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Valves clearence too tight

grepper

Old Bike Nerd
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I'm rebuilding a basket case engine, 81 cb750k. The valves were not labeled or numbered. So I just cleaned everything and gave them a lapping.
I'm at the point where I'm setting the valve clearences, yes I know, .005.
So, I couldn't get a .0015 in with a the smallest shim I have, 2.75. So I ordered even thinner shims and again, even with a 2.30 shim I can get my smallest .0015 feeler guage in. What the heck? Yeah I lapped them but not that much.

Any thoughts?

 
Death. When you mismatch used valves to the holes they no longer fit tightly on the seat and lapping in enough to make them so WILL remove the nitride coating. Why one NEVER mismatches the valves, it leads to insurmountable running issues. With nitride gone the valve is so soft underneath any valve work lasts maybe 5K miles-10K max. I personally have witnessed 6K miles and most of the valves burned.

Your valves are now too long from mismatch, seat recession and the added lapping. The mismatch may have others too short. You may need to shorten the valve tips, which will lead to other issues as the ends are nitrided too.

That problem shows up when head has the seats cut too much too.
 
Well AMC, I've been reading your responses to threads for a couple of years here. There's always a lot of doom and gloom from you, but most often I agree in your assignments.

So, I have another engine I could pillage, an 82.

Do you think I should try to use that head? I'll have to check the clearences before I pull it.
Or, try something else, like all new valves?

 
Use the other head, new valves will kill you in price.

I apologize for all the 'doom and gloom' but I figure people like the truth straight up and quick rather than being mislead in a friendly fashion to find out after all the misleading things still won't work. That outlook while very pessimistic got me paid pretty well when I used it to sort out many issues others could not on hundred foot long printing presses with 5 million moving parts. Machines will always do what they will according to the physics and human feeling one way or the other affects them not one little bit. Why one often must divorce themselves of the human thing to fix the problems. A healthy dose of reality squashes most dreams. Showing of course the dreams were not based in reality to begin with. What we do.

Not saying I like that but I used it to further me pretty good. Murphy's laws are in reaction to the human condition which is commonly not one of sound logic.

Not only that but my being wrong means that usually something good has happened as versus what I predicted, it is SO much easier accepting being dead wrong when something good has happened as versus something bad...............it works out all the way around, at least for me.
 
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The problem with mismatched valves can be shown like this.....................you have a valve that has worn in to a seat perfectly. With Honda cheap crap valves that are super soft steel with only a .003"-.005" max thick heat treat (the nitride) over them, you have a dead soft valve material that under the seat flows somewhat with heat to allow the seat which is relatively cooler because of the head material surrounding it as a heat sink to sink into the valve a bit. Like a stamping process. At higher mileages that exhibits as a slightly sunk in ring on the valve where the valve contacts. That ring will be the exact location, OD, and ID the valve touched on. The valve right next to that one will in all probability be sunk in a different spot wider or narrower and no way will simple lapping make the two rings converge on each other to match without you having lapped too much to remove the nitride first. You will be removing 7, 8, 10 thousandths there, the nitride was gone at 3. Even worse will have the ring half on and off the other ring and practically zero seal there at the stairstep where the two meet, or a seat half the normal width to burn almost instantly when put in use. 3 or 4 valves MIGHT come really close (luck of the draw) but many will be off, there is no way to mix the valves up and EVER get back to full sealing, the odds against it being............I can't think in numbers that big.
 
It's a pain in the -ss but the first thing I do on one of these is to get 16 heavy duty freezer quart bags and every valve, lock, retainer, and spring/tappet, the whole nine yards on each valve gets marked and kept separate until re-assembled back into the original location. Unless the valves/seats got cut you need to do it on every engine out there, I did it for years working on car engine rebuilds, sometimes part location doesn't matter so much but on these little sewing machines it does. You lose your butt too much with setting up running clearances if you don't. (sound familiar?)

Just wait until you change cams.............more resetting fun. Many don't do that necessary attention to detail and then the cam change just results in all kinds of problems. Burned valves way early, dead cam lobes, the works. Thank God for rocker arm motors where you don't have to do all that.
 
Yup, you are right. I'd have $300 into new valves with no guarantee the old seats would still be an issue. I pulled the valve cover off the 82 engine and at initial inspection everything looks good. Of course I'll have to take everything apart and do a thorough inspection.

So, given the thin nitride coating, should I lap them at all?

 
.................until you accidentally knock them on the floor to scatter parts in a million directions. BTDT, that habit went in trash years ago. I want those parts positively contained to be in place still a long time later if needed, projects sometimes being what they are. I consider going in and out of plastic bags necessary insurance. Part of the attention to detail that always has the finished product run excellently. What has to happen with too many irons in the fire, sometimes you have to leave it for longer than you'd like, and still pick back up where you left off.

Grep, lap them LIGHTLY.
 
OK, Lap them lightly, got it.
I'm going to be able to build this top end in my sleep. Fist time I realized I forgot the O-rings at the bottom of the studs. Took the top end off and ripped both the base and head gasket in the process. New gaskets, put in 0-rings and reassembled again and discovered my valve length/clearance problem. This time I'll be taking apart the 82 top end and cleaning it up, plus the bad top end of the 81. Then reassemble. I may use the 82 jugs too. During the last reassemble the jugs fell of the bench and I broke a couple of fins. I went ahead and ordered a head and base gaskets, plus I ordered a new set of vinton valve seals. Figured I shouldn't risk unseen damage from taking them off one head and putting them on an other.

BTW AMC, as for the doom and gloom comment, I get it. No point in sugar coating it. I'm a software developer, so an engineer of sorts. I know that machines do things for a reason, either by faulty hardware or bad instructions, there is not supernatural forces at work or free will. Just because we don't understand it, doesn't mean there isn't a real reason for it.
 
You got it. There ain't no magic about no thing, all the rest is simple lack of knowledge, so jump in there and make that part not so any longer. There are reasons for everything, it's just that we commonly don't take the time to understand them.

FYI, most of the work on the head setup like valve setting clearances you can do like one cam at a time with the head off the engine. I use like Moroso light tension checking springs for race cars, you can then pop valves in and out (and shims) easily by hand as well as turning the cams by hand. Set it all up then go back and replace the light springs with your normal ones and then put head on engine. Then a final recheck which will go quick. 99.9% of the time if you have carefully done the work the valve clearances will be the same with the tighter springs. The 4 valve head rebuilders do it all the time.
 
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