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250 engine question

Discussion in 'Chevy Trucks' started by joseph, Oct 8, 2001.

  1. joseph

    joseph Junior Member
    Messages: 9

    On my 81 chevy C-10, I just put a new head on my 250 engine. After I put everything back together, I had a very loud exhaust leak at the pipe/manifold connections. Fixed that, then I was able to hear how bad my rocker arms were ratteling. Took the valve cover off (not an easy job on this engine), retightened all the rocker arms until they were quiet. Put it all back together, put it in drive and the engine started shaking so bad, I thought it was going to jump out of the truck. Before I tightened the rocker arms, the truck was driving great, just real loud because of the exhaust leak. But I only drove it for about 5 miles. It acts like a vacuum leak, but I was wondering if it could be that I tightened the rocker arms too much. I did put alot of torque on them. I've checked for vacuum leaks, but cant find any. Any suggestions would be appreciated. Thanks
  2. plowjockey

    plowjockey PlowSite.com Sponsor
    Messages: 622

    Are the rocker arms on a solid tappet cam or hydraulic lifters? I know if you jus tighten them down they will hold the valves open when they should be closed. If it is solids then they are set with a feeler gague. If hydraulic then they are set with the engin running since they use the oil pressure as the cushion? I'm a little rusty on this one.
    You should get a manual before you run it any further and check the proper procedure for valve setting.
    Been a long time since I had my hands on a six.

  3. joseph

    joseph Junior Member
    Messages: 9


    Thanks Bruce,,,,I'm not too familiar with the terminology, but my manual says to tighten the nut on top of the rocker arm until the push rod stops rotating completely. Then tighten one full turn. I did this, but later I was told that in order to do it right, you need to bump the engine to raise and lower the push rods to the correct position before you tighten them. Does that make sense? Anyway, I'm not sure what the correct position would be since my manual assumes I already know this. To answer your question, the pushrods sit on top of a pedestal that has a threaded stud coming out of it's center. I drop the push rod down a hole, set the rocker arm on top of the push rod, stud and valve, drop a washer and a nut over the threaded stud and start tightening. Nothing was mentioned about a gauge. Hope I explained it well enough. Thanks again.
  4. plowjockey

    plowjockey PlowSite.com Sponsor
    Messages: 622

    Yes you answered it perfectly.
    OK here we go. You need to bump the engine so that the pushrod in in the lowest position. Then you tighten the nut down until you just barely can NOT turn the pushrod. Thei means that there is no slack or (proper term) lash in the pushrod. That way when the oil pressure in the engine comes up it cushions the lifter end of the pushrod. If this is too tight whjen the engine comes up to pressure then the valve will gap open just a bit allowing your compression to bleed past the valve either out the exhaust or into the intake. The reason why I asked if it had solids was that they are set with a feeler gague rather than the other way. Goo luck.

  5. joseph

    joseph Junior Member
    Messages: 9


    Thanks for your help. Do you think this is whats causing the engine to shake so bad when I put it in gear? Having the rocker arms too tight against the push rods and valves?

  6. plowjockey

    plowjockey PlowSite.com Sponsor
    Messages: 622

    It's very likely that this is what is causing your problem. One thing that I forgot to mention is that you must do this when your engine is COLD. The reason is that when it is warm the block and head expand and any adjustments that you make will of course change as the engine cools and contracts. Therefore you need to do it when it is all the way cold.
    I hope this helps some. I used to have a Harley and it gave me fits until I learned all of this.
    75 knows these engines better than I do and he may be of some help also.

  7. plowking35

    plowking35 2000 Club Member
    from SE CT
    Messages: 2,923

    I went through all of this about 10 yrs ago, and the long and short of it all, was I should haev just small blocked it.
    Well anyway, you are on the right track, somewhat.
    I cant remember the order in which to do this, but here are the general ideas.
    1 Find top dead center #1 cyl. When you have that, certain exhaust and intake valves can be adjusted, I just cant remember which ones. Anyway it will be half of the valve train.
    2- Rotate till #6 is TDC and then tighten remaining valves.
    A shop manual will have the order in which to do this. It should be under valve lash adjustment.
    This is true of all hydraulic valves, with a v-8 you just do it with #1 TDC and then #8 TDC.
    That engine should have the integral head, meaning the intake and exhaust are together, very bad design. Head is prone to cracking, and doesnt make a lot of power. I ended up spending like 700 on parts and machine work and the truck was never the same. Then another 1000.00 converting to a small block in the end.
  8. 75

    75 PlowSite.com Addict
    Messages: 1,382

    Actually Bruce, I gotta be honest here - I don't have much experience with 6-bangers, except the big Mack ones and that was driving 'em, not fixing 'em! ;)

    I got a GM service manual down off the shelf, it's for the '77 model year but I don't think the procedure is any different, I've copied portions out of the book:

