Posted by Mark Taylor (Bright Racing Engines) on January 31, 1998 at 06:28:24:
In Reply to: Re: Damaged Valve Seats? posted by garrison on October 16, 1997 at 08:24:24:
: Probably, without a proper seat grinder, your best bet is to buy a new valve, and then get out the old valve grind stick, available at most better parts stores, ( you know,the thing with suction cups at both ends) some valve grinding compound, available at the same place, and go to work. however, BE careful not to get the compound in the guide. (putting a couple of small o-rings on the valve may help this.) and have a happy day grinding away. this job will require perserverance, but your other choices are: see if you can con a machinist to come to your house and do it with his grinder, but a seat grinding tool yourself $$., or have the truck towed to a machine shop. have a day!
: : In a previous posting I detailed my prob with no compression in one cylinder after installing new rings
: : I now recall that piston having several small "dings" in the top surface. Most likely something went thru there and
: : damaged the exhaust valve seat. As I'm still awaiting a manual to arrive, I'd appreciate any info on how best
: : to fix a damaged valve seat. Thanks for any and all advice.
Be aware that laping a new valve (or an old one)to correct a seat into a worn seat is asking for trouble, #1 it will sink the valve into the seat and can restrict airflow....reducing the performance of any engine, also this reduces the valve spring presure by bringing the
valve stem up higher....and combined with #2 the seat width becomes wider, this becomes a problem beacuse the wider the seat,the less seat presure per serface is on the seat....IE. if your spring produces 100.lb's at say 1.80 and the seat width is .050 the seat will have
less surface sealing pressure than a seat width of say .020
this reduced seat pressure and wide seat will allow hot spots and carbon buildup in the seat area...thus causing verry rapid valve and seat failure. also if the guide is worn very much the seat runout will be to much making the problem worse. in my oppion it is wirth the
cash to have a machine shop remachine the valves and seats....and replace the guides for maxium performance and durabilty.
on the other hand if you cant afford the machinist price, there is an alternatve to just laping the valve (I belive valve laping should only be used to check a seat and NOT regring it!)
if you own a bench grinder, 2 old valves and one new valve you can
get good results by doing the folowing.
first, take an old valve and grind it down @ 60deg. (when I did it this way, I used an old valve guide welded to a small post in front of the bench grinder allowing me to set any angle to gring the valve at.)
second, take the other old valve and grind it's seat at 30deg.
the new valve should be a perfect 45deg angle.
take the 60deg valve and use some sticky back sandpaper...cut some small peices and stick them to the 60deg seat, drop the valve into the hole and hook a drill to it's stem on the other side. use the drill to rotate the valve untill
the grinding marks are all the way around the bottom of the old seat. Now do the same thing with the 30 deg valve and some sticky back sandpaper, bring the 60 and 30deg almost to a point.
now take a black marker and mark the seat on all 4 sides, use the new 45deg valve and sticky back sand paper to grind the new seat, the marker will show you how wide your new seat is, now peel off the sandpaper from the valve, and add some FINE valve laping compound, use the drill one last time to lap the valve in....youre all done!
this prosess is not perfect, but it is better than "Lap Gringing" the seat, it will lower the valve in the head a little, so make shure to shim up the spring to get back the installed spring pressure. one last thing....MAKE SHURE you clean the valve guide and valve and lubracate both before putting the head back toghther,(I recomend polishing the valve stem as well)
hope this helps some.
MT.