600 and 650 and SS and Atlas engine differences

pierodn

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Nov 10, 2012
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Hi All.
Please, can you explain the main differences of these engines.
Not the barrells and the heads (i know) but about the crankcases, cranckshaft, cam, conrods.
Thank you.
Piero
 
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Model 99
Displacement 597 cc
Bore 68 mm
Stroke 82 mm
CR 8.2:1

Model 650
Displacement 646 cc
Bore 68 mm
Stroke 89 mm
CR 9:1

Model Atlas
Displacement 745 cc
Bore 73 mm
Stroke 89 mm
CR 7.5:1

Inlet valve head diameter: Atlas 1.50", others 1.406"
Exhaust valve head diameter: All 1.312"

Edit: typo corrected. Actually, cut and paste gone awry ....
 
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texasSlick said:
Model 99
Displacement 597 cc
Bore 68 mm
Stroke 82 mm
CR 8.2:1

Model 650
Displacement 646 cc
Bore 68 mm
Stroke 89 mm
CR 9:1

Model Atlas
Displacement 745 cc
Bore 73 mm
Stroke 82 mm
CR 7.5:1
Click to expand...
In addition, Atlas came with a end of camshaft timed breathing, Competition magneto as standard, as well as other differences.
 
Almost correct, except that 650 and Atlas has same stroke, 89 mm.
The 600 has smaller big end journals.
The flywheels are different on 650 and Atlas, possibly only balance factor different.
Same conrods on 650 and Atlas, different on 600.
650 and Atlas crankcases different, due to bigger bore. They had to move breather pipe. Also different studs.
600, 650 and Atlas, all had different pistons.
Easiest way to find out is checking parts lists.
Could have missed some.
 
Where does the Mercury engine fit into this? (1968-1970).
The cases are stamped 650SS, but I keep finding references to how elements of the Commando bottom end were introduced. But I am not sure what.
 
Stephen Hill said:
Where does the Mercury engine fit into this? (1968-1970).
The cases are stamped 650SS, but I keep finding references to how elements of the Commando bottom end were introduced. But I am not sure what.
Click to expand...
The basic NHT castings are the same from 650 1960 on. This may not consistent for 500 or 600 based on production rumors.
The basic cylinder deck machining is similar.
The front 5 studs are positioned the same for both small barrel and large barrel engines. On the large barrel (desaxe 750/850) 4 rear studs are moved back about .085".
The case opening is bored to suit the 750 or 850 barrel spigots.
20M3S, 200K, 300K, 325K engines are all post small bore engine and not suitable as such.

On Anna assertion that there is desaxe on 650, I do not believe it, but so far only have measured 500 and 600 barrels resulting in .004" center difference which is easily explaned by OEM and rebore tolerance and not due to desaxe ..
 
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You can use 650 pistons in the 99, but not 99 pistons in a 650 unless you cut the skirts.
 
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