Rocker arm geometry | The Z Club of Great Britain
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Rocker arm geometry

Discussion in 'S30 (240Z,260Z,280Z) Engine' started by dotMorse, May 16, 2019 at 10:22 PM.

  1. dotMorse

    dotMorse
    Z Club Member

    Evening all. I am currently rebuilding my head after a skim and general tidy up. Did not have much history with the car so wanted to strip and rebuild to make sure everything was in order, replace gaskets etc. Seemed like a good thing to do at the time.

    I started replacing the rocker arms tonight and the first is already causing a head ache. I am using the original cam shaft, rockers, springs etc all bagged and marked to make sure they go back in the right order. Set my spacing to 0.025mm on the first exhaust valve which only was the setting with the pivot bolt all the way down so no room for adjustment there. Did 2 test rotations and the marking is way off. Looks like the cam rotates initially off the pad.

    Don't think I can fit lower lash pads as these already are almost flush with the spring top.

    Anything stupid I am missing here?

    After looking at the rocker arm it looks like this is how it was running before. Is there a new cam, spring and arm purchase on the horizon?
     

    Attached Files:

  2. jonbills

    jonbills
    ZClub Administrator
    Staff Member Moderator

    0.25mm, not 0.025mm right?

    That looks odd. Did you skim both faces of the head? Or the cam towers?
    What's the height of the installed valve springs, (from head to cap) should be about 40mm I think. (41 rings a bell!)
    What head is it?

    Not sure if you're going to need new cam, rockers etc but new stuff alone won't fix it. The cam is too near the head or the springs and valves are too tall.
    Cam tower shims might fix it. Not sure how many you'd need though.
    https://www.thezstore.com/page/TZS/PROD/10-1151

    I think others sell them too.
     
  3. dotMorse

    dotMorse
    Z Club Member

    Thanks Jon. Sorry yes, 0.25mm and an E88 head. Just the underside was skimmed and not the top face. I left the cam towers in place as I didn't see the point of removing them. I wonder if it was skimmed previously? Does anyone know what the stock head height is? Ill disassemble and check dims again tonight.
     
  4. jonbills

    jonbills
    ZClub Administrator
    Staff Member Moderator

    Measure the valve lengths when you do, that might tell us something. I'm wondering if it's got the wrong valves installed.
    Do you have any photos of the installed valves and combustion Chambers?
     
  5. Huw

    Huw
    Z Club Member
    Staff Member

    Do you know if the valve seats have been recut at all? I ask because I had the same symptoms after I sent a head to have the valve seats dressed. The machine shop cut them back too far giving me the same problem you look to have. I 'fixed' the issue with cam tower shims, but in the end I opted to have new seats installed because I was not happy with the machinists work.
     
    jonbills likes this.
  6. dotMorse

    dotMorse
    Z Club Member

    From what I know the valve seats haven't been recut but part of this exercise was to find out what has been done and what has not. Managed to find some photos from the disassembly of the valves and valve seats.

    What was the cost of new seats as its not something I have done before? I guess a machinist job? Looking at the head again I wonder if they have skimmed too much of the top side at some point? Ill get the gauges and out and check the sizes of everything tonight and report back.

    Tom
     

    Attached Files:

  7. Huw

    Huw
    Z Club Member
    Staff Member

    Hi Tom.

    I not suggesting you will need to have your valve seats replaced. I had to through choice. I could have just had the cam towers shims installed, but I wanted reparation from the machine shop who screwed my head up. They did the seats for a very very reduced rate after much heated debate :rolleyes:. If they do need replacing it is a machine shop job.

    I can only think of three things that would reduce the distance between the cam and follower as you have described (the more enlightened probably know better than me). As Jon suggested the valve stem hight is incorrect due to wrong valves, or the top of the head has been skimmed reducing the hight of the cam, or the valve seats have been over cut raising the valve stem height. My bet is on a top skim or combination skim and seat cut. Worth trying a set of head saver shims. I can loan you a set of shims if you wish to try to see if it alleviates your issue.

    Best of luck and keep us posted.

    Huw
     
    jonbills likes this.
  8. dotMorse

    dotMorse
    Z Club Member

    So some measurements. I am writing these as I take them for my benefit as well. Working on the number 1 exhaust valve.

    Head overall height - 107.36mm - Stock - 107.90mm
    Outer Spring - 47.80mm - stock - 50.00mm
    Inner spring - 42.78mm - stock - 44.80mm
    Valve height overall - 117.31mm - 117.50mm
    Valve diameter - 32.88mm - stock - 33mm

    Would these differences make enough of a change to cause the issues?
     
  9. dotMorse

    dotMorse
    Z Club Member

    Might have got some of my stock numbers wrong so correct me if so.
     
