New lister CS SOM project ..Thorghts

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i get a lister cs from the bay 6month ago and just managed a look at it ..it was sold as a 6/1 with a 4.5kva generator and not a SOM ...alarm bell was ringing as i know you can not get 4.5 kva from 6hp but maybe it was to start say a 3 hp motor under load or something ..anyway it is a 4.5 kva and it IS a SOM but with a bit of linkage missing from the solenoid but other wise complete ..the engine is a 5/1 at 6hp at 650rpm made in 1952 according to the SN
The engine has had a rebuild at some time as the big end is +0.10 but its gone(i can make it clunk just rocking the flywheels)
Having measured the size of the flywheels and the pulley size on the gen it only take simple math in my head to determine the RPM of the engine needs to be 800 to 850rpm for it the run the gen at the needed 1500RPM....so i bit of a miss match
So i see it i have 3 option
1 rebuild the engine as a 8/1 and run it at 850rpm ...this is where i need help.. is it possible to do that? i know injector pressures are much higher
2 rebuild as it is and change the pulley so can run at 650rpm and only use 3kv.....which is a wast as i could do with mooooore POWER as jeremy clarkson say
3 buy a 8/1 for it but they seam a bit thinner on the ground

i have a 4th option but that involve another 6/1

Having found a brass plate on the base plate the original engine indeed was a 8/1 of 1961