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Need Help w/ Valve Grinding Basics


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  #1  
Old 03-12-2005, 08:50:11 PM
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Jim McIntyre Jim McIntyre is offline
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Default Need Help w/ Valve Grinding Basics

I'm working on a 17HP Ruston diesel. The intake valve casting was cracked, so I replaced it, and the intake valve, spring, etc. with a pulloff from another engine. The replacement valve and seat looked pretty good to me as I inspected it even under a 10x loupe- no pitting, rust, cracks, etc. But it leaks. Badly. The clearance is OK.

So, does it need to be lapped or ground?

Any tips on doing that, compound to use, etc.? - I've not done it before.

Thanks
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  #2  
Old 03-13-2005, 10:30:47 AM
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Keven Withers Keven Withers is offline
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Default Re: Need Help w/ Valve Grinding Basics

Lap it in with grease mix lapping compound you can purchase at auto parts store. When it is lapped in you should see a nice grey ring around the valve head and seat. Good Luck, Keven
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Old 03-13-2005, 10:41:21 AM
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Andrew Albrecht Andrew Albrecht is offline
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Default Re: Need Help w/ Valve Grinding Basics

There is a tool that is a wood stick with a suction cup on each end (2 sizes) and you wet the cup and stick it to the flat surface of the valve. apply a little lapping compound to the seat area. and place the valve in the guide, roll the stick back and fourth between your hands raising up and down after every few laps or rolls of the stick. Like it was said a small grey line will apear on the valve and seat all the way around when it is nicely lapped in for a good seal.
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Last edited by Andrew Albrecht; 03-13-2005 at 10:42:30 AM. Reason: spelling mistakes
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Old 03-13-2005, 11:47:08 AM
Randy L Peterson Randy L Peterson is offline
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Default Re: Need Help w/ Valve Grinding Basics

Another thing make sure the valve isnt slightly tweaked or bent... But it looks like lapping the valve in is the best way... Sounds like a fun project
Havin a blast preservin the past
Randy L Peterson
South Dakota
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Old 03-13-2005, 12:36:43 PM
Andrew Mackey Andrew Mackey is offline
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Exclamation Re: Need Help w/ Valve Grinding Basics

