>How can you tell if you need new pistons or if just the rings need to be replaced. The motor has low compression like 75's on a couple of cylinders and smokes pretty bad. It's a 1300CC case will 85.5 pistons just drop in like I hear?
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Dear Jim (and the Newsgroup)
Easy question. You don't replace the rings. You replace everything... you install a set of NEW pistons & cylinders (they come with new rings... which you still must dismantle & check).
(Ed.Note: Why dismantle the rings from your new pistons? Three reasons: They need to be cleaned of all cosmoline before they can be balanced, you still need to check that the ring-gap is correct, and finally, the edges of the ring-gap need to be stoned to remove the fine feather of hardened metal left by the gapping operation. It is that tiny feather of metal that produces those vertical scratches in the freshly honed barrel.)
The reason you need to replace the jugs as well as the pistons has to do with an idiosyncracy of horizontally opposed engines, be it Franklyn, Corvair, Onan, Lycoming, VW or what-have-you. The problem is that the cylinders do not wear in a perfect circle, they wear more on their downward-side, ending us as a measurable oval. Since the typical cylinder hone, which was designed to be used on monobloc engines, follows the bore of the cylinder, honing does nothing but smooth your already eccentric, oval-shaped bore.
To achieve a properly centered, truly round cylinder you must re-bore. But before doing so you must install the jug into a special fixture.
Back when Volkswagen overhauled their engines at the factory they spent millions trying to get the same service from overhauled jugs... and finally gave up when some bean counter pointed out it was less expensive to simply melt down & recast the old jugs rather than try to re-machine them. (A factory overhauled engine had the same warranty as a new one. But the ones with used jugs had a habit of crapping out early forcing VW to honor the warranty... and spend millions trying to figure out why the reconditioned jugs failed.)
The fact the cylinder walls kept getting thinner had a lot to do with this too -- 85.5's are just bored-out 83's (which are just bored-out 75's!). The thinner the wall, the more difficult to maintain alignment during the re-bore.
You'll hear all sorts of tales to the contrary -- how someone threw in a set of rings and ran another zillion miles. Such stories are usually bullshit. Hone the jugs, throw in a set of rings, it'll run nice... for a while. Easy way to make a quick buck, if the customer is some kid; kids seldom keep their bugs or buses more than a year or two.
As to your pistons, the wear-surfaces are the ring-lands. That’s where the top & bottom of the piston rings form their seal. Putting in a new set of rings doesn't make any sense unless you overhaul the pistons. That means having the grooves re-machined to be perfectly perpendicular to the axis of the piston. The tricky bit here is that the inner corner of the groove must have a subtle radius. Cut a square corner, as you see all the 'experts' doing when they use an old piston ring as a carbon scraper, and you create a stress-riser -- the piston will crack right there in the corner. (BT, DT :-) Usta be, all automotive machine shops overhauled pistons. Nowadays the cost of labor is so high and the price of replacement pistons so low, the only slugs that can justify being overhauled are high-buck forged racing items.
All of that should give you some idea why the smart money is on a new set of P&C's rather than overhauling the old ones.
-Bob Hoover
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