Biggest piston

[Santana]

What is the largest piston size I can put in a flathead 5?

 

[bowtiebutler]

The largest size you can get in a regular piston is .030 oversize. Now you can get forged racing pistons all the way up to around .080 oversize.

Matt

 

[Santana]

Where can I find the forged racing pistons

 

[bowtiebutler]

Santana, why would you want to bore one over that far? The thinner you make a cylinder, the more prone to distortion it is. This can cause poor ring seal, as well as reduced engine life. I only have an engine bored as far as it needs to go for the cylinder to clean up. The slightly larger displacement the engine gets from a bigger piston is not worth the trade off, of having a thin cylinder wall.

Matt

 

[Santana]

I have a badly scored cylinder wall so you're saying going maybe to a .030 over raptor piston would do it good? Is there a such thing as that

 

[creia]

I have a badly scored cylinder wall so you're saying going maybe to a .030 over raptor piston would do it good? Is there a such thing as that

Is this a "Kool Bore" (aluminum cylinder wall) engine? If so, unless you are married to that 5 HP block for some reason, it would most likely be more cost effective to get another good used 5 HP block or engine for what you will spend boring, honing, new piston, rings, etc.
Michael

 

[bpoirier]

Gotta make sure you use a chromed piston in a cool bore. Burris racing makes them.

 

[philip metzner]

I would stay within the .030 so you can use briggs parts. I have run them at .080, but the rings that they use are problematic. They use a three piece oil ring and they will tear hunks of the cylinder out sometimes. You are supposed to trim the expander to get the right tension, but your pretty much forced to cut one loop out, then the tension is to low. Its not to much problem on a race kart cause the oil is changed so frequently. On a mini, or kart it may get low to quickly.