Sunday, October 14, 2007

Piston Survival Training


Any time you start dealing with old Harley engines with cast iron cylinders, there will be some concern about piston "scuffing", "scoring", or "sticking". All pretty much the same incident, just a matter of degree. In this installment of my series on the Stroker Flathead Engine Build , I would like to address this issue.

I have touched on this issue in another post, The Great Knucklehead Cylinder Controversy . Scored pistons can be a heartbreaking proposition when it leads to a broken cylinder. Its not much fun even if it only causes enough damage to require an overbore. Having seen so many badly scored pistons come out of cast iron cylinders, I was naturally concerned about how the pistons would hold up in this Flathead.

Pistons stick, score, or scuff for one reason. The piston gets hot enough that it expands to the same size as the bore, leaving no clearance. Having too little clearance initially is only one of a number of possible causes. Lack of oil on the skirt can be one culprit. Lack of cooling (sitting in traffic) can be another. Retarded ignition timing and lean mixture are others. Add to that the fact that cast iron motors like Knuckleheads, Flatheads, and Iron Sportsters just tend to hold a lot of heat and you can see the potential for disaster!


In recent years I have been getting the piston skirts for Knuckleheads Teflon coated. The Teflon does a couple of things. It gives an extra margin of "lubricity", but it also allows you to run a little more clearance without fear of getting noise from piston slap (the Teflon coating "dampens" the sound). When Joe (the owner of the Flathead) brought up the idea of having additional coatings done, it sounded like a good idea.

What we wound up doing was having the piston skirts coated with a Teflon and the domes with a ceramic. The ceramic reflects the heat rather than letting it "soak" into the part. Most of the heat which ends up in a piston comes through the top due to being exposed to combustion temperatures. The ceramic should keep the pistons cooler, and thus less likely to stick. The Teflon on the skirts will give an extra level of protection in case of a break down in the oil film.

Added to this we did a ceramic coating on all parts of the combustion chamber (on the Flathead that means both the head and cylinder) including the face of the valve. We also ceramic coated the back side or "tulip" on the exhaust valves and the exhaust ports. Now with all those surfaces having a ceramic coating on them to limit the amount of heat soaking into them, this should be one of the coolest running Flatheads around.

One last thing. Likely not needed, and something you would want to do on a stock looking motor, but I added piston skirt oilers. With some careful measurements I determined that oil ports could be added to the cylinder flanges front and rear of each cylinder. Not any extra room here, so if you choose to do a similar modification be careful ! The holes need to be low enough that they are below the oil control rings with the piston at bottom dead center. Whatever fittings you use have to be small enough to clear each other in the "V" between the cylinders. I used some fittings from Clippard which were a #10-32 thread, 90 degree with 1/8" hose barb. Thses fitting are normally used in pneumatic air control devices. The actual hole going into the cylinder I limited to .050". Like I said, there is not much room for error here. I fed the oilers by way of a "T" in the oil pump return line.


Between all the teflon/ceramic coatings and the piston oilers this Flathead should prove to be about as bulletproof in the area of piston scoring as you can make it!

1 comment:

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