Piston ID?

Brettm57

Member
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?
 

flattop1

Dawg 89
Yes .
.020 with oversize top ring .
Or .020 .
 
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?

-----
?Thanks and God bless,
Brian Carlson
Carlson Racing Engines
Vector Cutz
www.CarlsonMotorsports.com
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30 years of service to the karting industry
Linden, IN
765-339-4407
bcarlson@CarlsonMotorsports.com
 

Brettm57

Member
I'm getting 30% leakdown at 100 psi. The engine was cold and the cylinder was pretty dry.

This is my first time ever using a leakdown tester, so I'm still learning how to use the tool. I'm thinking 30% has to be bad though
 

flattop1

Dawg 89
You did lock it at tdc ?
30% isn't too good .
Can you hear it coming out the breather or either port .
 

Brettm57

Member
Yeah I locked it. It's blowing big-time out the oil fill. I just got it apart and I have .007" piston to cylinder clearance. A little loose me thinks.......
 
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.)
 

Brettm57

Member
Is there a recommended pressure to use for a leakdown test on these engines? I'm thinking the more pressure I use the worse the percentage of leakdown is going to be.

How does the Burris piston compare to the Briggs piston for quality? Is one better than the other?

Thanks for your input!
 
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.
 

flattop1

Dawg 89
Aren't the 555470-484 pistons chrome ?
At least thats what on my note pad .
With the 555672-678 unchromed .
 

WPaul

Member
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%.

Best, WP
 
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