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Thread: hand lapping valves and checking seal

  1. #1
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    hand lapping valves and checking seal

    So, two questions for discussion:

    1- What are peoples opinions of hand lapping valves. Used and/or freshly ground. And by hand lapping I mean, compound goes on valve, power drill spins valve while tapping against seat.

    2- How is valve seat to valve seal checked using a spotting agent like Prussian blue/HiSpot Blue? Without being able to control film thickness I dont see how you can know what sort of seal you're getting. Maybe something like a permanent marker would work better because it would seem like its film thickness would be extremely thin and more consistent, but it dries so fast that might be an issue.

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    turbo addict Turbo Mopar Contributor iTurbo's Avatar
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    Re: hand lapping valves and checking seal

    I found what I thought was a pretty good video on YouTube about hand lapping valves. Using a power drill doesn't sound like a very good idea because it looks like the process takes some finesse and listening.

    https://www.youtube.com/watch?v=7GEmuQa3dPY

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    Re: hand lapping valves and checking seal

    Quote Originally Posted by iTurbo View Post
    I found what I thought was a pretty good video on YouTube about hand lapping valves. Using a power drill doesn't sound like a very good idea because it looks like the process takes some finesse and listening.

    https://www.youtube.com/watch?v=7GEmuQa3dPY
    Thats a fun video.

    The last time I did this I was trying to get a head back together just to be drivable again at minimum cost. The exhaust valve was so badly pitted that it was taking forever to hand lap it, I could have spent probably 20 minutes on one. So thats when I used the drill. Otherwise I wouldnt suggest it if its a matter of 30 seconds versus 2 minutes doing it by hand. I agree the sound and feel method is going to be a big advantage.

    - - - Updated - - -

    Although Im not so sure what that guy is hearing as far a pitch change is the change in the seat/valve versus just the lapping compound breaking down into a finer grit like its supposed to.

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    Re: hand lapping valves and checking seal

    I see no need to hand lap valves & seats that are freshly ground. Checking those parts you don't need or want to turn or twist the valve. Just put bluing on the valve face and pop the valve against the seat a couple times and check the witness mark on the valve face.

    As for putting used parts together out of necessity, I'd be very hesitant to use a drill with lapping compound. Slow & steady by hand would make me much more comfortable. I've used an antique pnuematic valve lapper in the past and it didn't turn the valve much, just some short but reasonably slow back&forth motions, figure about 1/4 of a turn. Use the same procedure above to verify the witness mark for contact between the valve face & seat all the way around. Don't twist/turn the valve with the bluing on it while trying to verify the contact area since that will mask low areas that aren't making contact.
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    Re: hand lapping valves and checking seal

    I just did this on a GRM Challenge motor...it was pretty easy. I did use a drill and then did a final clean up by hand, but my valves and seats were pretty crappy to start. Even with the drill though, it barely cut at all. I went very slowly and wasn't using much pressure. I then tested the seal on each valve by filling the chambers up with water and pumping compressed air into the port. If I saw bubbles, I did a bit more work. If I didn't, I called it good.

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    Re: hand lapping valves and checking seal

    I've got three of those little steel tubs of Clover compound, in three grits. Probably bought them in the mid '70s and I've done a number of touch ups with them, mostly on motorcycles. Seems like I'll never use them up as you need just the slightest dab on each valve face. The fine grit is what I use mostly.

    The thing is, as the valve to seat face wears, it gets wider, and with lapping it gets even wider, and a wider valve to seat contact area is usually a step backward in terms of flow. For that reason it is sometimes best to re-cut all angles of the seat and valve. That said, contact that is too thin breaks down quickly and is unable to transfer heat to the head.
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    Re: hand lapping valves and checking seal

    Are you lapping valves just as service or part of valve grind job valve and seat ?

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    Re: hand lapping valves and checking seal

    I have lapped valves when I have swapped valves from the old head to a new head but that's about it. I have also not lapped valves at all doing that and had no issues.

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    Re: hand lapping valves and checking seal

    Every head I've ever had apart, I always hand lap the valves back in.
    If I get an engine back together only to find that
    one cylinder has low compression due to a valve seat not sealing.... AAGGGHHHH!!!
    For the amount of time it takes to lap the valves in, you'd be a fool to NOT do it.
    It's just part of being thorough.
    No questions after the key is turned; but that's just me.