There are several companies that make lapping compound. You want either a combined tin that has both a coarse and a fine mix compound, or a small container of each type. Also, you will want to go to a good hardware, or home improvement store and pick up a weak spring that will fit loosely around the valve stem, and will reach the bottom of the valve pocket. The purpose of this spring is to lift the face of the valve off of the seat, as you are lapping the valve in. The procedure is as follows: 1) Make sure there is no carbon or debris under the valve face and seat, even a tiny bit will cause compression leaks. Make sure the valve mating face is flat, and that there is not a groove worn into it. If the valve face is grooved it will not seat properly. Have a machine shop check the valve for condition (stem wear, true head, etc.), and have them re-face the valve. 2) Check the valve and guide for wear. With the valve open, if you can shake the valve head more than an 1/8 of an inch, a new guide and maybe a valve are recommended. This is due to the fact that if the stem or guide is badly worn, one side of the valve will impact the seat off center, and this will cause deformation of the seat - the seat will be beaten out of round. I have seen engines with a 1/4 inch of play run, but the compression will be erratic as sometimes the valve does not seat properly. 3) Check the valve for square. Place the valve stem on a pane of glass, or a surface that you know is flat, with the valve head off the edge and clear of the side. Roll the valve for several revolutions, and note if the stem lays flat on the surface, and if the head of the valve stays steady. If the stem or the head appear to wobble, the valve is probably bent, and no amount of lapping will seat it. Be careful handling the valve. A fall of even a few feet can bend the stem! 4) If all these tests are satisfactory, then you are ready to start lapping! Place a thin bead of the course lapping compound on the face of the valve, on the mating surface, about 1/8" thick. Place the spring on the valve stem and put a little light oil and then the valve into the guide. BE VERY CAREFUL that you get no compound on the valve stem! That will be the quickest way to wipe out the stem and guide imaginable! Push the valve against the spring pressure, until the head meets the seat, and rotate the valve back and forth, a complete 2 or 3 turns in each direction. Let the spring lift the valve off the seat a little bit, and repeat. After 4 or 5 times of repetition, carefully remove the valve from the guide, and wipe off the compound carefully, making sure to keep it off the valve stem and guide. I can't emphasize enough to keep this stuff away from anywhere but on the valve face and seat! After wiping the surfaces off, look at the face and seat. The seat should have a band of wear that ie even in width around the entire circumference, on the mating surface. The valve head, in turn should have an even wear and width on the mating face. Check the valve first. If the face is not worn completely around the contact edge, then the valve is likely to be bent, and will have to be replaced. If the valve looks OK, then inspect the seat. The wear here should also be even and of equal width. If the wear is not even (one side worn, the other only a thin line, or several heavily worn spots, then the seat will have to be reground. A machine shop will have to do this.
If the wear is pretty even or if there are only slight gaps, then you can proceed with the lapping operation Again, you re-apply the course compound to the valve face, and apply pressure to the valve to make contact. Roll the valve several turns in both directions and releave pressure. Turn the valve 1/4 turn. repeat the lapping. Continue this for several minutes, (about 20 repeats). You should feel the resistance to turning the valve decreasing as the seat and head align themselves. When you are done, again clean off the valve head and the seat, making sure to remove all of the course compound. Even 1 grain left will ruin the next step! 5) Inspect the surfaces for even wear and spacing. If all is OK, apply the fine lapping compound, and repeat the lapping process. As the surfaces clean up, the valve resistance to turning will decrease as the faces wear in, and the compound is depleted. After about 40 rounds with the fine compound, again check the faces for even wear. You should see a light grey clean face on each surface. If that is true, remove the spring from the valve, wash everything thoroughly with kerosene, and then spray thoroughly with WD-40, and assemble.
I have found that the lapping stick works well, but there are alternatives. If the valve head has a slot cut into it, you can use a screwdriver. A better solution is to obtain a valve lapping tool! This can be found at a good auto supply store, like NAPA auto stores, or perhaps a good mechanic or fellow engine collecter may have one you can borrow. This tool automatically rotates and backspins the valve, as you crank the handle, to insure proper wear in at the faces. It comes with different tips designed to engage the valve head. Some are the rubber cups, as previousely mentioned, some have 2 pins that set into holes in the valve (early B&S for example), some are blades that fit into slots in the valve , and last, some valves are actually held together with a nut, and the tool will be fitted with a socket. If you have further questions please EMail me.
Andrew
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Old 03-13-2005, 11:29:16 PM
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Default Re: Need Help w/ Valve Grinding Basics

Wow. Andrew - thanks for that detailed procedure. I'll stop at NAPA tomorrow and pick up some grinding compound - looks like they have two types - fine and coarse, I hope.

The valve has a tapped 3/8 hole in the surface and I threaded a short shaft in it, so I'm all set to go.

On further inspection today, I found the stem and guide to be in great shape - hardly any side-to-side wobble at all. But, I put a straightedge along the valve face and discovered that the valve face is concave. The mating surface on the seat is convex (bowed out a bit.) I wouldn't guess that any amount of lapping will correct this, right? Does this mean I need to have face and seat reground at a shop, or should I go ahead and try lapping?
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Old 03-14-2005, 01:07:07 AM
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Default Re: Need Help w/ Valve Grinding Basics

Andrew
I too would like to thank you for that complete explaination of the valve lapping procedure! You wrote it so even a rookie like myself understands what's going on. Have lapped in valves before but never really knew what was going on totally, like I believe I do now!
Thanks again
Jim Hunter
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Old 03-14-2005, 10:21:40 AM
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Default Re: Need Help w/ Valve Grinding Basics

Lap it in some first this will show if you need to get the valve machined You probably wont need to machine the seat. Keven
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Old 03-14-2005, 11:10:20 AM
Andrew Mackey Andrew Mackey is offline
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Exclamation Re: Need Help w/ Valve Grinding Basics

JIM,
It all depends how deep and pronounced the relief is. Try this: try opening and closing the valve in place with no spring on it. As you close it, apply slight pressure to one side of the valve head, in different locations. If the valve closes smoothly, just lapping should be OK. However, if the valve seems to rest and then jump closed, then the valve should be refaced. The edge of the relief is catching on the valve seat. Is the indentation on the valve face really noticable? If so - reface. After facing and lapping, your valve stem clearances will also have to be adjusted in order to bring the clearances back to spec. On most engines, there is an adjustment to be made on either the pushrods or the rockers. On most smaller, more modern engines (B&S, Kohler, Techumpseh, etc.) the ends of the valves must be ground to regain lost clearances!
Andrew
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