I just took the head off my flathead, and the piston has the part #555470. I take it that is an OEM part#? It says it's a .020" oversize. A leakdown test tells me it needs new rings. What ring set do I need to go with this piston?
If the rings are leaking down excessively, it likely needs honed (possibly bored to the next piston size.)
Remember, most builders will use low tension rings in their engines to reduce drag -- they do leak considerably in a dry cylinder.
How much percentage leakage are you seeing?
Yea, especially with the new(er) style R3 pistons. You can run that rascal as tight as .002" with no problems (just keep an eye on temps.)
Loose is fast, but when the piston rocks, it will unseat the rings and you lose seal - that is probably what is happening to see as much leak down as you're getting. 2% on an adult stocker is good with low tension rings. For the small plate stuff we run undersized/shrunk rings, that'll leak considerably more (esp. the bottom two rings.)
I've always used 100psi - just a source of habit. I get the same measurements as when checking with 50 psi..my thought is that leakage should be more easily detected using higher psi -- that's certainly the case when measuring airflow on the bench.
Burris is fine. Their rings are better than Briggs oem rings.
You can still get chromed Burris pistons, that's a bonus.
Be sure to use the Briggs wrist pin though.
When leak checking an engine be sure to lock the crankshaft but after the pressure is applied allow the engine rock or the crank to move very slightly. The top ring will not be seated on the bottom of the top ring land and the leaking will be around the back side of the ring. With just a slight movement of the piston, POP, the ring seats and you get your true leakage. I use 100 PSI,, and NO it will not seat that ring.
Example; an excellent prepared Briggs FH cylinder that leaks 1% or less will show about 13% under the above conditions. as soon as the ring touches the lower land, POP,, 1%.