Cams road test PW3 vs JS2

Wicking it up when cold can cause damage anywhere in the motor because of cold oil, parts interference etc. Its a bad idea and I have seen scored Harley pistons caused by hard acceleration on cold motors (pistons heating up before the cylinders can warm and expand). There have been no reported issues caused by rocking and remember that even if a piston rocked and touched the head - it will level out. There is no hard unyielding force that is going to cause damage unless your squish band dimension is inadequate and then you are going to see damage everywhere on the piston including the area near the pin where there can be no rocking.

I will consult with the piston manufacture before spending time on things that have already been worked out.

Side Note.

One of my early race bikes with Hepolite pistons was run at Daytona with new carbs that were not jet tested (plug color not checked). The pistons actually melted and collapsed around the rods. This makes it hard to determine what temp pistons should be measured at.

I never use high rpm on a stone cold engine, but might get on it now and then before it is fully hot, as in the condition you describe with pistons heated to the point of reducing bore clearance to almost nil. I see that only happening when really hot, as in pulling a mountain grade in summer with a laden bike. The pistons have to be sized to allow clearance for this type of heat and load or they will seize. The rest of the time there is less heat, varying amounts of expansion/clearances , and of course, piston rock. Sorry to have taken up so much of your time, however whereas you say piston rock is not real or worthy of consideration, every article I read on the subject says that it most definitely is.
Here is one such article taken from the Corvette owners of America library

nother consideration is piston "rock". At TDC as the piston transitions from upward to downward movement, the piston will tip on its wrist pin. This causes one edge of the piston to be a small amount higher than the other edge. The exact amount will vary with how much piston to wall clearance there is; more clearance means more piston rock. Forged pistons generally have a looser piston to wall clearance than cast pistons, but newer design forged pistons have tighter clearances than was used in days gone by. This is another thing that's basically accounted for if a 0.040" quench distance is maintained. Only if less than 0.035" would this possible be an issue

Glen

If .035" squish is used then you're going to have damage to your piston no matter what brand it is.

Other Norton owners have reported that damage occurs with Hepolite pistons using .038" squish clearance. You can bet that damage will also occur with Stock Hepolites with .040" clearance if raced. I have personally seen piston interference with the head with .050" squish clearance using Hepolite pistons.

To tighten up a piston and check clearance when hot may not be good enough. The piston manufacturer needs to see a used piston before making changes. Hundreds of sets have been sold but used JS pistons are extremely rare unless they are in a running motor. I have one from a motor with a broken crank but that's it. I don't know of any others. If someone has a damaged piston from a dropped valve etc - please send it to me.

Hehe of course only drunk or drugged or beat up Harley and Honda riders would blast away full bore on cold engines. Do consider that its universal instructions to initial cold start with new flat lifter cam to run straight over 2000 rpm and vary it above that for 20 min only stopping to let cool if needed then start again to finish seating.

I been thinking [owe] on how to nail down piston clearance in actual or close to it measuring. I'm thinking of torque plate or bored out trashed head so guages can reach in, then direct flame down at piston crown till head gets over 300' F and piston 500' F then pull piston and mic it and then the bores. This would reveal out of roundness too with the head clamping and steel fasteners. i need to learn what gauges to get maybe digital with tablet display. IR temp sensor too.


Short skirt pistons are know to act similar to race tires, trades some wear life for higher performance. If ya take full advantage of what the short skirt piston offer then expect to dive in more often too but so far none the street bikes have reported rocking wear power loss.

jseng1 said:

If .035" squish is used then you're going to have damage to your piston no matter what brand it is.

Other Norton owners have reported that damage occurs with Hepolite pistons using .038" squish clearance. You can bet that damage will also occur with Stock Hepolites with .040" clearance if raced. I have personally seen piston interference with the head with .050" squish clearance using Hepolite pistons.

To tighten up a piston and check clearance when hot may not be good enough. The piston manufacturer needs to see a used piston before making changes. Hundreds of sets have been sold but used JS pistons are extremely rare unless they are in a running motor. I have one from a motor with a broken crank but that's it. I don't know of any others. If someone has a damaged piston from a dropped valve etc - please send it to me.

Click to expand...

As mentioned, the article is from the Corvette Owners of America. I included it to show that piston rock is a "real"
consideration for others, not an imaginary problem or time waster. Of course the clearances quoted in the article are for Corvette engines not Norton Commandos, but their bit on piston rock as a factor in allowing for squish clearance was the take away point in the article.