Interchangability of parts, Commando-Atlas

Hi folks, Aside from the obvious, what parts can be used in an Atlas motor that are used in a Commando motor? The same question goes for the transmission?
The most recent Atlas project has both the motor and the transmission stripped to bare cases which were just filled with Texas dried mud hornets nests. The tennants fortunately had vacated already. The crank shaft is very rusty.
The good news is that I have several options. I can sell it as parts or rebuild as stock Atlas motor or spend a fortune with Jim Schmidt and put together a screaming hot rod antique sleeper motorcycle.
Thanks for the help and input. I have never rebuilt one but with Mick Hemmings dvd, I think it is worth a try. It's already apart so why not at least try it?

I run an Atlas oil pump. Other than the clearance angle cut near the fastener, the internals and externals measured up to be identicle. except for the maring and slop of my original.
Both pressure and return are right on. Only slight rework to accomodate the seal for the MKIII timing cover, which was also required for the old pump (flushing an end cap for the wider surface).

kingdaddyo said:

Hi folks, Aside from the obvious, what parts can be used in an Atlas motor that are used in a Commando motor? The same question goes for the transmission?
The most recent Atlas project has both the motor and the transmission stripped to bare cases which were just filled with Texas dried mud hornets nests. The tennants fortunately had vacated already. The crank shaft is very rusty.
The good news is that I have several options. I can sell it as parts or rebuild as stock Atlas motor or spend a fortune with Jim Schmidt and put together a screaming hot rod antique sleeper motorcycle.
Thanks for the help and input. I have never rebuilt one but with Mick Hemmings dvd, I think it is worth a try. It's already apart so why not at least try it?

Click to expand...

Generally all early 750 Commando engine parts could be used in an Atlas engine. Oil pump gears on Commando are 6 start, while Atlas is 3 start giving much more oil pressure with the Commando pump...not desirable in an Atlas. Commando crankshaft has a different balance factor that will increase vibration in a rigid mounted engine/frame - not disirable in an Atlas. If you have the internals out of the Atlas gearbox, all of the Commando internals will fit in the cases. Not sure about the mainshaft with regard to clutch fitment as these are quite different between Atlas and Commando. Outer gearbox case for the Atlas has the clutch cable boss in a different position than Commando.

Thank you for the information. I guess I thought the Crank shaft might be a direct swap. The piston rods look like they got hot as they are discolored towards the top near the wrist pins. Maybe a problem with oiling? Will the Commando Rods work in the Atlas?
This may be a little more ambitious a project than I expected. The pistons are 20 over hepolites and appear to be ok.. Clearly the top end was rebuilt at some point.

i use Atlas AMC shell in my Combat w/o mods except missing the stupid spacer. The Atlas pinon idler gear is more robust than a Commandos the racers chew up.

Can I use a Combat head on regular 71 750 cylinders? I have a C head but will only use it as a back up provided it fits.

BC

You can use the combat head on regular commando barrels. Because the head is shaved .040" you should install slightly shorter push rods from a combat engine or cut yours down. The "C" head has 932 carbs (32mm intake ports).
Atlas heads and barrels are different because they join with a spigot join. It's ok to use commando barrels and head on the atlas case. You can use the commando con rods. They are drilled to splash oil up to the sides of the cylinders but if you use the atlas big end bearings that are not drilled to link to these oilways it will be the same as running atlas rods. If you use commando big end bearings that are drilled for the oilway I would use the commando 6 start oil pump too.

Note the Commando rod oil hole piston jets are useless feature mistaking done when seizures occured but turned out to be bad patch of pistons but Norton had ordered all the rods with holes. Rod big ends self limit oil flow so only the head flow needs restricting which is done in Cdo's by installing rocker spindle notch away from the head oil feed vents and intakes get valve seals. Racers orient the bearing shells to close off rod jet holes as not enough to cool piston and no need for piston pin lube.

Stillreel said:

Can I use a Combat head on regular 71 750 cylinders? I have a C head but will only use it as a back up provided it fits.

BC

Click to expand...

yes you can use this combination.....the .040" shorter head is only HALF the reason to use shorter push rods...the high lift combat cam is the other half (2/3 actually ) of the reason....It seems not many people realize this... look at the intake lift and multiply by 1.13 http://atlanticgreen.com/images/cam111244.gif
Since you will not have the combat cam you might also not put longer push rods in.

With this combination you will potentially have pinging problems unless you're an expert on custom ignitions to avoid the detonations....

edit/correction..... longer to shorter push rod.......good catch

Generally all early 750 Commando engine parts could be used in an Atlas engine. Oil pump gears on Commando are 6 start, while Atlas is 3 start giving much more oil pressure with the Commando pump...not desirable in an Atlas. Commando crankshaft has a different balance factor that will increase vibration in a rigid mounted engine/frame - not disirable in an Atlas. If you have the internals out of the Atlas gearbox, all of the Commando internals will fit in the cases. Not sure about the mainshaft with regard to clutch fitment as these are quite different between Atlas and Commando. Outer gearbox case for the Atlas has the clutch cable boss in a different position than Commando.[/quote]


Pre 66 Atlas had a smaller oil block and smaller oil holes in crankcase, they were changed in 66; and post 66 bikes, I believe, had 6 start worm, but I could be wrong on the last bit.

Hey Dyno, don't you mean shorter push rods? The other thing to note on the 2s cam is it has a .021 larger base circle. All of this, shaved head, higher lift, bigger base circle will have a tendency to open the valves unless you compensate by shortening the push rods.

cams....2 hot rod cams I have are base circle cut downs.
I have probably only tested 4 combat cams(long time ago) . Just coming back in from the shop....I've got 24 cams on the shelf outside of what are in motors. I have not found any of the gear tach drive cams to have bigger than aprox .895" base circle.... including my factory original (202206) combat cam. The next/only noticeable base circle step-up is going back to old atlas or early commando (pre "S") with .910+ base circle, but nothing bigger at all up to the last of 75's.
Always interested in new stuff.......
How many of these factory original big base circle 2S cams you got ????

Don't mix Commando and Atlas timing covers & gasket - the oil ports are not compatible.

Mick

dynodave said:

cams....2 hot rod cams I have are base circle cut downs.
I have probably only tested 4 combat cams(long time ago) . Just coming back in from the shop....I've got 24 cams on the shelf outside of what are in motors. I have not found any of the gear tach drive cams to have bigger than aprox .895" base circle.... including my factory original (202206) combat cam. The next/only noticeable base circle step-up is going back to old atlas or early commando (pre "S") with .910+ base circle, but nothing bigger at all up to the last of 75's.
Always interested in new stuff.......
How many of these factory original big base circle 2S cams you got ????

Click to expand...

Regarding the base circles, thanks for your wisdom Dave, I was going by the technical notes. I have 2 -2S cams at .892 and one at .887. The widest was a non geared cam out of a 68 commando and it was .898.