leak down test procedure

[TeddyG]

I tried a leak down test on my 750SF2, but was unable to lock down the motor to prevent the pistons from moving at 100 lbs pressure, so what’s the best method- putting the bike on the side stand and then putting the front wheel against a garage wall? Trying to use a breaker bar on the crank bolt doesn’t work.
Thanks

 

[piranha-bro2]

. Trying to use a breaker bar on the crank bolt doesn’t work.

Why not? Should work with the breaker arm (and extension?) is locked against the footpeg. If the nut is undoing , then can't you set it so that the breaker arm is on the opposite side of the foot peg? I've never done a leakdown, but do the pistons have to be at TDC?

 

[TeddyG]

Yes, the pistons need to be at TDC and 100 lbs of pressure is hard to counter w a breaker bar, so I’ll try locking the motor w the front wheel pressing against the garage wall w bike on the side stand and in gear. I want to do the leak down test to determine the state of the motor after seeing those wear marks on the cylinder that I showed you. Wolfgang said there should be no cylinder marks nor the uneven compression I’m seeing after the top end rebuild, and thinks the marks are from either too loose or too tight rebore. He asked if I’m hearing piston slap, and I am hearing a sort of metallic “pinging” near the top end, but I thought it was valves.

 

[Exmoor beast]

If your on TDC compression then you won’t need to lock the engine- your obviously not on TDC, your just near. Never had to lock an engine to do a leak down.

 

[motoddrob]

Don't rely on the case/starter clutch marks for TDC unless you have checked them with a dial gauge or piston stop. I found mine was 5deg retarded

 

[Dellortoman]

Mr Beast is right. Get it close enough to TDC and it'll stay put.

But if you want to check leakage with the piston at other positions in the bore (bore wear is generally more in the middle of the stroke where the side load on the cylinder wall is greatest), just lock the crank with a some kind of wrench on the primary drive nut.

I don't know why your breaker bar on the crank nut (primary drive side) isn't working. 100psi on the piston won't generate enough torque to undo the nut. But just to be sure, I'm a great believer in doing the arithmetic. In this case it's simple enough. You've got 100psi on an 80mm dia piston. With the crankpin at 90° to the cylinder bore, the lever arm on the crank is 37mm (half of the 74mm stroke). Do the arithmetic and you get a torque of 128 Nm or 94.5 ft.lb. If that unscrews the nut on the primary drive end of your crankshaft then it was too loose to begin with. But if you're worried about undoing the nut, just make sure the crankpin is toward the front of the engine. That'll tend to turn the crank in the normal direction of rotation and will tighten the nut rather than loosen it.

 

[Exmoor beast]

Leak down is great for finding a leaky valve that won’t show up on a compression test but not going to tell you if it’sbeen rebored too tight .
You’ve got a marked bore, that’s not right- it’s rattling, that’s not right. Time to take it apart.