Important points on fitting Pistons

**Mike Wienandt** · 04-18-2020

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.

**Mike Wienandt** · 04-19-2020

Buy three different brands of pistons for the same model engine.
Measure and record the A,B, C and D dimension. You will find a difference.
Some is due to the different alloys may have different expansion qualities. But some run better than others,
These are some of the reasons

**NERSTROM** · 04-20-2020

Mike, well written.

**Mike Wienandt** · 04-20-2020

More to think about:
Fuel and compression ratios. I'm assuming most motorheads here are playing with these feature.

If a Gas piston temp is a base, Alcohol piston temp are much cooler so for best performance Spec A and B can be much tighter. These clearances will certainly fail in gas engines though, even race gas. And some race gases burn hotter than others. So just understanding that can put you a head of your opponent.
The goal to good performance is complete combustion. Sounds simple, it is not.

Compression: This term to me has two main parts involved. Squish clearance. This is the measured distance between the piston and the outer portion (about 50%) on the head. We are talking about loopers not deflectors here. The Tighter the squish clearance, the more heat is forced into the piston. Ex pipe temp actually can go down. This puts the piston at risk. The A and B spec are again critical.

The looser the squish clearance blows the heat out into the pipe raising pipe temp and lowering piston temp putting the piston at less risk. It is a balance calculating the maximum squish velocity to get best/safe performance.
A tight narrow squish band may work as a wide loose squish band, but calculations are require to minimize damage and get best results.

Total Tapped Compression ratio. This is the other part of compression. This determines the fuel Octane/quality that is required. Pump gas runs fine at 6.0:1 Low grade race gas at 7.5:1 in the 100ish range. 8.2:1 is 105 or batter. Higher compression is more heat in combustion. This can effect your piston fits.
These different levels of fuels have different burn rates and temps so getting the timing set correct for each fuel is important.

Know your engine. the more you understand the longer you can go fast vs paddling.

**zul8tr** · 04-20-2020

Originally Posted by Mike Wienandt

More to think about:
Fuel and compression ratios. I'm assuming most motorheads here are playing with these feature.

If a Gas piston temp is a base, Alcohol piston temp are much cooler so for best performance Spec A and B can be much tighter. These clearances will certainly fail in gas engines though, even race gas. And some race gases burn hotter than others. So just understanding that can put you a head of your opponent.
The goal to good performance is complete combustion. Sounds simple, it is not.

Compression: This term to me has two main parts involved. Squish clearance. This is the measured distance between the piston and the outer portion (about 50%) on the head. We are talking about loopers not deflectors here. The Tighter the squish clearance, the more heat is forced into the piston. Ex pipe temp actually can go down. This puts the piston at risk. The A and B spec are again critical.

The looser the squish clearance blows the heat out into the pipe raising pipe temp and lowering piston temp putting the piston at less risk. It is a balance calculating the maximum squish velocity to get best/safe performance.
A tight narrow squish band may work as a wide loose squish band, but calculations are require to minimize damage and get best results.

Total Tapped Compression ratio. This is the other part of compression. This determines the fuel Octane/quality that is required. Pump gas runs fine at 6.0:1 Low grade race gas at 7.5:1 in the 100ish range. 8.2:1 is 105 or batter. Higher compression is more heat in combustion. This can effect your piston fits.
These different levels of fuels have different burn rates and temps so getting the timing set correct for each fuel is important.

Know your engine. the more you understand the longer you can go fast vs paddling.

Mike

Great info.

Total Tapped CR? Do you mean Total Trapped CR? Which I know as cylinder volume above the top of the exhaust port to TDC and is much lower than based on total cylinder volume from TDC to BDC.

Thanks
Pete

**Mike Wienandt** · 04-20-2020

Originally Posted by zul8tr

Mike

Great info.

Total Tapped CR? Do you mean Total Trapped CR? Which I know as cylinder volume above the top of the exhaust port to TDC and is much lower than based on total cylinder volume from TDC to BDC.

Thanks
Pete

Effective Compression Ratio is the same as Trapped compression ratio. ECR or TCR. It is when the exhaust port closes. If you raise the E port you are compressing less, so you can make the head tighter to make up for higher ports. A little math required.

Geometric compression ratio is also call four stroke compression ratio. That takes into account the entire cylinder displacement compared to head volume. This is useless in my mind. I need to know what is it really. as so I work with so many different engines and port duration s.

Great question.