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Since the 1977 model serial numbers start at: 9A254045, and the 1978 model numbers start at: 9A276935, my best guess would be that your MF-285 is a 1977 model. Source page: http://www.tractordata.com/farm-tractors/000/7/7/775-massey-ferguson-285.html
A Perkins 4 cylinder diesel engine comes standard with a Massey Ferguson 285 tractor. Other engine specifications include: 2000 RPMs, 17:5:1 Compression, 4.5 in x 5 in Bore/Stroke and 5.2 L displacement. The engine has a Measurement of 12.5"W x 16"H. You can read about this at http://www.alibaba.com/product-free/104938109/Massey_Ferguson_MF_285_tractor_gear.html
You can purchase a manual for your tractor online. The online store is located here: http://www.ssbtractor.com/tractor_manual_display.cgi?m=Massey%20Ferguson&o=MF%20285.
to the left of the gear lever is a cap with a 1 1/16 bolt head on it. remove that cap and fill there. To check the level the is a small dipstick under the lift on the frame.
This model is an older model and to buy one doesn't require too much money and would not be considered an expensive tractor. The general price for this model now would be between $5,000-$10,000 which really isn't too much.
There are many online used tractor part websites, vintage tractors are becoming more popular. I suggest http://www.tractorpartsinc.com because they have a substantial selection of Masey Ferguson parts.
You slide down the the bottom half of the front the bit with the see through window until it is clear of the unit the batteries can the be removed and replaced. then re slide up to close
5% of 285= 5% * 285= 0.05 * 285= 14.25
285
1x285=285 3x95=285 5x57=285 Need any more? :)
570
20 out of 285 or 20/285 = 7.0175%