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  1. #1

    Default valve clearance question

    Hey all,

    thinking about doing a top end on my stock 1979 302

    what types of pistons are in there? ( they're not notched, right? just flat no valve reliefs)

    and do you think a 1.94 valve 60cc combustion chamber head like the brodix st5.0R with a Howard's cam like this would clear if I check P to V?

    adv270 278 @ .050 217/225 .528/.533 108LSA 104ICL

    thanks

    J from WA

  2. #2
    FEP Senior Member gt4494's Avatar
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    What lift camshaft are you planning?
    Are you going to use stock rockers or 1.6?
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  3. #3

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    Quote Originally Posted by gt4494 View Post
    What lift camshaft are you planning?
    Are you going to use stock rockers or 1.6?
    All the info is above for the cam .528/.533 217/225 duration 108 LSA off the shelf. it will be 1.6 roller rockers from the same manufacturer for the camshaft howards purple aluminum 1.6 roller rockers

  4. #4
    FEP Member bkm's Avatar
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    Absolutely no way for anyone to give you an answer without it being a guess. You're going to have to check ptv clearance the old fashioned way.

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  5. #5

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    Quote Originally Posted by bkm View Post
    Absolutely no way for anyone to give you an answer without it being a guess. You're going to have to check ptv clearance the old fashioned way.

    Sent from my SM-G960U using Tapatalk
    Ok I understand p to v has a lot to do with cam timing but what about the piston and the 1.94 valve? I know I can't run the 55cc version of the head unless I had an aftermarket piston so the 60cc version should not smack the piston?

  6. #6

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    IF original, or original type replacements, a '79 will have flat-top pistons with valve notches, with 1.585" compression distance (wrist pin center to piston top), which puts the pistons about 0.030" "down the hole" in an untouched block... which sucks for compression, but can be a bit of an advantage for a higher lift camshaft...

    What does cylinder head chamber volume have to do with up/down valve head locations? Nothing, usually...

    Reread bkm's post, numerous times if necessary... get your things, put together what you want, and then your answer will be, WHEN you check piston-to-valve clearance, that if there isn't sufficient running clearances, to create or have created sufficient clearances... deepening of the piston's valve notches...

    Better yet would be to disassemble what you're starting with and the cylinder heads that you want to use and do some measuring before throwing money at it for a camshaft that may or may not work...
    Last edited by Walking-Tall; 01-15-2021 at 08:38 AM.
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  7. #7
    FEP Super Member bwguardian's Avatar
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    More than likely you will run into an issue because of the larger valve size given the piston valve reliefs need to be cut wider to accommodate the larger valve. I ran into this with my then 82 GT with a smaller Lunati Street Master cam. But as stated above, you need to mock it up with clay and check it.
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  8. #8

  9. #9

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    sounds like i need to find a newer roller short block and make that work and check the p to v on that vs the old flat tappet block i have that may or may not work

  10. #10
    FEP Member bkm's Avatar
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    Quote Originally Posted by hazelj80 View Post
    Ok I understand p to v has a lot to do with cam timing but what about the piston and the 1.94 valve? I know I can't run the 55cc version of the head unless I had an aftermarket piston so the 60cc version should not smack the piston?
    It has everything to do with the cam events. You could make a cam with .300 lift smack a piston if you wanted to. The bigger valves like 1.94 on an inline head become very iffy. There are lots of guys running the AFR 165 heads on stock pistons,, but i think it's very close. If you haven't bought the heads yet and are dead set on using the oem short block, buy a set of twisted wedge heads. The 11r heads have great valve angles that will work with stock short blocks and decent camshafts. I'm running the 11r 190's 56cc with a cam around 240 duration and .607 lift on the intake. I'm running a completely stock 88 short block and have .090 clearance on the intake valve. The intake valve is no where near the valve pocket on the piston. I could run the 86 true flat tops without issue.

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    Last edited by bkm; 01-16-2021 at 04:13 AM.

  11. #11

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    Quote Originally Posted by bkm View Post
    It has everything to do with the cam events. You could make a cam with .300 lift smack a piston if you wanted to. The bigger valves like 1.94 on an inline head become very iffy. There are lots of guys running the AFR 165 heads on stock pistons,, but i think it's very close. If you haven't bought the heads yet and are dead set on using the oem short block, buy a set of twisted wedge heads. The 11r heads have great valve angles that will work with stock short blocks and decent camshafts. I'm running the 11r 190's 56cc with a cam around 240 duration and .607 lift on the intake. I'm running a completely stock 88 short block and have .090 clearance on the intake valve. The intake valve is no where near the valve pocket on the piston. I could run the 86 true flat tops without issue.

