51 B Movement of crankshaft?

ndgregor

Member
I was attempting to use a cylinder leakage test kit on my 51 B yesterday. The procedure is to thread an adapter into the spark plug hole and supply compressed air at 60 psi into the cylinder when the piston is at or near TDC. I had the tractor in 6th gear, clutch engaged. When I would begin to build pressure, the belt drive would rotate counter clockwise until a valve would begin to open allowing the air to escape. I would estimate the belt drive would rotate counter clockwise one eighth of a turn.

I would try to hold it in place, but the pressure would get close to 60 psi and I would no longer be able to keep it from rotating back. My question is how can the belt drive crankshaft be able to rotate without the tractor moving? In 6th gear, clutch engaged. I would suspect any movement of the crankshaft belt pulley would result in tractor movement via the wheels. The clutch was just rebuilt so it should not be slipping. Is this normal?
 
Lock your brakes and try with your piston just past TDC. If you test it
with the piston coming up to TDC, you push the piston down causing the
engine to turn backward.
 
Normal backlash in the gears will allow the movement you are describing. The piston needs to be at exactly TDC or it will rotate the crankshaft when pressurized. Also if you rotate the engine one revolution you will not have valves opening right away. On compression stroke it will have to turn 180 degrees before a valve will open.
 
(quoted from post at 04:44:45 04/03/17) I was attempting to use a cylinder leakage test kit on my 51 B yesterday. The procedure is to thread an adapter into the spark plug hole and supply compressed air at 60 psi into the cylinder when the piston is at or near TDC. I had the tractor in 6th gear, clutch engaged. When I would begin to build pressure, the belt drive would rotate counter clockwise until a valve would begin to open allowing the air to escape. I would estimate the belt drive would rotate counter clockwise one eighth of a turn.

I would try to hold it in place, but the pressure would get close to 60 psi and I would no longer be able to keep it from rotating back. My question is how can the belt drive crankshaft be able to rotate without the tractor moving? In 6th gear, clutch engaged. I would suspect any movement of the crankshaft belt pulley would result in tractor movement via the wheels. The clutch was just rebuilt so it should not be slipping. Is this normal?

For a leakdown cylinder check, the rocker arms both need to be completely backed off. Cannot guess where the cam and crank might be, never would know if the test is valid. Set the regulator to 100 psi. Let the piston move until it is at BDC. Read the difference in the gages and that is your percent leakage. Listen in the exhaust pipe for exhaust valve leak, in the carburetor for intake valve leak, crankcase vent or cover for piston ring leak, look in the radiator (full of water) for bubbles, head gasket or crack. 20% or less on an old tractor would be good. Over 50%, something needs attention. Can't make high performance less than 10% out of these antiques. Of course any % with bubbles in the radiator should be corrected.
 
Thanks for all of the advice. I made sure it was at TDC and did not have any problems. I am detecting about 20% on the LH cylinder and 25% leakage on the RH cylinder. The gauge says that is acceptable, do you agree?
 

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