  10. #10
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    Re: hand lapping valves and checking seal

    Valves spin so not much good to only check in one position. You are then assuming your machine shop guy is perfect. Some are. I have had one shop chatter my seats and they assembled it. Later for repairs, I had a cylinder head only shop explain without me even asking how they would keep things lined up including how they reset for every valve seat due to the fact no head is perfect. Then I watched him grind a few valves at a 47 degree angle because we were trying to save the heavily damaged valves and retain some margin/thickness at the circumference of the face.

    If you are slapping junk back together it sounds fine, if you are trying to slap "good stuff" back together without really fixing it, then I would take note of the advice that heavy lapping will increase the width of the seat. It will also eat up your margin on the side of the valve face.

    Does the valve seat get chewed up a lot more than the valve when lapping?
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    Re: hand lapping valves and checking seal

    Quote Originally Posted by Ondonti View Post
    Valves spin so not much good to only check in one position. You are then assuming your machine shop guy is perfect. Some are. I have had one shop chatter my seats and they assembled it. Later for repairs, I had a cylinder head only shop explain without me even asking how they would keep things line up including how they reset for every valve seat due to the fact no head is perfect. Then I watched him grind a few valves at a 47 degree angle because we were trying to save the heavily damamged valves and retain some margin/thickness at the circumference of the face.

    If you are slapping junk back together it sounds fine, if you are trying to slap "good stuff" back together without really fixing it, then I would take note of the advice that heavy lapping will increase the width of the seat. It will also eat up your margin on the side of the valve face.

    Does the valve seat get chewed up a lot more than the valve when lapping?
    The point of valve chatter during grinding is an excellent comment.
    There are a wide variety of stones that can be used and the finer stones while making minor cuts vs 'one-pass' grinds can strongly influence the success of the mating surfaces.
    It is good practice to lap the valves once the seat/valve grinding is completed if only to insure that the contact width and location are correct and consistent throughout the entire diameter - on both surfaces.

    The video covers the concept very well with the exception being the change in audible 'grit' noise while lapping.
    This more to do with the compound granules breaking apart and becoming finer during the process rather than the two surfaces being mated.

    The picture below (although being a non-multiangle cut) exposes the way the compound witness mark should appear throughout the entire valve and seat diameter.
    Any pitting or variation in width exposes damage to either surface that needs to be reground while variation in thickness questions whether either were ground off center due to a bent valve, poor tooling, excessively loose guide or a loose pilot tool.

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  12. #12
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    Re: hand lapping valves and checking seal

    There's some permatex valve lapping compound to avoid, it's supposed to be multigrit.... in that it breaks down to finer grit as you use it.... but what happens is that you've just got it getting a nice fine finish, then it pulls in a "rock" from the edge and scores it up. It's okay for busting the carbon off and mowing down pits though.

    Anyway, I think it was those "Help" products, had two grades of compound on a card. The fine on that was pretty good for finishing up.

    When I thought I was done, I wiped it all off and dripped water around to see if it wicked in or leaked.

    I did use a drill for the "mowing down" stage, for carbon and pit removal, but on the fine grit, it seems better to hand lap with a stick tool to get feedback.
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    Re: hand lapping valves and checking seal

    Quote Originally Posted by RoadWarrior222 View Post
    There's some permatex valve lapping compound to avoid, it's supposed to be multigrit.... in that it breaks down to finer grit as you use it.... but what happens is that you've just got it getting a nice fine finish, then it pulls in a "rock" from the edge and scores it up. It's okay for busting the carbon off and mowing down pits though.
    Anyway, I think it was those "Help" products, had two grades of compound on a card. The fine on that was pretty good for finishing up.
    When I thought I was done, I wiped it all off and dripped water around to see if it wicked in or leaked.
    I did use a drill for the "mowing down" stage, for carbon and pit removal, but on the fine grit, it seems better to hand lap with a stick tool to get feedback.
    The Clover compound mentioned earlier in the thread works rather well - stir it well before use.

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