Adjustable Cam Pulley & Head skimming | The Z Club of Great Britain
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Adjustable Cam Pulley & Head skimming

Discussion in 'Engine' started by Turn & Burn, Oct 30, 2019.

  1. Turn & Burn

    Turn & Burn
    Z Club Member

    I'm nearing the point of commencing rebuild on my L28 motor and wondering what people have done with regards to cam timing. The block is slightly skimmed to clean the face up and the head will be getting an amount of skim to enable the higher compression, aiming for 10.8/1 on an N42 head.
    I'm running oversize valves with a 1mm longer stem than the usual N42, so it may be best practice to shim for this reason alone and maintain original geometry.
    What's the usual approach to skimmed heads, do people shim the cam towers or fit an adjustable pulley?
    How much head skim can the chain tolerate before it runs slack, so will I need to shim regardless?
    If I need an adjustable pulley does anyone have one they don't require?

    Thanks
     
    Last edited: Oct 30, 2019
  2. richiep

    richiep
    Z Club Member

    The adjustable cam sprocket doesn't really figure into this situation; what you need is either cam tower shims to compensate for the loss in head thickness, or a Kameari Twin Idler Gear, which negates the need for tower shims having up to 4mm of adjustability. Of course, its super expensive compared to the tower shims!

    The longer valves will require you to check your installed spring heights too. You may need thicker washer/shims at the bottom of the springs to maintain the correct heights.
     
  3. Turn & Burn

    Turn & Burn
    Z Club Member

    Thanks Richie, surely reducing the chain sprocket centres will cause the cam to retard which can be corrected by advancing the cam on the adjustable sprocket? It may be that with 1mm of tower shims for the long valves that everything comes back to position give or take a thou’

    I’ve got an extra set of 1mm spring seats to compensate for the longer shafts.
     
    Last edited: Oct 30, 2019
  4. franky

    franky Well-Known Forum User

    you'll need an adjustable cam sprocket.
     
  5. IbanezDan51

    IbanezDan51
    Z Club Member

    Using shims would surely require a line bore after? Which is very expensive ?
     
  6. Turn & Burn

    Turn & Burn
    Z Club Member

    Not sure on this, I would have hoped using shim stl accuracy could be retained, the towers are on spigots so should maintain alignment. I’ll have to read up it’s not something I’d considered.
     
  7. jonbills

    jonbills
    ZClub Administrator
    Staff Member Moderator

    There's two issues;
    1) Richie's point: the chain tensioner can only take up a certain amount of slack, and a big skim creates a lot more Slack to take up (maybe more than the chain tensioner can handle)

    2)Andrews point: moving the sprocket centres closer together retards the cam timing.

    The 2nd might be no bad thing, if the engine is well balanced. It'll just mean more power at higher revs. (prob best to measure it though than assume its fine) and you can advance it on the 2nd or 3rd positions on the standard sprocket or get an adjustable sprocket. (my memory is that my Kameari one was about £100 delivered)

    The first problem needs tower shims or the twin idler or the cheaper option the Kameari adjustable chain tensioner.

    And yeah, longer valve stems will have to be dealt with in the spring height and the tower height or lash pad thickness.
     
    richiep and Turn & Burn like this.
  8. jonbills

    jonbills
    ZClub Administrator
    Staff Member Moderator

    I think you're OK on it, lots of people in the US seem to fit them and don't talk about having to line bore the cam towers.
     
    Turn & Burn likes this.
  9. richiep

    richiep
    Z Club Member

    Tower shims are very common - my Fairlady Z’s N42 has nearly 2mm of them stacked under the towers without issue for example. The whole “don’t remove your cam towers or the world will implode” thing that surfaces on forums every so often is overly cautious. As long as the towers are always kept as a set and in the correct order and the cam is checked to turn easily enough, they can be removed and the shims used.

    Kameari also make thicker tower spacers, e.g solid 1mm or 2mm thick items, negating the need to stack 15thou shims.

    Good point about the cam sprocket Jon; and yes, the Kameari is in the hundred quid region - I think mine for the stroker project was £115 plus post?
     
  10. Turn & Burn

    Turn & Burn
    Z Club Member

    Thanks for replies, I’ll go with the shims and see where I end up. Probably end up ordering the adjustable sprocket too, but reckon I could make one in an afternoon on the lathe using a normal sprocket outer.
     
    Last edited: Oct 31, 2019
  11. Mr.G

    Mr.G
    Z Club Member

    Did you get an adjustable cam pulley in the end or make one?

    Motorsport Auto have the Kameari one for 169USD are there any other better sources? I know theres RHD Japan but anyone else I should consider?
     
  12. Turn & Burn

    Turn & Burn
    Z Club Member

    I did make one by making a new hub and boring out the centre of a new one from an engine kit. After that I didn’t use it and bought a Kameari one instead.
    The cam drive rebuild kit replacement pulley appears to be cast. I was really unimpressed with the quality, that’s why I gave up with it.
    A couple of engine builders said they wouldn’t have bothered fitting one anyway and once mine was assembled it came within a degree of original position so I guess if you’re thinking whether you need one and your not in a hurry build your engine on a normal pulley and check ur timing before spending money.
     
    Last edited: Nov 22, 2020 at 9:27 AM
  13. Mr.G

    Mr.G
    Z Club Member

    At least you tried making one and sounds like you would have been successful if the material was different.

    Thanks for the advice but this is a performance build and I will eventually be asking my dyno tuner to retard and advance and see where peak power/torque occurs.

    I'll probably be getting mine from RHD Japan, unless anyone has any other sources to mention.
     
  14. Turn & Burn

    Turn & Burn
    Z Club Member

    If u do start doing cam adjustments make sure when you do your build you have sufficient gap between valve and piston, as you retard the cam things can get close very quickly.
     
    Mr.G likes this.
  15. Mr.G

    Mr.G
    Z Club Member

    Yes, I will be checking that when i clay my pistons
     

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