The Stovebolt Site Forums The Shop Area Engine Shop 402 into a '86 Silverado

Hey guys,

Helped a friend pull the 305 out of his silverado, and drop in a '71 402 we pulled out of a '73 truck. It was almost a bolt-in swap except for the d-shaft.

The 402 runs cool, has good oil pressure & doesn't smoke, but it has 50 PSI of compression on the #2 cylinder. We checked compression on #6 and it had 125 PSI.

Would this be valvetrain related? The only other thing I could think to mention is for some reason, the fuel pump pushrod isn't moving. So we used an electric fuel pump.

What do we need to look at?

thanks
chip

Likely to be a worn cam lobe, but bad valves and or damaged rings in a cylinder is a possibility.Pull the valve cover and watch the valve action.You can add a little oil to the bad cylinder just as you test the compression to see if it temorarily seals the rings.Its better to do a leakdown test, you can hear the air escaping past the valves or rings.

We're hoping its a bent pushrod or flat cam. The donor sat up for a year before it was given to my buddy. The guy thought it was a worn out SBC, was he pi$$ed when we ran the numbers.

Since the mech fuel pump doesn't work, my money is on a bad camshaft. The motor appears to be bone stock except for an edelbrock carb and long-tube headers.

Questions- If its a flat cam, should we pull the heads and have them reworked? Should we flush the motor with diesel to remove any metal from a failed cam? If the cam is flat, are we looking at rebuilding the top of the motor? I know my buddy doesn't want to pull and rebuild the entire motor.

thanks

chip

Take an airhose coupler and a spark-plug with the porcelin guts removed(be careful, it will make VERY sharp chunks). Solder the airhose coupler to the metal base of the spark-plug.
Set your piston at TDC for the cylinder you want to test. Install the fitting you just made in place of the spark-plug. Hook up the airhose to the fitting and listen for air at the carb(intake valve leak), the oil fill(rings leak)or the tail-pipe(exhaust valve leak).

A flat cam doesn't happen over night. Flushing the engine should not be neccesary. Check the oil for metal. If it is a cam, check around for a good replacement with matching lifters and go with it. Joe

I agree with Mech but I would suggest that you buy the part that he suggest you build. A good parts store will have an air hose spark plug fitting and they will cost you less then $10.

Good luck

Oh yeah. remember to put the truck in gear or park with the working parking brake engaged, and keep your hands away from the fan. The compressed air will rotate the engine.

What usually causes a low compression reading in a motor like this? No idea of miles, but it seems to be stock and well taken care of.

I went and worked on this engine last weekend. But I'll be diagnosing it over the phone. If vinnie can't find damage in the upper valvetrain, I should have him pressurize the cylinder and listen for leaks then?

I've seen the air chuck tool that screws into the cylinder. Where can vinnie get one? There is a local NAPA, Autozone, Advance, and O'Rilies (sp?) local to him.

thanks

chip

I would go with opinions above on valves or rings but there could be many more. Like blown head gasket, cracked cylinder head or cracked valve seat. One of the hydraulic lifters could have lost it's seal or almost gone. Does it have a lot of clacking noise from that side? If the pressure test is done with the air put water in the cooling system then have your friend check the radiator at the same time for bubbles. Also on the fuel pump is the rod actually not moving or could it have to big a spacer between the block and pump? It doesn't take much to reduce the action on that lever.
Anyway just a few other things to think about. Those 396(402)'s are pretty nice engine's to have and getting scarce.

Found part of the problem: The #2 exhaust rocker adjusting nut had backed all the way off. Will post when we get it screwed back together.

chip

Come on now Chip, the rocker adjustment stud was backed off enough to kill the cylinder, and you didn't hear it clacking? That 402 must have a rowdy exhaust sound.By 402, you mean a big block, right?

Chip, do you have any pics of the 402?
Make sure the stud is not pulling out of the head.
clacking noise who heard a clacking noise :p

Pushrod was sitting in guideplate, rocker was twisted off to side doing nothing. Screw in studs with guideplates & dual wound valvesprings. No clacking was heard when it was running with #2 dead. Exhaust not that loud.

My buddy called me and I told him how to adjust valves once he bought a new pushrod and adjuster nut.

After he adjusted all the valves, compression on #2 came back, compression on #5 went away. He pressurized the #5 cyl and heard air escaping from the exhaust. So he backed off the nut and the exhaust leak stopped. When he went to adjust the valve 1/2 turn past zero lash, the exhaust started passing air again and lost compression.

So he started it up and set the valves again with it idling. He said if he adjusts most any valve past 1/4- 1/3 turn the cylinder starts missing. When he did get it adjusted to 1/4 turn, he let it cool. He says when the motor is cold it runs great, but a few minutes later one or more cylinders go dead and the motor loses power.

Weak valve springs?

Any other ideas?

chip

PS- Holy combustion batman--- Did any of these motors have solid lifters????

Chip is he sure it's a hydraulic cam system in there and not solid lifters? If they were solid then you have to set the gap with feeler gauges. Just a thought.

Yes robin,,, The X-Ray Bat-Goggles may have detected a solid-lifter configuration
Time to break out the feeler guage and see what happens