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    Yeah that seems to be the way to go is to just use twisted wedge heads with the 61cc chamber because I read that 58cc trickflows does require you to run their tw piston which is why I mentioned the combustion chamber sizes it's probably when you get too small they get too close is why I think they mention that in their instructions

  12. #12
    FEP Member bkm's Avatar
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    The 58cc heads probably have the same clearance that the 61cc heads have. They don't mill the 61cc heads to 58cc. They're probably a different casting or the combustion chamber is machined smaller with the same free drop on the valve. The 11r heads have more clearance than the FAC 170 heads due to the valve angles. The 11r 170 head uses a 2.02 valve that will give miles of clearance with healthy camshafts.

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  13. #13

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    Quote Originally Posted by bkm View Post
    The 58cc heads probably have the same clearance that the 61cc heads have. They don't mill the 61cc heads to 58cc. They're probably a different casting or the combustion chamber is machined smaller with the same free drop on the valve. The 11r heads have more clearance than the FAC 170 heads due to the valve angles. The 11r 170 head uses a 2.02 valve that will give miles of clearance with healthy camshafts.

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    Now I'm wondering if I should get into the short block and put different pistons in it?

  14. #14
    FEP Power Member Ethyl Cat's Avatar
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    BKM is on to the important part of the equation. Valve drop.

    The amount of distance the valve will travel before breaking the plane of the cylinder bore.

    The flatter the valve angle the better this number is and the better the chance PTV will be ok.

    If I know valve drop, head gasket thickness, deck clearance and cam specs I can usually tell if it is going to clear or not. In your case it would be treating the piston as a true flat top because I don't think the pockets are cut big enough diameter wise to accommodate the valve.

    If you had an aftermarket piston and knew pocket depth, that could be included in the calculations.
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  15. #15

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    Quote Originally Posted by Ethyl Cat View Post
    BKM is on to the important part of the equation. Valve drop.

    The amount of distance the valve will travel before breaking the plane of the cylinder bore.

    The flatter the valve angle the better this number is and the better the chance PTV will be ok.

    If I know valve drop, head gasket thickness, deck clearance and cam specs I can usually tell if it is going to clear or not. In your case it would be treating the piston as a true flat top because I don't think the pockets are cut big enough diameter wise to accommodate the valve.

    If you had an aftermarket piston and knew pocket depth, that could be included in the calculations.
    Very informative! thanks!! more to think about. I was looking at 11r heads too. since that valve is twisted pretty far onto a flat plane if i get what you and bkm are saying? so knowing that my first post was about putting a 20 degree conventional SBF head on with a 217/225 .528/.533 108/104 cam assuming that the piston has no valverelief and stock short block height and i measure a total quench area of .040 or so, i may not clear if it was measured?
    Last edited by hazelj80; 01-18-2021 at 05:54 PM.

  16. #16
    FEP Power Member Ethyl Cat's Avatar
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    It is pretty simple
    If you keep the point where the stem meets the valve at the same height and start at 0 degrees valve angle you are tipping the edge of the valve towards the intake side of the chamber .0175" per degree on a 2.0" diameter valve.

    11r head= 11 degrees
    stock = 20 degrees

    9 degrees X .0175" per = .1575" of clearance gained or lost depending on which way you are going.
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  17. #17

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    Quote Originally Posted by Ethyl Cat View Post
    It is pretty simple
    If you keep the point where the stem meets the valve at the same height and start at 0 degrees valve angle you are tipping the edge of the valve towards the intake side of the chamber .0175" per degree on a 2.0" diameter valve.

    11r head= 11 degrees
    stock = 20 degrees

    9 degrees X .0175" per = .1575" of clearance gained or lost depending on which way you are going.
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    that makes sense to me better to start with a valve that is at less of an angle (11) vs more (20).

  18. #18
    FEP Power Member Ethyl Cat's Avatar
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    If PTV is your only criteria I guess so.

    Use an LS for example. True flat tops and a 12-15 degree valve angle. .600" lift without issues .
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  19. #19

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    Cam timing is the other part of the equation, correct? probably the really important part?

  20. #20
    FEP Power Member Ethyl Cat's Avatar
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    Yep

    It all important dude, I was just talking about the relationship between valve angle and valve drop.

    All of the inputs needed to calculate PTV are important.
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