Hello I have a 1984 isuzu medium size cab over with a 3.9 and dont have a repaire manual. I replace the head gasket and replaced the turbo but I have not been able to find the adjustment proceedure any help would be appreciated.

You'll need a 15 thou (0.4mm) feeler gauge, a flat head screwdriver, 10mm ring spanner, a flat surface and enough tools to take the rocker cover off.

It's also done when the engine is cold.
Take the cover off, only do cylinders where you are sure the valves are completely closed.
place feeler gauge between the rocker and the valve stem, place ring spanner on 10mm locknut and flat screwdriver in the slot on the top.
Crack the locknut with one hand and set the gap by turning the screwdriver, tighten the locknut and check.
Repeat as necessary.

Repeat that on all the valves you know are closed, then put the truck in top gear and push it to turn the engine over. When it's closed the valves that were previously open, you can do those too.

My 4BD1T is usually a lot smoother and quieter when the valve clearances are done.

Which turbo did you replace and whereabouts are you?
I'm looking for parts for the japanese IHI-RHB6 turbo.

HI DOUGAL HERE IS THE PROCEDURE FROM THE SHOP MANUAL , NO MORE ACCURATE THAN YOUR WAY BUT IS A LITTLE EASIER WHEN YOU MAY NOT BE FAMILIER WITH THE ENGINE
TURN THE CRANK TO TOP DEAD CENTER ON NO ONE CYL BOTH INT AND EX VLVS SHOULD BE CLOSED ROCKERS WILL BE LOOSE

ADJUST INT AND EX ON NO 1 CYL
ADJUST INT ON NO 2 CYL
ADJUST EX NO 3 CYL
ROTATE CRANKSHAFT ONE COMPLETE TURN BRINGING BACK TO TOP DC

ADJUST INT AND EX ON NO 4 CYL
ADJUST INT ON NO 3 CYL
ADJUST EX ON NO 2 CYL

FOLLOW THE REST OF DOUGALS PROCEDURE AND YOU SHOULD BE FINE AND I WOULD READJUST THE VLV A T ABOUT THE 1000 MILE MARK OR AT LEAST CHECK THEM .. JERRY

Thanks Guy's I wasn't sure I thought I could get by with one up and one down untill I got them all but wasn't sure. Im loacated in North Carolina,USA What parts do you need Dougal? Thanks again...Cal

I'm looking for a better compressor wheel and maybe matching housing. Mine got eaten by dust from a dodgy air cleaner.

Oh and for Jerry. What's the best/easiest way to know #1 is at TDC?

Hello. A little warning here with the 3.9 engine.....I have found several that the "timing hash mark" on the front pulley to be "slipped" out of the correct timing position...........I now always remove the center crank bolt to see the crank key way.........I bring the key way to a 12 o clock position and then compare the hash mark on the pulley to the TDC mark on the cover............

regards,
crossbones

TO BE ABSOLUTE CERTAIN FOLLOW JAMES INSTRUCTIONS BELOW BY LOOKING AT THE KEYWAY IN THE CRANK HUB, IF THAT IS ALL OK AND THE MARKS LINE UP, TURN THE ENG IN A CLOCKWISE MANNER AND HOLD YOUR FINGERS ON THE INTAKE AND THE EXHAUST ROCKER TIPS JUST OVER THE PUSH RODS WHEN THE INTAKE VL STARTS TO OPEN ON THE INTAKE STROKE THE EX HAUST VL WILL BE CLOSED AND AS THE INTAKE VALVE GOES ALL THE WAY DOWN AND BACK UP IF YOU PAY CLOSE ATTENTION JUST AS THE INTAKE IS COMPLETELY CLOSED THE EX HAUST WILL STILL BE CLOSED KEEP TURNING THE ENG UNTILL THE EXHAUST OPENS AND THEN CLOSES AND JUST AS THE EX IS ON IT S SEAT THE ROCKER SHOULD BE AT REST IF YOU TURN THE CRANK ANY FARTHER THE INTAKE WILL START TO OPEN AGAIN, AT THIS POINT YOU SHOULD BE AT TDC BOTH VLV ON THEIR SEAT AND THE ROCKER S WILL BE LOOSE . NOW CHECK THE TIMING MARKS EITHER ON THE CRANK PULLEY OR ON THE CRANK HUB MARKS THE CRANK HUB MARKS MAY BE FORWARD OR BACKWARD JUST A TINY BIT , SO JUST MOVE THE CRANK A LITTLE TO LINE UP THE MARKS, , USUALLY AS JAMES MENTIONED IF THE CRANK DAMPNER DOES OR HAS SLIPPED IT MAY BE OFF 2-3 INCHES , I HAVE SEEN THEM ON THE 2.6 GAS ENG BE SLIPPED A FULL 90 DEGREES .. JERRY