Can anyone advise what piston to head measurements are pushing the limit of safe for an 84mm GSR motor with steel turbo tuff rods?

JE recommends .040" minimum P2H steel rods and .060" for aluminum rods but I was curious what some have gotten away with and if anyone has run tighter?

Thanks in advance to anyone who wishes to share.

Maybe if nothing else we can just have an open discussion to help others in the future whether it be steel or aluminum rods.

 

Their recommendation is correct. Any closer and you run the risk of a piston touching the head. 5 thou closer than that may not be a big deal 90% of the time but a missed gear or a little more crank flex over time and you run a piston into the head.

 

Thanks for the reply!

Obviously the closer you can get the piston to the head the better the quench effect you receive. This being said too much and you end up with contact and smashed ring lands. I know some machinists have talked about getting tighter with it and that's what experience I am hoping some will share.

So you are saying you have run .035" without issue on certain motors? I know there are a lot of variables that need to be considered but just looking for honest info from those who have been there done that.

 

Yes I've run .035 without issues...and have also had the piston kissing the head just right. The problem is when it kisses just too much and you lose a head gasket and it costs you the race.

Big stroke motors have a lot of crank flex. High rpm engines with heavy turbo pistons and rods can have a lot of crank flex. If you have a good rigid setup that doesn't have much movement you can get away with a lot. Also...some small bore motors have pistons that fit right up into the chamber so make sure you check your piston to head in the right spot....closest thing to the piston. You have a turbo car...don't even play with it. We run 60 miniumum in the big power motors. 55 WILL hit

 

What is the variable that affects needing so much extra clearance in big power turbo motors? Is it the excess heat from such high power levels? Or are you factoring this based on the aluminum rods that are typically used in such high power engines.

My current motor is steel rods and JE says .040" for steel and .060" for aluminum.

Just wanting to clarify if you are recommending the greater .060" clearance for steel rod "big power" turbo motors. If so do you recommend even greater for aluminum rod "big power" turbo motors?

 

60 minimum on alu rods you mean!?
I really hope NOT 55 will hit on steel rods!

 

Do you take how far the piston is in the hole @TDC and add the head gasket thickness to get piston to head clearance?

 

Yes that is the process. Piston to deck height + HG thickness = Piston to head measurement.

To elaborate on that (from my research), you also need to account for the piston "rock" back and forth that is created by the piston to wall clearance. The greater the P2W the greater the "rock".

Push the piston top on intake and exhaust side until the skirt contacts the cylinder bore and measure the high and low measurements during the process. Then, according to my sources, the median of the two numbers would be used as your piston to deck height in the equation above.

For example; if your measurements come out as .010" (high) in the hole and .020" (low) in the hole then your piston to deck measurement used for this calculation would be .015" in the hole.

Maybe 4Piston can confirm or deny that this is the process they use as well. My information came from a trusted machinist.