Lap it - Make That Flat

If you've calculated the proper mean squish velocity for your engine (you are using the software programs indicated on my web page for doing this aren't you - the software is free) you may be able to achieve the desired head or top of the barrel milling/decking by careful work using a granite surface plate, some 3/8 thick glass, some various grades of wet/dry sand paper, some 2" wide masking tape and some valve grinding compound. This is the poor mans milling machine - it is more time consuming that having it done at a shop but once you buy the tools you could do a bunch of them if you wanted. Additionally, lapping heads, barrels, reed cages and just about every engine mating surface will almost insure that you'll never suffer a leak due to a warped surface. Simply put when things are VERY flat there is no place for a leak to occur. You'll soon find yourself lapping everything in your garage.

A small 2" to 3" thick by 14" square toolroom grade granite surface plate (or the largest size you can afford) can be had at some of the catalog tool sales depots for not a lot of cash. I have had a great one for many years. The trick to using them and making them last is to never grind against the granite. That's what the wet/dry paper and 3/8" glass is for.

If I knew I had to remove .005" or .010" from the surface of a head I would first measure its thickness at the bolt hold down bosses and record it. Then I'd take out my surface plate and put it on my bench to attach a piece of about 240 grit paper to it using 2" wide masking tape. One inch on the paper and one inch on the granite. Spray it liberally with WD or similar. Carefully, smoothly and slowly - use rhythm - move the head across the paper taking care to not damage the masking tape - since it will make it release. The idea is to move the piece in circles and rotate the piece to be sure the material is coming off evenly. Avoid rocking the piece so it doesn't get rounded off. Let the paper do the cutting - there is little to gain from pressing hard against the plate, in fact this could make the proces take longer. If it's not going fast enough you can use a rougher paper if there is sufficient material to be removed - then go back to the smoother paper.
Remove it and check its thickness periodically - say every 100 rotations to get an idea of how fast the material is coming off. When the dimension comes to within .001" of the destination, switch to 400 grit and "take it home." Many parts will need no further smoothness than what this provides. Stop the procedure when the dimension has been reached and there are no surface scratches. Aluminum cuts faster that aluminum/iron - that is to say if the job requires the barrel be decked a few thousanths, it is wise to start with rougher paper - say 120 and work it there until the dimension is within a couple of thousandths. It is more difficult to work with rougher paper since it wants to bite more at first. Pressing evenly and rotating the piece back and forth with help. Then switch to 240 for a thousanth, then to 400 for the rest.
Some parts can be further enhanced by lapping against glass attached to the surface plate. In this case it is necessary to use valve grinding compound as the cutting medium. Many companies sell their compounds in a small set of "course" and "fine." Parts like reed cages usually require the fine compound only. The glass will wear out after a while so it's a good idea to get several pieces made at once - I also use 1/4" thick glass with good results when I can't get the thicker stuff when I want it.
Any part that should be flat can be made really flat using this technique. It is an old one and is still performed on every component of every professional race bike on the track. It is a proven problem reducer and adds the benifit of becoming "one with" your engines pieces. I always lap heads, barrels, the reed cage surface on the barrel, the surfaces of the reed cages, and I'll radius the edges of the reeds themselves against very fine paper and then finish off their corners with the paper in my hand. If an engine is disassembled further I'll lap with compound the lower deck of the engine. It is necessary to remove head or cylinder studs before performing the procedures. Double jam nutting them will work if you don't have a stud puller.

Rick

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