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  1. #1
    Join Date
    Nov 2004
    Location
    Portsmouth, Hampshire
    Posts
    295

    Valve length question

    Looking for some advice and experience from any mechanics out there. I have a damaged valve on my 429 Ford engine and Im really struggling with finding an exact replacement. The engine had bigger valves fitted to match 'Cobra Jet' Spec, I have found the right diameter valves but the length is slightly different. Im wondering on an engine like this with hydraulic lifters and stamped rockers if a couple of millimetres would be a problem ?
    I understand there is a balance about the position of the rockers and where they touch the valve but Im not sure how sensitive that is, a couple of mm doesnt seem like a lot of angle change to me and presumably the hydraulic lifters will just to their thing ?

    Any words of wisdom appreciated, I want to get the car back together but also dont want to wreck it.
    Paul

    SRV8 - 429 Ford Big Block, 4 speed toploader

  2. #2
    Join Date
    Aug 2004
    Location
    Christchurch, NZ
    Age
    59
    Posts
    2,400
    I guess one important thing to check is whether you are likely to get valve spring bind (if the new valve is shorter) at full lift.
    Cheers

    Myles D-W

  3. #3
    Join Date
    Nov 2004
    Location
    Portsmouth, Hampshire
    Posts
    295
    The valves I am finding are 2-3mm longer than the ones fitted. In my mind I cant think of a big problem but I am no expert on valve trains and I am sure there are all these different lengths for good reasons! What I am surprised about is there are so many for the same engine, also lots of push rod lengths, you would think the two items would be relatively stable, Im guessing it relates to cam lift and rocker type.
    Paul

    SRV8 - 429 Ford Big Block, 4 speed toploader

  4. #4
    Join Date
    Nov 2006
    Location
    Shropshire/Wales Border
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    49
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    6,015
    Its worth checking the location of the collet retaining groove on the valve as this is what will actually set your spring installed height rather than the valves overall length. Shorter could cause spring bind for example.

    Its an interesting question re the rocker to valve stem tip location, it is considered pretty important, if nothing else you could lose a small amount of valve lift if its not set correctly (how noticeable that would be on a street car?) plus you add more side load to the valve.

    Just pondering but armchair theory would say a stamped rocker "floating" on a stud would end up 2-3mm higher to regain the correct geometry for the rocker to valve stem. However that leaves you with too short a pushrod. Assuming enough hydraulic lifter range that might be taken up, or you do whats probably the right thing and fit slightly longer pushrods. The worry I would have is that without very careful checking you're not going to know where in the lifters range the cup is sitting, and bear in mind as the engine heats up that range moves anyway as components expand.

    Its a complex one and I'd be very tempted to talk to Real Steel and ask their advice.
    Lloyd B
    Current: Crendon #54 in build - 427 Side Oiler/Cobrajet Heads/Dual 600cfm Holleys/4 speed toploader/Vintage cast knock on wheels
    Dedion Dax/Clarkson 383 Chevy with roller 4/7 swap cam, AFR195 heads - SOLD

  5. #5
    Join Date
    Aug 2007
    Location
    Bournemouth
    Posts
    340
    You could see a difference in lift ratio as the geometry between the rocker and the valve will change as the datum will be different, albeit very slightly. But in these situations a small change in length could result in a big change elsewhwere. If the valve lift is affected then you could end up in an unbalanced engine with that particular cylinder having either more, or less charge than the others.
    This is just theory but worth a consideration. Others may have a different opinion.

    David.
    Life is driving the Cobra. The rest of the time is just waiting.

    Wessex region caretaker rep.

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