DB780 Hydraulic Pump Outlet Diameter.

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DB780 Hydraulic Pump Outlet Diameter.

Post by Richard » Fri Jan 11, 2019 8:12 am

I am the carer of a DB 780 with an unknown history, white bonnet, s/n 780'1'604291's. The hydraulic pump was noisy and the pressure poor, so I removed the PTO housing and found the pump bearings worn and the straight tooth spur gear and helical cut idler damaged. My guess is that the entire PTO housing was replaced with that from a tractor having a helical cut pump gear train, but that the straight cut spur gear on the pump was not changed at the time.

I need to replace the pump, gear and the idler gear, so I purchased second hand straight cut spur gears and a pump from I believe a DB 995 tractor. I was also given a rusted and slightly damaged helical idler and pump gear.

When I tried to install the replacement pump it would not slide onto the high pressure outlet fitting. On the old pump the HP outlet bore was about 0.84” (21.34mm), the replacement is smaller being only 0.812” (20.62mm).

While I work out what to do, I have reverted to the old pump, now fitted with all helical gears, but still with poor hydraulic oil pressure.

Apart from straight or helical cut drive gears, I see only one standard pump being listed in the spare parts lists. That item is listed as fitting models 770, 780, 880, 885, 990, 995, 996, 1190, 1290, 1194, 1294. David Brown part numbers; K962635, K918995, K928575, K962688.

I cannot tell if it is the pump or the pressure pipe that is non-standard. I could replace the big oil pipe, which might be quite involved if the diff must come out to do that. I could bore out, or replace the pump. I don't want to shuffle the pump bearings and body between the different pumps as there are too many possible differences.

So why do the pumps have different outlet bore diameters?
What outlet bore diameter might I expect on a new pump?

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Re: DB780 Hydraulic Pump Outlet Diameter.

Post by db2d » Sun Jan 13, 2019 1:45 pm

I have measured the outlet bore on a pump from an 885. 0.800 inches.
I will do some research on new pumps

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Re: DB780 Hydraulic Pump Outlet Diameter.

Post by philedge » Mon Jan 14, 2019 9:38 am

The outlet on the spare 880 pump I have is 20.6mm or 13/16 inch. There was a change of pump manufacturer on the 880 in Feb 67, but I think the old and new pumps were interchangeable so there wouldnt gave been a change in outlet bore??
'66 880 Selectamatic rat.

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