How do I know what lash pads to put in? | The Z Club of Great Britain
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How do I know what lash pads to put in?

Discussion in 'Engine' started by Fastededdie, Sep 29, 2020.

  1. Fastededdie

    Fastededdie
    Z Club Member

    Hi all I 'v just bought a second hand reconditioned engine and head. The head came with a nice looking cam that read e30 and a number 7 on it. I am installing it into my 260Z. Long story but I won't go into it.

    The head has been cleaned and shaved but it has not been installed and the guy Mark nice fellow and he is on here who I purchased it from. Hi Mark if you are tuning in. Excuse the pun . Basically I am looking to install lash pads and I am not sure to use my old ones or get some new but I have seen there are loads of sizes and I am lost to know what to do. Can anyone advise? Regards Eddie
     
  2. johnymd

    johnymd
    Z Club Member

    The theory is that you increase the size of the lash pad by the amount proportional to the reduction of cam base circle size if the valve seat height has not changed from factory spec.

    The above is the main reason why you will need bigger lash pads to bring the rocker wipe pattern back to the centre of the pad. The reason the cam base circle can change is due to increase in cam lift or duration. The lift is the difference between the smallest part of the cam lobe and the largest. On a regrind you can’t make the lobe bigger so the solution is to make the base circle smaller. Also if you increase the duration this may be outside the shape of the old cam lobe so removing base circle material will also be required to achieve this. Some cam manufacturers will supply the correct size lash pads to suit their cams because they know how their base circles differ from factory spec. Some will tell you that you always need to get the measurement from wipe pattern testing but if the valve seats are still at factory height then it can be calculated. If your wipe pattern with factory cam is perfectly centred then the lash pad can be calculated by base circle size. I will say though, it will always be worth doing a wipe pattern test to verify your lash pad size is correct.

    Hope this helps.
     
    AliK, Mr Ex Jnr, uk66fastback and 3 others like this.
  3. Rob Gaskin

    Rob Gaskin Treasurer Staff Member Moderator

    John, I've not worked on lashpads but I know that when my trackday engine was built all this was 'sorted'. I would imagine that it could be botched by adjusting the valve clearances but of course rocker angle and 'wipe' would all wrong.
     
  4. johnymd

    johnymd
    Z Club Member

    I can’t see how you can adjust this to correct the wipe pattern. The only adjustment is for valve clearance and this needs to be set correct. The adjustment for the difference in base circle can only be done at the lash pad.
     
    jonbills likes this.
  5. Fastededdie

    Fastededdie
    Z Club Member

    OMG How confusing. Will get on youtube to see )
     
  6. Rob Gaskin

    Rob Gaskin Treasurer Staff Member Moderator

    John, I wasn't saying that 'wipe' could be corrected with valve clearances in fact I said it would be all wrong. What I was saying was that the gap between the base circle and the lash-pad could be adjusted but would be a botch.
     
  7. ben240z

    ben240z
    Z Club Member

    Ideally you need a set of different thickness lash pads to drop into each valve to check the wipe patern. Then you buy the corresponding set of pads ( usually the same all the way through but depending on the work you have done it may be that you need one size for the inlet and another for the exhaust)

    I have a set that you can borrow but would need them replacing if lost as I got them from the usa years ago
     
  8. richiep

    richiep
    Z Club Member

    The other approach to help decide on the correct lash pad thickness is to do a test assembly using a standard lash pad and then layers of shim stock placed underneath it in the valve spring retainer to test height/wipe pattern combinations. You can get brass or steel shim stock in 0.1mm sheets. Make a bunch of little shims from that. Once the wipe pattern is correct with the shims, you then know which size lash pads to order. Saves you having to hunt around for or invest in expensive sets of pads in different sizes. I have sheets of 0.1mm steel precisely for this purpose.
     
    Mr Ex Jnr and Mr.G like this.
  9. Albrecht

    Albrecht Well-Known Forum User

    For a low-tech fudge, comparing the installed valve height* between the donor head and the recipient head should give an idea of whether a straight swap of camshaft, cam followers and valve lash caps from one head to the other will be possible.

    *Installed valve height is essentially the measurement of the amount each valve stem 'sticks out' of the head casting when the valve is in the closed position.

    If the installed valve heights are within a few mm of each other head-to-head, then a straight swap over may be feasible. Not recommended, but feasible.
     
  10. Huw

    Huw
    Z Club Member

    I used an old set of feeler gauges and cut them up to make little shims, a bit like Rich above said, to get the correct wipe patterns before ordering the shims. Just has to be careful to make note of the thicknesses being used!
     
    Mr.G likes this.
  11. gregwni

    gregwni New Forum User

    My machine shop did a valve job and they appear not to have adjusted the valve lenghts on all occasions, so in a couple of the exahust valves, im finding that i dont have enough adjustment range to get the correct gap (e.g. where i should be able to get 0.25mm cold gap for the exhaust valve on valve 12 - i can only fit a 0.13 mm feeler gauge in when i have used up all the adjustment available to me).

    Id rather not have to take the head off - am i right in saying that i should be able to "fix" this by installing different sized lash pads on the offending valves?
    (and then, i presume id have to try a few to make sure i get onethat i can get a decent wipe pattern on?)

    And if so, do you know anywhere in the uk i can buy them?
     
  12. jonbills

    jonbills
    ZClub Administrator
    Moderator

    What was the job they did on the valves?
    have you got any pics? although smaller shims would give you more clearance I think you must be very close to edge of the rocker pad. The valves should be set to the same height as each other and that height should be basically the target installed height of the springs. It sounds to me like its beyond just a few new lashpads.
     

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