    Adjust valvles when lifter is on base circle of camshaft lobe as follows:

    a. Mark distributor housing, with chalk, at number one and number six positions (plug wire) then disconnect plug wires at spark plugs and coil and remove distributor cap and plug wire assembly.

    b. Crank engine until distributor rotor points to number one cylinder position. The following valves can be adjusted with engine in number one firing position:
    Number one cylinder - Exhaust and Intake
    Number two cylinder - Intake
    Number three cylinder - Exhaust
    Number four cylinder - Intake
    Number five cylinder - Exhaust

    c. Back out adjusting nut until lash is felt at the push rod then turn in adjusting nut until all lash is removed. This can be determined by checking push rod end play while turning adjusting nut. When play has been removed, turn adjusting nut in one full additional turn (to center lifter plunger)

    d. Crank engine until distributor rotor points to number six position. The following valves can be adjusted with engine in number six firing position:

    Number two cylinder - Exhaust
    Number three cylinder - Intake
    Number four cylinder - Exhaust
    Number five cylinder - Intake
    Number six cylinder - Intake and Exhaust

    e. Back out adjusting nut until lash is felt at the push rod then turn in adjusting nut until all lash is removed. This can be determined by checking push rod end play while turning adjusting nut. When play has been removed, turn adjusting nut in one full additional turn (to center lifter plunger)

    That's from the GM manual, hope it helps! Here's a few more out-takes from the same manual, on possible causes for the "rattling" noise:

    The general types of valve lifter noise are as follows:

    Hard rapping noise - Usually caused by the plunger becoming tight in the bore of the lifter body to such an extent that the return spring can no longer push the plunger back up to working position. Probable causes are:

    a. Excessive varnish or carbon deposit causing abnormal stickiness.
    b. Galling or "pickup" between plunger and bore of lifter body, usually caused by an abrasive piece of dirt or metal wedging between plunger and lifter body.

    Moderate rapping noise - Probable causes are:

    a. Excessively high leakdown rate
    b. Leaky check valve seat.
    c. Improper adjustment.

    General noise throughout the valve train - This will, in most cases, be caused by either insufficient oil supply or improper adjustment.

    Intermittent clicking - Probable causes are;

    a. A microscopic piece of dirt momentarily caught between the ball seat and check valve ball.
    b. In rare cases, the ball itself may be out-of-roound or have a flat spot.
    c. Improper adjustment.

    Again, taken from the GM manual! :)
  9. joseph

    joseph Junior Member
    Messages: 9

    intake & exhaust

    Thanks Rob for all the info. All I have to do now is figure out which ones are the intake and which ones are the exhaust. This is my first time inside an engine and it has definitely been a learning experience. Had I known the 250 was prone to head cracking, I probably would have switched over to a V8. But for now, I'll just keep driving it (I hope) until the next major break down. I cant put all the blame on the engine. I knew the radiator was on it's way out, but I kept thinking I could get one more day out of it. When it finally went, it went quick. Thanks again for all your help.
  10. plowjockey

    plowjockey PlowSite.com Sponsor
    Messages: 622


    To figure which valve is which is really quite easy. You are I hope familiar with which is the intake manifold and which is the exhaust manifold. Simply look at the valve location in the head and how it coincides with the port on the outside. Whichever manifold that valve is in relation to is what kind of valve it is.

  11. raceman6135

    raceman6135 Member
    Messages: 61

    Another method for adjusting valves with STOCK camshafts:

    Make sure the piston is on TDC (top dead centre) on the firing/power stroke (determined when the rotor points directly to the corresponding terminal in the distributor cap) -- you can then set both the intake and exhaust on that cylinder.

    An excellent method for adjusting valves on any MODIFIED engines (i.e. non-stock camshaft): rotate the engine until the intake valve is almost completely closed. You can then adjust the EXHAUST rocker arm on that cylinder.

    Once the exhaust is set, rotate the engine until the exhaust valve just starts to open. You can then adjust the INTAKE rocker arm on that cylinder.

    I've had to use this last method on many of my racing engines where camshafts can have an "advertised" duration of close to 360 degrees, because this is the only method (short of using a dial indicator) that will make sure the lifter to be adjusted will be on the base circle of the camshaft.

    Just a few notes you can add to your list of "fool proof tricks!"

  12. joseph

    joseph Junior Member
    Messages: 9

    250 engine

    Okay, I readjusted the pushrods using the No. 1 and No.6 cylinder method. I noticed when I was adjusting one of the No. 2 rocker arms, I had to tighten the nut forever before the pushrod stopped turning. This wasnt the case with any of the others. Have I done some really bad damage to this valve? Anyway, after I did all this and cranked it up, the engine's still running rough (shaking). Especially when I put it in gear. When I was trying to put the exhaust pipes back onto the manifold, before all of this shaking started, I had to use a jack to get the pipes up high enough to get the nuts started. Could I have jacked everything too high and bent something, which is now causing a vacuum leak? I'm about ready to pull the head and start all over.