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  1. #1
    125ccCrazy's Avatar
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    Valve clearance on a 5hp flathead????

    Ok I'm putting my 5hp flatty together, I put a small Precision cam in it (233 lift), heavier springs, locks and retainers.... I have 0 clearance bewteen the valve and lifter, I didnt think I'd have a problem since the cam is so small, nearly stock specs..... What would be the recommended clearance or is it 0 lash?? If I have to open the clearance what would I do, a little off the valve until I get the clearance??..

    Thanks in advance for any help

  2. #2

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    Can't remember right off what the clearance is, but you do need some. You grind a little off the valve stem to gain clearance. If you have none, when things heat up and expand, your valve will be slightly hanging open.

  3. #3

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    .005-.007 for the intake .009-.011 for the exhaust. Measured when the piston is down about 1/4" past TDC on the compression stroke.

  4. #4
    Senior Member Jay Wrix's Avatar
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    jimh has the right idea, I probably run the same cams, and those are my specs, and take it off the valve

  5. #5
    125ccCrazy's Avatar
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    Thanks guys for the help, I thought that was the procedure but wanted to find out for sure and what the specs should before I jumped into something I wasn't sure that I fully understood LOL.... Now when I turn the engine over I can see the lifter rotate as it starts to close the gap before coming in contact with the valve....... I must have filed.025-.030 off the exhaust valve before getting .010 lash and nearly the same on the intake so the valves were overly tight and probably would have caused some catastropic damage in a short time.. Now I can breath a sigh of relief..

    Thanks again...

  6. #6
    Supporting Speed Nut! minidragbike's Avatar
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    Wait!!!!!
    The less the better. Maybe 0.002" intake and 0.004" exhaust.
    You can grind either the valve, or lifter. It doesn't make a difference.

  7. #7
    Senior Member Oldsalt's Avatar
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    Quote Originally Posted by jimh View Post
    Can't remember right off what the clearance is, but you do need some. You grind a little off the valve stem to gain clearance. If you have none, when things heat up and expand, your valve will be slightly hanging open.
    You are dead right about zero clearance, or less, hanging the valve open. Especially the exhaust valve because it tends to sink in the block due to it's high temp and close up the clearance. I have run across flat head briggs motors that would run and be good engines if a simple valve adjustment is made. The last time I did it I cheated...I didn't remove the head. Took off the tappet cover and wedged open the exhaust valve with a screwdriver and ground some clearance between the lifter and the valve stem with a die grinder. It worked fine although this cure was on a rota tiller and for a mini I'd pull the head and remove the valve for clearance grinding.

    Another trick that I have used on briggs flat heads that have a valve frozen in the guide, usually from long storage or water getting in the motor, is to remove the head, cam and lifter. If an attempt is made to pry up the stuck valve useing a screwdriver the result will be a bent valve. However, if a hole is drilled in the bottom of the block, directly below the stuck valve, a long punch, with a diameter less than the lifter bore, can be inserted and used to tap the valve out the top. Clean it up and re-assemble. The hole in the bottom of the block is then tapped and a plug installed. Have saved a couple of engines this way.

  8. #8
    Member big"D"'s Avatar
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    I have a Briggs flat-5 that was running great. It came off a snow thrower and is now on my Cat 400x... I had a problem with the valve sticking. The sludge on the intake valve was so much that it would cause the valve to stick when cooled after running. I took the valve out, cleaned everything up and it ran great.

    Now that same motor started running terrible. Spitting and sputtering, backfiring. So I rebuilt the diaphragm carb... Twice. Still ran the same. Terrible. I checked the points, wires but all where fine. Then I pulled the head. I noticed the intake valve appeared close but would still easily spin untill the piston was 2/3 up on the compression stroke, then would finally close tight. This would allow 2/3 of the air and gas the engine just sucked into the cylinder to blow right back into the carb. Then when it warmed up a little, it would backfire and stop running...

    I just checked the valve clearance as prescribed here and the exhaust is fine but the intake is tight. I will Grind a little off the intake valve...

    What I don't get is this motor ran fine with all kinds of power all last year and part of this year them BAM... I've known for a while that I have gremlins in my garage but I didn't know they could get inside things sealed tight... What a clusterfuffle this has been...

    I love this forum. Most time the answers I need are right here in this Old Mini Bike forum... Thanks to all...

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