Sunday, July 22, 2007

Basic Jugs - I

HEADZUP! The fins on your cast iron cylinder barrels are brittle. Drop one, the fins are going to snap off and you're gonna have to buy another set. So put down some cardboard. And be willing to sacrifice a toe if a jug gets away from you.

After-market VW jugs come in two basic flavors and a variety of sizes. The two flavors are A-types, meaning they're to be used with the stock crankshaft throw, and B-types for stroker cranks. A-types have a greater crown height; the distance between the center-line of the wrist-pin and the top of the piston. Stroker cranks shove the piston right out of the bore. By using a lower crown height the piston sits lower in the bore and will not protrude as far, meaning you can use a thinner spacer under the jug, your heads won't be pushed so far away from the center-line and your valve-train geometry won't be as badly out of whack. Of course, that also means the pistons will sit slightly deeper into the crankcase at BDC, where the piston's skirt may interfere with the flange of the opposing cylinder. So B-types also have shorter skirts in that area, although not short enough for a really aggressive stroker. As the Mechanic-in-Charge, part of your job will be to ensure there is no interference.


Today, buying pistons by mail order is a crap-shoot. To see why, go read...

When buying pistons what you wanna do is stand right there at the counter, open the carton and inspect the color-dots on the tops of the pistons. If all four don't match, don't buy. It's hard enough building a good engine with good parts; it's virtually impossible if you start with bad parts. And mis-matched jugs are bad, bad parts. (When you do find an honest dealer, buy as many sets of jugs as you can afford. [Figure 1] Good investment if nothing else. [I'm sad to announce that OVW will be closing its doors on 25 Dec 2007. Nancy will continue to accept drop-orders but the friendly little store with its honest and honorable people will be no more.])

But let's say you've found a suitable set of jugs, no broken fins (feel them), color-dots are okay like the set of Mahle's in Fig 2. (Yeah, they're cast. No, they'll do fine at propeller speeds.) The first thing you want to do is number them. Use a marking pen or crayon to put a big, bold number on the inner flap. This is called a work number; it doesn't have anything to do with the engine's method of designating cylinders. But the pistons are matched to your barrels and the rings are matched to your pistons. You can't let them get mixed up. And they will if you don't mark them. (See Fig 3)

Go find a container that will hold and protect four pistons. Turn on your air and rig a die-grinder with a narrow cut-off wheel. (No got? Then use a rat-tailed file.) Find your vibrating scriber and have it handy. (No scriber? Then get a round-nosed punch. [Make one out of a nail or something.] ) All tooled-up? Okay, pull a jug outta the carton and use a hammer-handle to push the piston out of the cylinder. Put the piston into the plastic bag and fold it over on itself; the next step calls for spraying abrasive grit around and we want to keep it off the piston. Find the flat section of fins and use the file or die-grinder to transfer the jug's work-number to the jug. It should look like Fig 4 when you get done (only prettier). That is, three notches means '#3,' one notch means '#1,' and so on. Before you put it back into the box, mark the piston. Herezhow:

Lookit the face of the piston. Find the arrow. That shows you which way the piston has to be installed on the engine. Turn the piston over and scribe the work number on the wrist-pin trunnion under the arrow. Herezwhy: If you're building a really good engine you're going to have your valve heads, combustion chambers and piston-tops treated with a ceramic-metallic thermal barrier coating. When the parts come out of the oven the coating will have obscured any markings on the crown of the piston -- you won't know which way to install the thing until you measure the off-set of the piston-pin. So you put your work-number on the trunnion that must point toward the flywheel-end of the crankshaft.

So... write the work-number onto the underside of the piston under the arrow, put it back into its plastic bag and put it into the second box you've provided. Now you can put the barrel back into the carton. (But there you are, die-grinder at hand, and you just know you could do a better job cleaning up the parting-line flash on those fins.... Don't. No air flows across the parting-line. [Think about it.] It doesn't matter if the fins are open or closed at that point. Grind on the things and you'll just be spraying lotsa abrasive grit around.)

Okay, got the gen? Then do the same procedure to the other three jugs and whistle when you're done; I'm gonna go cop a smoke.

You'll love this next step! A chance for you to exercise your artistry in paint. Herez whatcha need: Some flat black paint. If you don't have any, make some by mixing a tad of naptha with glossy black paint. If using Rustoleum Flat Black in the half-pint can as shown in the photo, you'll need to thin it with about an ounce of mineral spirits. But before opening the can, make sure it is at room temperature. Then shake it for at least two minutes. Don't guess; check a clock and give it an honest two-minute shake-up. Provide yourself with a stirring stick, open the can, add the mineral spirits and stir for at least one minute.

Obtain an inexpensive 1" paint brush; something cheap enough to throw away after using. (Why? Because it the cost of the mineral spirits needed to clean the brush is more than the price of a new brush. Using a pair of heavy shears cut off about two-thirds of the width of the bristles. You've now made a fin brush :-)

As you can see in the photos I've threaded the barrels onto a piece of extruded angle supported at both ends; a broom stick or piece of plastic pipe would work as well. Do not use anything that can scratch the barrels.

This particular set of pistons & cylinders was free of cosmoline. Some sets are not. If the jugs are greasy they must first be washed in solvent to remove the grease.

All set? Then go ahead and paint the cylinders. Try to keep the paint off of the machined surfaces. Use a paper towel dampened with mineral spirits to wipe off any mistakes; the machined surfaces must be perfectly free of paint when we assemble the engine. Removing it now is easier than removing it later.

It generally takes me about half an hour to paint a set of jugs. If this is your first set, you won't need that long. When you're done, wrap your paint brush in plastic and allow the paint to cure for about half an hour. Now take a strong flashlight and inspect your work for holidays. Surprise!

Okay, so it's a bit harder than it looks. (Which is why it takes me about half an hour.) Unwrap your paint brush, stir up the paint and do the job properly.

Once the jugs are painted it will take a day or two for them to dry sufficiently to be baked. Baking hardens the paint. It also causes it to shrink. If the paint is not fully dry, baking will cause the paint to crack, rust will form in the cracks and your engine will look like hell. And run hotter than it should, since rust makes a dandy insulator.

Another reason we must have a good paint job is because the next step in prepping our jugs is to give them a bath, complete with lots of scrubbing and a boiling water rinse. If you've missed a spot with your painting, your jugs are going to start to rust even before you get the engine assembled.


NOTE: You also need to check the length of the barrels within the set. All should be the same to within about .001" This may be checked with a surface gauge (ie, you don't have to actually measure the things -- just make sure they are all of equal height between the sealing surfaces). If the height is out by no more than .0015" you can live with it. Up to about .003" you can shorten the three longest cylinders. But any error greater than .003" is simply too much work; it would be best to find a more accurately made set. When checking for height measure at least three points of the circumference. You will occasionally find a barrel in which the sealing surfaces are not parallel to each other. If you have a shop full of equipment you can re-machine the barrel to make it square then adjust the length of the other barrels to match. But on the whole, you'll be miles ahead if you start with more accurtely made parts. -- rsh