Pistons and Cylinders

by Al Brase


There are 4 main reasons to replace pistons and cylinders:

  1. Worn ring groove lands. Put a new ring in the groove and check the space that's left with a feeler guage. I'm a little rusty on numbers but I think that anything more than 4 thousandths (.004) inch is too much.

  2. Cylinder wall taper. To measure this it's really nice to have a bore taper guage. They're expensive and few shops have them, so what I use is an inside micrometer. This takes a lot of fiddling around and a lot of "feel" to do a good job of determining the wear. The wear mostly will be at the top of the ring travel, just below the ridge. Engines can tolerate some taper (up to .010 inch) but bear in mind that this makes the rings go in and out as the pistons go up and down their travel, and a lot of taper is going to wear out the rings and lands pretty quickly.

  3. Broken fins. The reasons for this are obvious.

  4. Good opportunity to upgrade to a larger displacement. There is a point of diminishing returns with this plan. (some think it's 85.5mm, others may feel up to 90.5mm)

What it mostly boils down to is economics. If you're a shop, you just sell the customer new parts, make the markup on them, save them the labor charge of cleaning the old ones, and the rebuild may last 100,000 miles instead of 50,000 miles (I said may). If you have more time than money, and the p&c's meet the above specs, then go for it! Personally, I'd rather spend a couple hours doing something more fun.

Type4 cylinders will have a good life after re-ringing. The existing GEOMETRY of the pistons and cylinders makes a big difference. If the motor being rebuilt was ever fried thru lack of cooling air, oil, too lean or too much advance, they may be damaged. But my experience is that p&c's always outlast exhaust valve guides and if you've got the heads off anyway....

A good inside mike is really helpful. Well, a bore guage is REALLY helpful, measuring anything with an inside mike takes a lot of feel and practice (but there's only one way to get the practice, right?). Cylinder wall taper shouldn't be more than 4 or 5 thousandths of an inch (.004-.005). I've assembled chevys with 10 but the rings only lasted 20k miles or so. The other three sides of GEOMETRY(that does describe the essence of it) are the piston ring grooves. With a new ring in place, there should be no more than .005 clearance between the groove and the ring.

Honing

Actually, glaze breaking. Real hones are rigid and used as the final step after a rebore to bring the bore to a precise size and finish.

Support the cylinder somehow. Best would be fastening into the case before you clean the case, but you'll NEED to clean the case then. It's my belief that you don't want to make the cylinders very rough, but you need some roughness to cause the rings to wear in. So, use a glaze breaker hone of 180-320 grit, slather the walls with honing oil (mix solvent or kerosene and motor oilso it's pretty runny) fasten the glaze breaker into the slowest drill you can find and move the hone the hone top to bottom in the cylinder while running the drill. Remember you're not trying to take out a lot of metal, just giving it a new finish.

The ideal that all the books called for was a 30 degree crosshatch. 25 degrees or 50 degrees, I don't think the rings know the difference, but try for some crosshatch. Maybe if your drill isn't very slow you might try triggering it as you move the hone in and out--30 to 60 seconds in each hole is enough. To keep the stones wet, I put the oil in an oil can (pump oiler) and slather it continuously.

Wait, you're not done! Now you've got to get that cast iron grit out of the pores of the cylinder walls. Chevy books said soap and hot water were the answer and it's sure easier to put VW cylinders in the kitchen sink than Chevys, but I think that clean oil-wetted rags used to wipe them down will work too. When they don't come out grey, they're clean.


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