"Getting competitive power is clearly much more than just phoning up the local speed shop and ordering one of each. Every part of Gietl’s engine is the most recent chapter in an entire book of development as yet unfinished. The pistons and their rings are fine examples, They are full-skirted Venolia/Alcoa forgings, machined to Gietl’s drawings. The top ring is stainless, of Dykes L-section type. The second ring is rectangular in section, and of cast steel. The oil ring is a three-piece affair.
Why? He had noticed in dyno testing of newly completed engines that the weaker ones tended to blacken their oil far more than the good ones. Poor ring performance was allowing oil to pass into the chamber, carbonize on the wall, and then be swept back into the sump. The rings from such engines showed less than ideal contact with the cylinder wall. The rings hadn’t seated and sealed very well at all.
Gietl examined the break-in process. He decided that the familiar cross-hatch hone pattern left by proper cylinder preparation is nothing but a one-time double-cut file to shave the rings into intimate contact with the wall. Once break-in was complete, the wide ring contact would be lubricated well enough to glide over the hone pattern, which would have worn down almost completely anyway. He reasoned that too good an oil film could stop the break-in prematurely. He therefore adopted a dry break-in procedure. The pistons, rings and cylinder bores are solvent- cleaned and assembled dry save for a drop of oil on each piston skirt. The engine is started and run at half red-line for nearly a minute. Upon teardown, the rings were seen to have seated very nicely.
Because they are air-cooled, BMW’s cylinders are subject much distortion. The fins toward front get cold air while those at back get hot air. The tops of cylinders are closer to combustion chamber than are the to the the the the the bottoms. Therefore the rings must conform to a cylinder both bulged and tapered. Standard rectangular section rings usually have a good deal of spring tension. This is what presses them against the wall to form a seal at low speeds, below 3500 rpm. It was Paul Dykes who showed that combustion gas pressure, acting behind the ring, supplied the sealing force at higher speeds. A rectangular ring is too stiff radially to be flattened out onto the cylinder wall by gas pressure if that wall is the least bit wavy. Hence the Dykes, or L-section ring works better in such circumstances. Gietl has traced through all these developments in his own work. His present top ring is a stainless automotive L-ring.
There is a temptation to use only one top ring. After all, what can the second ring supply but drag? The G-50 Matchless used a two-ring piston and so do some of the Yoshimura kits. Gietl tried pistons machined for two rings and discovered that they needed much extra clearance to avoid seizure. Evidently, he reasoned, that second ring wasn’t a gas ring at all, but a heat transfer ring. Shielded from combustion heat by the effective top ring, the second ring could pour out piston heat through its broad face to the much cooler cylinder wall. Very well then. With three rings, clearance could be reduced to a very small value. Gietl’s second ring is a broad-faced rectangular steel ring.
The bottom ring is for oil control. Oil in the combustion chamber can lead to detonation, to plug fouling, to valve stem deposits and piston ring sticking. The stock BMW oil ring is one-piece, which Gietl finds works only in truly round cylinders. He uses a limber three-piece ring consisting of two thin rails separated by a spacer. There is the usual row of holes in the ring groove to drain oil scraped by the top rail, but where goes the oil scraped by the lower rail? In conventional designs, nowhere, but here there is another row of holes below the groove. Oil control is very good, and the piston crowns are a dry, creamy gray because of it.
A further impediment to the use of really tight piston clearances is excessive piston rigidity. Gietl removes a further 50-60 grams of weight from the pistons with handwork, concentrating on making the skirt as thin as possible. In his view, the piston is simply a guide for the rings and must hold their seal faces in strict parallelism with the cylinder wall. This is the compelling reason for tight clearance. A thin, springy piston can spread out on the wall a bit under load, relieving the oil film of concentrated pressure and placing more heat transfer area in the thermal circuit between piston and wall. With a close clearance, slight piston overheating causes an immediate increase in heat flow to the wall, cooling the piston and controlling the situation. His clearance is indeed tiny: .0015inch. The pistons run like this for several Nationals and are discarded when clearance reaches .0025inch. Teflon buttons retain 2.5 mm-wall tool steel wristpins made by Amol Precision. The high ground finish used inside and out eliminates many surface defects and is part of the reason these pins survive where others have perished."