09:52 #5

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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???

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300000 Commando drive train/engine 17 Jan 2022 10:34 #6

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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.
Dave Comeau
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Drive Train-Commando 325000 23 Oct 2022 06:36 #7

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Quick and to the point...
The 325xxx NHT used in the S2"E" rolling chassis is a tremendously different variation of NHT with minimal compatibility with the prior 300000 family.
It is easier to list the parts that have carried over from the 828cc series, which is again different from the prior 200000 750cc series.

Head with internals
barrels with lifters
piston, rods
camshaft, timing chain
ignitions points assy

some other components are possibly useable only by re-engineering and machined modification

The primary and gearbox are also vastly different and not easily converted for use on earlier versions. Only the clutch, shafts and gears are carried over from earlier models.
Dave Comeau
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