Lapping Valves

Lapping Valves

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As I readily admit, I'm no master mechanic. I didn't take auto shop in high school, and while other guys were fixing up their 57 Chevy's, I was singing in the choir, taking advanced math, and in general being a nerd (except lettering in track). I assume many of you know more than I do about auto mechanics, and this is frequently borne out by responses to my posts to WillysTech.

But I have lapped my valves with considerable benefit, so for those of you that are unfamiliar with the process, let me explain.

My engine was running very rough. A compression test showed 90, 115, 60, 50, so basically it was running on 1 1/2 cylinders. I had adjusted the intake valves about a year before, but didn't do the exhaust valves because I didn't dare pull the exhaust manifold (on account of the badly rusted studs and nuts). Someone has since told me he adjusted his exhaust valves without pulling the manifold, but he admitted space was really tight.

I removed the head from my F4-134 to find mildly burnt valves, with the exhaust being a little worse than the intakes, as I would have expected with my neglect of them. If a valve doesn't seal tight, a small amount of hot combustion gases will escape. This high-pressure, hot gas erodes the surface of the valve and seat, leading to what is called burnt valves. Burnt valves don't seal tight, so the condition compounds itself. Looking at the beveled edge of the valves, I could see irregular dark blotches, not smooth shiny metal.

I got a lapping kit from Checker for less than $10, which consisted of a wooden stick with suction cups on each end and two tins of abrasive compound: coarse and fine. With the valve springs removed, the valves are free to come out. It takes a valve spring compressor tool to relieve tension of the springs so you can remove the clips that hold the springs to the valves.

I scraped off carbon deposits from the valves. I put a little lapping compound (about the consistency of toothpaste) on the valve surface, then put it in the head (or block for exhaust valves). I used a suction cup on the flat face of the valve and by twirling the stick between the palms of my hands, spun the valve in its seat. I used coarse first and then fine on the exhaust valves, but only fine on the intakes. I checked the surface of the valve and matching seat from time to time and repeated the process until each valve looked good.

When I got the engine back together, the compression was 112, 115, 115, 80. Number 4 cylinder has some bad scratches, perhaps made by a broken ring or a wrist pin that came out. Now I'm running on 3 1/2 cylinders. Some of the improvement could have been accomplished by a valve adjustment, but the lapping polishes the valve and seat, eliminating minor irregularities in the mating surfaces, leading to a tighter seal.

The 134 engines originally had a valve seat cut directly into the cast block. This metal is too soft and wears rapidly, especially with unleaded gas. My engine had been fitted with hardened valve seat inserts. Any machine shop can do this for you if your engine has not yet had this done. A seat insert cost about $5, but you have to cut a land in the block and press the seat in. I was glad to learn that someone had already done this on my engine.

The lapping could be done in a day, except in my case I broke a stud, then broke a screw extractor, then broke several carbide bits, and eventually put a helicoil and new stud in. I spent several days.

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Lapping valves is as old as the internal combustion engine. I've been told the owner's manual of the Model-T recommended lapping on a regular schedule, maybe once a year. A valve job at a machine shop puts a precise bevel on the valve and seat surfaces which is superior to what you can do with lapping. After you lap the valves a few times, you will have a groove in the seat which is not desirable, so you'll have to have them ground and or replaced. If a valve is seriously burnt, it must be replaced. Lapping is the least costly, least effective and easiest repair to burnt valves.

I'd guess (and this is only a guess since I haven't done it) that if you adjust your valves regularly (once a year?) and lap them when compression indicates it is necessary, you can go quite a bit longer between valve jobs.

Rick
9/10/98

what is accomplished by lapping your valves? : MGB & GT ...