Excuse my ignorance in advance I am totally new to these engines, John could I elaborate on the points you've made? I would like to do as much as possible myself, obviously things like crank balancing which I will certainly want to do I will have to outsource.
If you want to keep the Atlas from so called blowing up, then be very conservative, in how you build it.
Get the Paul Dunstal tuning book, as this has allot of upgrades that can be used on the Atlas engine.
Yes I would like to do a very conservative bulletproof build, the upgrades I'd want to do would be for the sake of reliability primarily, I don't need to squeeze out any extra power from the already stressed and overpowered unit! Does Dunstalls book cover this or is it (as I imagine) primarily concerned with racing and performance tuning (at the expense of reliability)?
This means the drive side crankcase's were machined differently to the standard Atlas drive side crankcases. There was also a large spacer that was used between the crankcases and the inner primary chaincase. This means the standard featherbed Atlas engine will not fit with the alloy primary chaincase into the AMC duplex frame.
The G15 Atlas engine has three holes, and the featherbed one has six holes.
So I understand that the G15 crankcases will probably fit the earlier G12 primary chaincase? And that the Atlas crankcases were probably modified to fit them?
Get Brightspark to do a magneto for you, and your ignition problems will be solved.
Unless I'm mistaken for the sake of reliability I'd prefer to go to a strong Alternator setup without the mag. What did they do later with the former Mag placement on the engine, did they just block off the port/hole to the crankcase? I'd also like to go to an electronic ignition.
For any engine up grades, the oil pump is perfectly adequate, but you can also buy the later Commando one with the same size feed and return gears.
I would want to do this, so the Commando one would be an upgrade right? And thats from a 750 combat?
This used longer valves and shorter push rods. The change of ration gave another 1 to 2 bhp power increase. Use Commando valves and pushrods with the chrome valve stems.
This all bolts on over from the Commando to the Atlas/G15 engine? Would it stress the engine more, if so I may stick to the older valves, I was thinking about bronze valve guides as well.
Use the later oil modifications. Six start oil pump gears, oil holes in the conrods, better oil control pistion rings, larger oil feed holes in the crankcases, and the change to smooth rocker arm spindles.
I didn't quite understand this part, the six start oil pump gears (what/where is this?), oil holes in the conrods (is this a mod or stock on the commando/combat conrods), piston rings, larger oil feed holes (is this a modification to the crankcases or again something that came in the later Atlas/G15 cases) and the change to smooth rocker arm spindles (again is this a later upgrade on the commando/combat engines or a case of an aftermarket upgrade or polishing etc.).
Is it a case of sourcing all these parts for a commando/combat engine and retrofitting them to the atlas head / crankcase etc.
Do not under any circumstances use the first Atlas issued conrods,as these can break. Used the later or Commando type. If you are worried about these, get them polished and shot peened as this is all the factory did when they raced this engine, or go and buy Corrello steel rods from America.
Use the later Commando head gasket with the flame ring, and get the crankshaft dynamically balanced. Change the oil regularly, and use SAE 50 grade oil, add a spin on oil filter kit to it.You can also use the later Commando barrels as the 750 ones were heat treated and were much stronger than the earlier type. -)
So from what you've said a lot of the original Atlas engine can be swapped out/discarded. If I read you correctly, I can use the cylinders from a 750 combat and they are identical in appearance and fit, just materially superior? The crank, is this also interchangeable from a Combat in which case I should use a Combat 750 crank and conrods and lightweight pistons (any idea from where?). Internal head parts such as rockers, valves, rods can be swapped over from a 750 combat?
Lastly the G15 gearbox, clutch basket etc, was this originally directly sourced from a G12 and if so can it bolt over from a G12 onto the G15, likewise are there any upgrades to the gearbox that can be done? Anyone tried a 5th gear or is this wishful thinking?
Slighly off topic, it's the G15 I'd like to build and lightly modify, one thing I'd like to do is to drop the front a few inches as it'll be primarily for road use and the bike is quite tall, has anyone tried this is this a good idea at all? I imagine it would reduce rake and increase the flickability (and instability) of the bike. Are the original G12 teledraulics shorter than the long roadholders used on the G15 CS?