Piston to cylinder wall clearance on a 1970 440

[Wing It]

Hi everyone. I am building a 1970 440 and I have Speed Pro L2355F pistons. On the box it says "Minimum Clearance - .001"" . An older Speed Pro catalog says these pistons should have a minimum clearance of .0015". We called Speed Pro and they stand by these specs although they will not comment on what the maximum clearance is. They told us that if we run .0035" they will likely crack.

I know the piston and skirt design matter as far as how they are constructed. We put the pistons in an oven and measured them hot and they grew about .0035" at 180 degrees.

These pistons are the VMS-75 alloy. Other manufacturers make pistons with the same VMS-75 alloy and they list clearances in the .0025" to .0055" range for a piston about the same size.

The engine is mostly stock for a 1970 440 Super Commando. 906 heads, six pack rods, .060" bore, slightly larger valves, mild hydraulic Lunati cam. Right now we have the block honed to .001" clearance. With only about a half thou on each side of the piston it sure feels tight. My machinist is thinking we might consider .002" - .0025". Has anyone have experience with these same pistons in a 440? If so, what clearance did you run? How long has it been? Any problems?

Thank you!

 

[R413]

That sounds awfully tight. Do you know what a Engine with tight pistons will run like? Not good. Run hot, down on power, and it won’t last very long, scuffing pistons and cylinder walls.

what kind of driving will this engine see?

I like to run .0028 to .003 on cast pistons in a 440. You need more clearance when the bore gets bigger.

 

[440 Charger]

That sounds awfully tight. Do you know what a Engine with tight pistons will run like? Not good. Run hot, down on power, and it won’t last very long, scuffing pistons and cylinder walls.

what kind of driving will this engine see?

I like to run .0028 to .003 on cast pistons in a 440. You need more clearance when the bore gets bigger.

You know your stuff Sir.

 

[440 Charger]

Hi everyone. I am building a 1970 440 and I have Speed Pro L2355F pistons. On the box it says "Minimum Clearance - .001"" . An older Speed Pro catalog says these pistons should have a minimum clearance of .0015". We called Speed Pro and they stand by these specs although they will not comment on what the maximum clearance is. They told us that if we run .0035" they will likely crack.

I know the piston and skirt design matter as far as how they are constructed. We put the pistons in an oven and measured them hot and they grew about .0035" at 180 degrees.

These pistons are the VMS-75 alloy. Other manufacturers make pistons with the same VMS-75 alloy and they list clearances in the .0025" to .0055" range for a piston about the same size.

The engine is mostly stock for a 1970 440 Super Commando. 906 heads, six pack rods, .060" bore, slightly larger valves, mild hydraulic Lunati cam. Right now we have the block honed to .001" clearance. With only about a half thou on each side of the piston it sure feels tight. My machinist is thinking we might consider .002" - .0025". Has anyone have experience with these same pistons in a 440? If so, what clearance did you run? How long has it been? Any problems?

Thank you!

Machinest is correct.

 

[Wing It]

That sounds awfully tight. Do you know what a Engine with tight pistons will run like? Not good. Run hot, down on power, and it won’t last very long, scuffing pistons and cylinder walls.

what kind of driving will this engine see?

I like to run .0028 to .003 on cast pistons in a 440. You need more clearance when the bore gets bigger.

Cast pistons? These are forged and the VMS-75 alloy. The driving will just be cruising in town and some freeway. It is a matching number 440 block in a Superbird. I will get on it from time to time but no racing. Stock performance or slightly better is fine with me.

 

[1 Wild R/T]

My first thought was .003+ but I did a quick search and found this from one of the pro engine builders here...

I would agree with your Shop (smart guys

),
no tighter than .0035" measured above the skirt coating

So I'm guessing the newer 2355's have that black anti scuff coating & Speed Pro anticipates you measuring the coating... On some late model OE pistons with the coating the bore clearance spec is actually a .0005 interference fit...

I'd go with the measure above the coating & run .003-.0035

 

[33 IMP]

I would never build a forged piston engine tighter than .003 .

But then, ive raced cast piston engines with .008, and a forged piston 426 that turned out to have .012. (They were both a bit "noisy").

 

[440 Charger]

That sounds awfully tight. Do you know what a Engine with tight pistons will run like? Not good. Run hot, down on power, and it won’t last very long, scuffing pistons and cylinder walls.

what kind of driving will this engine see?

I like to run .0028 to .003 on cast pistons in a 440. You need more clearance when the bore gets bigger.

I would never build a forged piston engine tighter than .003 .

Oil is always a key.

 

[Challenger340]

We really like the ICON IC9953 Forgings.... same 5/64. 5/64. 3/16 Ring Pack as the 2355's but a TON lighter
and,
the ICON IC9953's are typically built so that if you Bore & Hone Final Sizing around .0005" above nominal dimension ?
you end up bang on around .0035" Clrc with .018" Top & 2nd Ring End Gaps