Piston clearance

[Tom H]

I'm ordering custom pistons from Venolia, for my 1725. I plan to order them as 3.248 in dia, as this is a standard (82.5 cm) size and would then use readily available rings. One question Venolia has on their order form is "piston clearance". I was planning on having my machine shop set that clearance when they bore the block to match the new pistons - after I have the pistons in hand. But I was wondering where / how the clearance is normally achieved. Does the piston maker make the dia undersize by the clearance amount? Or does the block borer make the bore oversize by the clearance amount. It seems a moot point somewhat, since I plan to have the bore done AFTER the pistons are in hand, but I was unsure of what the normal expectation is.

My assumption is that I will tell Venolia to make the pistons exactly 3.248 in and the block machinist will set the clearance. Right?

Thanks ,

Tom

 

[RootesRacer]

Your bore clearance will have to do with application, piston construction and end use.

N/A applications will typically have tighter tolerances, 1.5 thou is common for a production engine.
Turbo engines need more clearance due to the higher temps and the coef of thermal expansion being higher on alum than iron.

Race engines will have even looser tolerances for lower friction, but these engines will be noisier due to piston slap.

Certain pistons are designed with slippers cast in to them to allow looser clearances, but prevent piston slap by focusing expansion to the skirts.
You probably wont have this to consider.

Have the pistons made first, then send them to an experienced shop that builds stock to race setups, then tell them what you want, they will determine the proper clearance.

Otherwise .002 is a safe clearance for a basic N/A street motor.

 

[sunbeam74]

I took the other approach. I had my machine shop clean up the bores in advance. Then communicated the finial bore dimension and application to Venolia.

I also bought my rings with the pistons from Venolia.

It just simplified everything.... at least for me.

Steve

 

[jmthehermit]

Tom, get the block done first! After all the cylinder bores are machined and identical in size, then order your pistons. Jeff

 

[Tom H]

Jarrid, Steve, Hermit,

Thanks for the advice. Between your answers and my machinist, I think I got the plan.

Assuming I get this thing together, I'm making plans to drive it to the west coast (from Cleveland) sometime late next summer. Maybe make the next Invasion too.

Tom