Important lash cap question

Discussion in 'Engines' started by Brian.G, Sep 24, 2009.

  1. Brian.G

    Brian.G Forum Member

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    Well important to me anyway......

    Lash caps were originally used so that the valve stem would not have to be head treated at the point of bucket/rocker contact.
    The lash cap was pressed on and the hardened cap acted as a bearing surface, OK?
    The cap stayed in place by interference fit on the softer valve stem material.

    Now, the big question I have is>
    Can an acceptable interference fit be obtained between the two if the lash cap is fitted to an already hardened valve stem originally designed with a hardened end surface.

    Or will the lash cap dislodge in time or crack, given that the two materials are of similar hardness?

    Im sure on the softer stem some galling/or compression occurs at time of fitting and this helps to aid fit.

    Can anybody give me a definite answer on this. I need to know should i remove temper and then fit, or if theres need.

    Thanks.
     
    Last edited: Sep 24, 2009
  2. A.N. Other Banned after significant club disruption Dec 5th 2

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    Do they definitely rely on interference fit? My thinking is if they were, you could never be 100% sure they were sat snug on the top of the valve.

    Since they have skirts which exceed the valve clearance in height, they won't dislodge for that reason, rather than requiring interference to keep them where they are.

    ?
     
  3. Brian.G

    Brian.G Forum Member

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    You see thats what im not sure of Chris, any drawings Ive seen of caps have a very shallow skirt, and have very tight tolerances with regards diameter, as in you can buy 10 different sizes between 7mm and 7.5mm so thats why i thought it was an interference thing?

    In your opinion > shim bucket, or lash cap?

    Thanks Chris.

    Im thinking lash cap as it would be lighter?

    Obviously a lash cap would lessen spring preload, but I can overcome that.
     
    Last edited: Sep 24, 2009
  4. A.N. Other Banned after significant club disruption Dec 5th 2

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    I've spent some recent time sorting lash caps on my motor, and have been over at a Vauxhall engine specialists (SBD, Chessington) who produce some proper motors and I believe are held in good esteem.

    I have simply bought 7mm lash caps, since that's what they're using on the Red Tops and subsequent motors.

    I also went to QEP in Sussex and the specs were again by the millimetre, rather than 0.1mm.

    I had a couple of snug/interference lash caps which I didn't think were sitting on the valves properly, so I opened them up until they were.

    Never a mention by SBD of 0.1mm interference and the rest.

    Skirt depths I decided myself - I just didn't want them anywhere near the collets, and cut them so they were well clear. 0.3mm off the collets was a quoted minimum skirt depth, but I just made sure they were well clear, 0.4mm or more.

    I was advised that if that wasn't possible, to turn them upside down and skirt them around the bucket, though I didn't need to.

    I've not seen bucket shimming - never had to touch this stuff before - but I believe lash caps are the proper solution.

    Needs a surface grinder or someone you can trust to grind them parallel to a precision.
     
  5. RobT

    RobT Forum Junkie

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    lash caps over shims any day - not definately sure about your main question though
     
  6. Brian.G

    Brian.G Forum Member

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    Right, all taken on board.
    I have seen lash caps sold in incremental sizes but that doesent seem to matter now given what you said above Chris.

    @ Rob, its for a head im doing, i need to make up some space/play in order to keep standard valve lengths.
    Perhaps I should drop my intake cam carrier journals the required amount, this would be the correct way, but I then loose symmetry and it makes journal machining harder as I then have to rejig when cutting the intake journals, as opposed to cutting exhaust and intake journals on the same plane.
    The bucket/carrier girdle then has to be stepped too.
     
  7. A.N. Other Banned after significant club disruption Dec 5th 2

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    So far I've seen lash caps up to 3.14mm.

    The problems arise when they're getting to short, ie there's not enough meat in them.
     
  8. Brian.G

    Brian.G Forum Member

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    Ok, well I need to make up 2.5mm, so is this an ok way to do it or is it a compromise given the fact I can do what I please?
    Should I drop journals or are you happy with lash caps Chris? Any horror stories?
     
  9. RobT

    RobT Forum Junkie

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    Depending where you get your buckets from, its possible you can get them with different stem lengths I would think
     
  10. RobT

    RobT Forum Junkie

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    my motor runs lash caps and regularly goes to 8.5K
     
  11. A.N. Other Banned after significant club disruption Dec 5th 2

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    As far as I can tell, this is the correct way. It is very time consuming, but the correct way for high revving 16V motors (> 7.5k rpm ballpark). They look like rubbish, but it seems this is what people use.

    I wouldn't attempt to suggest I'm experienced with this - I was just fixing overly wide clearances which should have been done by the originator, but as I say, it's time consuming and requires precision.

    Horror stories can arise from anything, installed heights, coilbound springs etc, and that could throw a lash cap out, but it's already self destructing by then.

    You can largely do what you please. Put in 1 valve which is 1mm shorter (edit: above the collet) if you wanted, and make it up on the lash cap. Makes no theoretical difference.
     
  12. RobT

    RobT Forum Junkie

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  13. Brian.G

    Brian.G Forum Member

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    Ok, a very big thanks to you both, Im going to think about this more later, at least now I have something to think about.
    What a brilliant forum the fact you can get answers immediately on something such as this.
    Thanks again, Brian.
     
  14. A.N. Other Banned after significant club disruption Dec 5th 2

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    Thanks Brian. Saw the article in Golf+. Keep at it. Can't wait.
     
  15. mickey marrows Forum Member

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    Interesting.. the lash caps in my KWL 8v head attach to the stem of the bucket.. not the valve..
     
  16. A.N. Other Banned after significant club disruption Dec 5th 2

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    If the bucket 'pin' has a different diameter, it's specific to that.

    If it has the same diameter as the valve, it can go in either way, subject to lash cap skirt => collet clearance.
     
  17. Brian.G

    Brian.G Forum Member

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    Oh I wil Chris, I just want to have all machining angles covered first before I start the thread, so as I dont run into a problem and have to go...um...ah...umm..:lol:
     
  18. Brian.G

    Brian.G Forum Member

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    One more question, what happens the lash caps if the engine is over revved and the valves bounce?
     
  19. A.N. Other Banned after significant club disruption Dec 5th 2

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    No certainty to that one (gravity and crossed fingers?), but if a piston has clipped a valve, separate problems are already developing and it's head off time anyway to start junking some valve springs and valves.
     
  20. Brian.G

    Brian.G Forum Member

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    Ya I suppose so, maybe Im just getting paranoid...
     

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