  10. Farmer42

    Farmer42
    Z Club Member

    If only the bottom of the head has been skimmed it would only affect the distance the valve opens. It would possibly foul the top of the block and/or the pistons it would not affect the rocker height.

    If as Huw has suggested the valve seats have been cut too deep, the height of the valve stem & springs would be greater. It could be a possibility that they were cut ready for installing hardened seats but were never installed. The outer spring height with the valve loaded up and closed should be 40mm from the bottom spring seat to top retainer. The clearance between rocker arm and cam lobe should be 0.008 inch for inlet and 0.010 for exhaust. Have you checked that the wipe pattern is centralised on each valve as this could make a difference with clearance?

    If you don't already have one, I would recommend getting a copy of 'How to rebuild your nissan/ Datsun ohc engine and work back through the head rebuild methodically following the guidance before spending on expensive parts or possible fixes.
     
  11. dotMorse

    dotMorse
    Z Club Member

    Thanks for the advice. That is the book I am working through step by step.

    The original post was due to the wipe pattern not being centralised with no room for adjustment.

    I am just in the process of checking valve heights so thanks for the dims. Saves me digging them out again.
     
  12. Farmer42

    Farmer42
    Z Club Member

    Doh!:banghead:

    You probably spotted my mistake. The valve lash clearance numbers should be mm not inches. Just looked at the pics again. It looks like you have seat inserts fitted so if the valve springs are sitting too high it could be that they have been overcut.
     
  13. dotMorse

    dotMorse
    Z Club Member

    So my height is 40.67mm. I think we might have found the issue.

    Is there an easy solution? Tower shims?

    Any near Warwickshire? Unfortunately on a bit of a schedule this weekend getting ready for the car to come back and one of the last periods I have free. Happy to drive to collect if anyone has any tomorrow morning?

    Will I also need some shims for the valves to brings these back to a gap of 40mm?
     
    Last edited: May 17, 2019 at 10:49 PM
  14. jonbills

    jonbills
    ZClub Administrator
    Staff Member Moderator

    What's 40.67? Spring installed height?
    Is the stock height 40? I wouldn't think a 0.67 mm difference would be the problem.
    I was expecting to find something a couple of mm out.
    It's not a p79 or p90 head is it?
     
  15. dotMorse

    dotMorse
    Z Club Member

    The height of 40.67 was the gap from spring seat to spring retainer. That's also including the washer that the spring sits on

    Definitely a E88 head.

    With the head height at 0.54mm less and the 0.67mm on the spring retainer would that add up? I agree that it seems like a small difference but do the small differences add up to a big change?
     
  16. jonbills

    jonbills
    ZClub Administrator
    Staff Member Moderator

    That's the right measurement I think.

    If the 0.5 off the head was all off the top (unlikely) then that's a total of 1.2. Multiplied by the rocker ratio that could be 1.8 at the adjustment stud. I reckon the adjusters have 5mm of adjustment in them (guess!) so still doesn't seem enough to me, but maybe :)
    I dont know how tall the cam towers should be, but might be worth measuring their height too. I have some in the loft I can check but not until next weekend.
     
    Last edited: May 18, 2019 at 1:14 PM
  17. Huw

    Huw
    Z Club Member
    Staff Member

    That’s sounds right to me 40mm for the spring and 0.6mm for the seat. I do have a set of shims you can try. If you pm me with your address I can post them to you if you wish.

    Huw
     
  18. dotMorse

    dotMorse
    Z Club Member

    Thank you all. Appreciate the input and advice. Hopefully it will sort itself out to a degree with some tower shims. Seems like the easiest first step.

    Thank you Huw. I will send you an address over now.
     
    jonbills likes this.
  19. dotMorse

    dotMorse
    Z Club Member

    So looking at the Rocker arms again the other day. There are some pitting on some of the faces along with wear off center from the misalignment previous. You can feel a small groove with your finger at the center of the cam compression point on the rocker arm face.

    I know if you replace the camshaft you should replace the rockers but if I replace the rockers do I need to replace the cam? I only ask as I can get a new set of Rockers from RockAuto for about £100 delivered. I am thinking of replacing the spring and rocker arms while I am at this point as I would like a base that I am happy with rather than chancing it with old gear.

    I had the cam looked at by the guy that did the head skim who said it looked absolutely fine and showed no signs of wear so I would like to keep this if possible.

    I have 2 sets of tower shims on their way to me (thanks Huw and MJP) but if I do raise the camshaft and it is then out of center with the existing wear pattern on the rockers will this be an issue?

    Thanks again....
     
  20. jonbills

    jonbills
    ZClub Administrator
    Staff Member Moderator

    No problem putting new rockers on with an old cam.
    Make sure there's zddp in your oil when you fit them.
     

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