Important points on fitting Pistons
Piston Fitting 101 from WPT, Mike Wienandt
Bore in mm 62.28 ,+.020
Stroke mm 55.6
Con Rod Length, mm 111.2
Cylinder # 1 Bore 2.7753 70.4926mm A----------- Above ring
A--Piston Dia @ above ring 2.7592 0.0161 B-----------Below Ring
B--Piston Dia. @ below ring 2.7659 0.0094 C-----------Across from wrist pin
C--Piston Dia @wrist pin 2.7703 0.0050 D-----------Bottom of shirt
D--Piston dia. @ skirt 2.7703 0.0050
Cylinder # 2 Bore 2.7760 70.5104
A--Piston Dia @ above ring 2.7587 0.0173
B--Piston Dia. @ below ring 2.7667 0.0093
C--Piston Dia @wrist pin 2.7703 0.0057
D--Piston dia. @ skirt 2.7700 0.0060
WIENANDT Performance Technology
When fitting pistons to an engine, there are many details that matter on the end performance. I will not give you the answers, but I will try to educate you on some of the guidelines and effects.
Above is the worksheet I created to understand some of the details. I measure the piston only with a micrometer, never a caliper.
Above the ring # A
Just under the Ring # B
At Wrist Pin # C
At the bottom of the skirt # D
Record those. The Skirt (# D) but almost all pistons bend, flex or just plain collapse the shirts. Most cases you will find the shirt collapse to be .003-.004” in a season of hard running. The Piston pivots on the # C spec and has only small wear/collapse at that point.
The # B spec is your magic. The closer that is to the bore, the better heat transfer from the piston to the cylinder. The closer also does not let the piston rock and pound the skirts to collapse as much. If this spec is larger, the piston will run hotter and expand more under load. Also looser can allow carbon to build up on the sides. Most people think it is blow-by. But in fact it is fuel baked on to the piston from being very hot in that area. Pistons have relief by the wrist pin area as there is more metal in the area to expand under heat. That relief piston to bore clearance increased at that point. Being further from the bore causing piston temp at that point to increase. Thus the build up on the sides.
If the # B spec is too tight the piston will sick/seize/fail/blowup, Stop motion as it welded itself to the bore. @#$%$%^^&. If this spec is too loose, performance will be less.
The bottom/skirt is larger/closer to the bore. The only way to stick the lower skirt is to have a cooling failure. The majority of the heat is coming from combustion, on top the piston. The way the piston transfer the heat through to piston to dissipate is dependent on fit and the structure design of the piston.
The # A spec is important in other way. Too small lets a lot of heat at the ring. The ring can break down from over heating. A big top diameter makes great power but must make sure it is not rubbing on the bore. This causes friction and heat with will over heat the piston and change the hardness of the heat treat. The now softer/weaker top can sag and then pinch the ring. Power gone.
Piston Coatings: Ceramic top coating is not thick enough to really do what they say. In my opinion, a waste of money.
Side coating has many types. Most are really break in coatings. The keep the piston from scuffing while the piston molds itself to the bore. When the pistons are broke in the coating is about worn away. So, I gues it did it’s job to that point.
I have found one coating that I fell in love with. It is much thicker and conforms to the bore. It can build .003 to .005 over size and fits tight in the bore. It will wear to the natural fit of the bore. At the # B spec, I have found pistons .003 larger than a new piston at the end of the season. This coatings transfers heat better, minimizes the pistons ability to rock back and forth so that makes collapsing the shirts much less. This is a friction reducing compound that conforms to the distortion. I have done tests for the company doing A-B-A tests. Results showed + 3hp on a 50 hp engine. I now use this on every engine I build where the rules allow. Great product for doing piston development
Let go back to clearances. A new piston might have .005 clearance at the #D and could be .010 @ the #B.
I have heard some guy like used pistons. If it was .005 on skirt and skirt collapsed .003. You fit to .005 on shirt, now you fit the #B spec too tight. Failure waiting to happen. You must think it through.
