I have an 88 Samurai with a newly rebuilt engine (1.3)
After it warms up the oil pressure drops low. Anyone know what would cause this?

It depends on what you call low. There will always be a difference between cold and hot at idle. If the oil pressure is like 5-7 lbs at idle this is not good and can be caused by a bad oil pump or loose bearings.

The pressure is like 5-8 PSI. Has new bearings and the oil pump is OK, checked with a feeler gauge. Someone told me a bad bypass spring could cause this

Yes, also, if the bypass valve is stuck open it will have the same symptoms.

Where is the bypass valve located?

It's located in the oil pump assy. Front of motor. If you look in the Factory Service manual (FSM) it will show exactly where and how to get to it. If you need a FSM you can download a copy from here; Ack's FAQ: Search

Ok thanks a lot! Where can I get replacements?

I am not sure but unless you change out the pump you may be able to get a spring and piston from the dealer or someone like Hawk Suzuki. Many times you can just remove the spring and piston and clean them up, clean out the bore and reassemble and your good to go. The by-pass valve sometimes gets stuck when the motor is apart and the pump is cleaned with solvent and the by-pass valve is not cleaned at this time. You may get lucky, also the bore that the by-pass is in will wear to the point that the piston is very loose and will no longer control the oil pressure, in that case the pump must be replaced.

great, thanks for your help!

Tim